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Journals
Argudo, Oscar; Chica, Antoni; Andújar, Carlos
Computer Graphics Forum, Vol. 37, Num. 2, 2018.
DOI: http://dx.doi.org/10.1111/cgf.13345
Despite recent advances in surveying techniques, publicly available Digital Elevation Models (DEMs) of terrains are lowresolution except for selected places on Earth. In this paper we present a new method to turn low-resolution DEMs into plausible and faithful high-resolution terrains. Unlike other approaches for terrain synthesis/amplification (fractal noise, hydraulic and thermal erosion, multi-resolution dictionaries), we benefit from high-resolution aerial images to produce highly-detailed DEMs mimicking the features of the real terrain. We explore different architectures for Fully Convolutional Neural Networks to learn upsampling patterns for DEMs from detailed training sets (high-resolution DEMs and orthophotos), yielding up to one order of magnitude more resolution. Our comparative results show that our method outperforms competing data amplification approaches in terms of elevation accuracy and terrain plausibility.
Besuievsky, Gonzalo; Beckers, Benoit; Patow, Gustavo A.
Graphical Models, Vol. 95, pp 42-50, 2018.
DOI: http://dx.doi.org/10.1016/j.gmod.2017.06.002
Solar simulation for 3D city models may be a complex task if detailed geometry is taken into account. For this reason, the models are often approximated by simpler geometry to reduce their size and complexity. However, geometric details, as for example the ones that exist in a roof, can significantly change the simulation results if not properly taken into account. The classic solution to deal with a too detailed city model is to use a Level-of-Detail (LoD) approach for geometry reduction. In this paper we present a new LoD strategy for 3D city models aimed at accurate solar simulations able to cope with models with highly detailed geometry. Given a Point of Interest (POI) or a Region of Interest (ROI) to analyze, the method works by automatically detecting and preserving all the geometry (i.e., roofs) that have significant impact on the simulation and simplifying the rest of the geometry.
Casafont, Miquel; Bonada, Jordi; Roure, Francesc; Pastor, Magdalena; Susin, Antonio
International Journal of Structural Stability and Dynamics, Vol. 18, Num. 1, pp 1--32, 2018.
DOI: http://dx.doi.org/10.1142/S0219455418500049
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The investigation attempts to adapt a beam finite element procedure based on the Generalized Beam Theory (GBT) to the analysis of perforated columns. The presence of perforations is taken into account through the use of two beam elements with different properties for the non-perforated and perforated parts of the member. Each part is meshed with its corresponding finite element and, afterwards, they are linked by means of constraint equations. Linear buckling analyses on steel storage rack columns are carried out to demonstrate how the proposed procedure should be applied. Some practical issues are discussed, such as the GBT deformation modes to be included in the analyses, or the optimum finite element discretization. The resulting buckling loads are validated by comparison with the values obtained in analyses performed using shell finite element models. Finally, it is verified that the buckling loads produced with the proposed method are rather accurate.
Diego Jesus; Patow, Gustavo A.; António Coelho; António Augusto Sousa
Computers & Graphics, Vol. 72, pp 106-121, 2018.
DOI: http://dx.doi.org/10.1016/j.cag.2018.02.003
Procedural modeling techniques reduce the effort of creating large virtual cities. However, current methodologies do not allow direct user control over the generated models. Associated with this problem, we face the additional problem related to intrinsic ambiguity existing in user selections. In this paper, we propose to address this problem by using a genetic algorithm to generalize user-provided point-and-click selections of building elements. From a few user-selected elements, the system infers new sets of elements that potentially correspond to the user’s intention, including the ones manually selected. These sets are obtained by queries over the shape trees generated by the procedural rules, thus exploiting shape semantics, hierarchy and geometric properties. Our system also provides a complete selection-action paradigm that allows users to edit procedurally generated buildings without necessarily explicitly writing queries. The pairs of user selections and procedural operations (the actions) are stored in a tree-like structure, which is easily evaluated. Results show that the selection inference is capable of generating sets of shapes that closely match the user intention and queries are able to perform complex selections that would be difficult to achieve in other systems. User studies confirm this result.
Díaz-García, Jesús; Brunet, Pere; Navazo, Isabel; Vázquez, Pere-Pau
Computers & Graphics, 2018.
DOI: http://dx.doi.org/https://doi.org/10.1016/j.cag.2018.02.007
Mobile devices have experienced an incredible market penetration in the last decade. Currently, medium to premium smartphones are relatively a ordable devices. With the increase in screen size and resolution, together with the improvements in performance of mobile CPUs and GPUs, more tasks have become possible. In this paper we explore the rendering of medium to large volumetric models on mobile and low performance devices in general. To do so, we present a progressive ray casting method that is able to obtain interactive frame rates and high quality results for models that not long ago were only supported by desktop computers.
A General Illumination Model for Molecular Visualization
Hermosilla, Pedro; Vázquez, Pere-Pau; Vinacua, Àlvar; Ropinski, Timo
Computer Graphics Forum, 2018.
Several visual representations have been developed over the years to visualize molecular structures, and to enable a better understanding of their underlying chemical processes. Today, the most frequently used atom-based representations are the Space-filling, the Solvent Excluded Surface, the Balls-and-Sticks, and the Licorice models. While each of these representations has its individual benefits, when applied to large-scale models spatial arrangements can be difficult to interpret when employing current visualization techniques. In the past it has been shown that global illumination techniques improve the perception of molecular visualizations; unfortunately existing approaches are tailored towards a single visual representation. We propose a general illumination model for molecular visualization that is valid for different representations. With our illumination model, it becomes possible, for the first time, to achieve consistent illumination among all atom-based molecular representations. The proposed model can be further evaluated in real-time, as it employs an analytical solution to simulate diffuse light interactions between objects. To be able to derive such a solution for the rather complicated and diverse visual representations, we propose the use of regression analysis together with adapted parameter sampling strategies as well as shape parametrization guided sampling, which are applied to the geometric building blocks of the targeted visual representations. We will discuss the proposed sampling strategies, the derived illumination model, and demonstrate its capabilities when visualizing several dynamic molecules.
A technique for massive sky view factor calculations in large cities
Muñoz, David; Beckers, Benoit; Besuievsky, Gonzalo; Patow, Gustavo A.
International Journal of Remote Sensing, Vol. 39, Num. 112, pp 4040--4058, 2018.
DOI: http://dx.doi.org/https://doi.org/10.1080/01431161.2018.1452071
In many applications, such as urban physics simulations or the study of the solar impact effects at different scales, complex 3D city models are required to evaluate physical values. In this article, we propose an efficient system for quickly computing the Sky View Factor (SVF) for a massive number of points inside a large city. To do that, we embed the city into a regular grid, and for each cell we select a subset of the geometry consisting of a square area centred in the cell and including it. Then, we remove the selected geometry from the city model and we project the rest onto a panoramic image, called environment map. Later, when several SVF evaluations are required, we only need to determine the cell that each evaluation point belongs to, and compute the SVF with the cell’s geometry plus its corresponding environment map. To test our system, we perform several evaluations inside a cell’s area, and compare the results with an accurate ray-tracing-based SVF evaluation. Our results show the feasibility of the method and its advantages when used for a large set of computations. We show that our tool provides a way to handle the complexity of urban scale models, and specifically allows working with geometry details if they are required.
Muñoz-Pandiella, Imanol; Bosch, Carles; Mérillou, Stephane; Mérillou, Nicolas; Patow, Gustavo A.; Pueyo, Xavier
IEEE Transactions on Visualization and Computer Graphics, 2018.
DOI: http://dx.doi.org/10.1109/TVCG.2018.2794526
Weathering effects are ubiquitous phenomena in cities. Buildings age and deteriorate over time as they interact with the environment. Pollution accumulating on facades is a particularly visible consequence of this. Even though relevant work has been done to produce impressive images of virtual urban environments including weathering effects, so far, no technique using a global approach has been proposed to deal with weathering effects. Here, we propose a technique based on a fast physically-inspired approach, that focuses on modeling the changes in appearance due to pollution soiling on an urban scale. We consider pollution effects to depend on three main factors: wind, rain and sun exposure, and we take into account three intervening steps: deposition, reaction and washing. Using a low-cost pre-computation, we evaluate the pollution distribution throughout the city. Based on this and the use of screen-space operators, our method results in an efficient approach able to generate realistic images of urban scenes by combining the intervening factors at interactive rates. In addition, the pre-computation demands a reduced amount of memory to store the resulting pollution map and, as it is independent from scene complexity, it can suit large and complex models by adapting the map resolution.
Visual Analysis of Protein-ligand Interactions
Vázquez, Pere-Pau; Hermosilla, Pedro; Guallar, V.; Estrada, J.; Vinacua, Àlvar
Computer Graphics Forum, 2018.
The analysis of protein-ligand interactions is complex because of the many factors at play. Most current methods for visual analysis provide this information in the form of simple 2D plots, which, besides being quite space hungry, often encode a low number of different properties. In this paper we present a system for compact 2D visualization of molecular simulations. It purposely omits most spatial information and presents physical information associated to single molecular components and their pairwise interactions through a set of 2D InfoVis tools with coordinated views, suitable interaction, and focus+context techniques to analyze large amounts of data. The system provides a wide range of motifs for elements such as protein secondary structures or hydrogen bond networks, and a set of tools for their interactive inspection, both for a single simulation and for comparing two different simulations. As a result, the analysis of protein-ligand interactions of Molecular Simulation trajectories is greatly facilitated.
Aguerre, Jose Pedro; Fernandez, Eduardo; Besuievsky, Gonzalo; Beckers, Benoit
Graphical Models, Vol. 91, pp 1--11, 2017.
DOI: http://dx.doi.org/10.1016/j.gmod.2017.05.002
Cities numerical simulation including physical phenomena generates highly complex computational chal- lenges. In this paper, we focus on the radiation exchange simulation on an urban scale, considering differ- ent types of cities. Observing that the matrix representing the view factors between buildings is sparse, we propose a new numerical model for radiation computation. This solution is based on the radiosity method. We show that the radiosity matrix associated with models composed of up to 140k patches can be stored in main memory, providing a promising avenue for further research. Moreover, a new technique is proposed for estimating the inverse of the radiosity matrix, accelerating the computation of radiation exchange. These techniques could help to consider the characteristics of the environment in building de- sign, as well as assessing in the definition of city regulations related to urban construction.
Coherent multi-layer landscape synthesis
Argudo, Oscar; Andújar, Carlos; Chica, Antoni; Guérin, Eric; Digne, Julie; Peytavie, Adrien; Galin, Eric
The Visual Computer, Vol. 33, Num. 6, pp 1005--1015, 2017.
DOI: http://dx.doi.org/10.1007/s00371-017-1393-6
We present an efficient method for generating coherent multi-layer landscapes. We use a dictionary built from exemplars to synthesize high-resolution fully featured terrains from input low-resolution elevation data. Our example-based method consists in analyzing real-world terrain examples and learning the procedural rules directly from these inputs. We take into account not only the elevation of the terrain, but also additional layers such as the slope, orientation, drainage area, the density and distribution of vegetation, and the soil type. By increasing the variety of terrain exemplars, our method allows the user to synthesize and control different types of landscapes and biomes, such as temperate or rain forests, arid deserts and mountains.
Argudo, Oscar; Comino, Marc; Chica, Antoni; Andújar, Carlos; Lumbreras, Felipe
Computers & Graphics, 2017.
DOI: http://dx.doi.org/10.1016/j.cag.2017.11.004
The visual enrichment of digital terrain models with plausible synthetic detail requires the segmentation of aerial images into a suitable collection of categories. In this paper we present a complete pipeline for segmenting high-resolution aerial images into a user-defined set of categories distinguishing e.g. terrain, sand, snow, water, and different types of vegetation. This segmentation-for-synthesis problem implies that per-pixel categories must be established according to the algorithms chosen for rendering the synthetic detail. This precludes the definition of a universal set of labels and hinders the construction of large training sets. Since artists might choose to add new categories on the fly, the whole pipeline must be robust against unbalanced datasets, and fast on both training and inference. Under these constraints, we analyze the contribution of common per-pixel descriptors, and compare the performance of state-of-the-art supervised learning algorithms. We report the findings of two user studies. The first one was conducted to analyze human accuracy when manually labeling aerial images. The second user study compares detailed terrains built using different segmentation strategies, including official land cover maps. These studies demonstrate that our approach can be used to turn digital elevation models into fully-featured, detailed terrains with minimal authoring efforts.
Gemelli-Obturator Complex in the deep gluteal space. An anatomic and dynamic study.
Balius, R.; Susin, Antonio; Morros, C.; Pujol, M.; Perez, M.; Sala, X.
Skeletal Radiology, pp 1--8, 2017.
DOI: http://dx.doi.org/10.1007/s00256-017-2831-2
Bendezú, Alvaro; Mego, Marianela; Monclús, Eva; Merino, Xavier; Accarino, Ana; Malagelada, Juan Ramón; Navazo, Isabel; Azpiroz, Fernando
Neurogastroenterology and Motility, Vol. 29, Num. 2, pp 12930-1--12930-8, 2017.
DOI: http://dx.doi.org/10.1111/nmo.12930
Background: The metabolic activity of colonic microbiota is influenced by diet; however, the relationship between metabolism and colonic content is not known. Our aim was to determine the effect of meals, defecation, and diet on colonic content. Methods: In 10 healthy subjects, two abdominal MRI scans were acquired during fasting, 1 week apart, and after 3 days on low- and high-residue diets, respectively. With each diet, daily fecal output and the number of daytime anal gas evacuations were measured. On the first study day, a second scan was acquired 4 hours after a test meal (n=6) or after 4 hours with nil ingestion (n=4). On the second study day, a scan was also acquired after a spontaneous bowel movement. Results: On the low-residue diet, daily fecal volume averaged 145 ± 15 mL; subjects passed 10.6 ± 1.6 daytime anal gas evacuations and, by the third day, non-gaseous colonic content was 479 ± 36 mL. The high-residue diet increased the three parameters to 16.5 ± 2.9 anal gas evacuations, 223 ± 19 mL fecal output, and 616 ± 55 mL non-gaseous colonic content (P<.05 vs low-residue diet for all). On the low-residue diet, non-gaseous content in the right colon had increased by 41 ± 11 mL, 4 hours after the test meal, whereas no significant change was observed after 4-hour fast (-15 ± 8 mL; P=.006 vs fed). Defecation significantly reduced the non-gaseous content in distal colonic segments. Conclusion & inferences: Colonic content exhibits physiologic variations with an approximate 1/3 daily turnover produced by meals and defecation, superimposed over diet-related day-to-day variations.
Coll, Narcís; Guerrieri, Marité Ethel
International Journal of Geographical Information Science, Vol. 31, Num. 7, pp 1467--1484, 2017.
DOI: http://dx.doi.org/10.1080/13658816.2017.1300804
In this paper, we propose a new graphics processing unit (GPU) method able to compute the 2D constrained Delaunay triangulation (CDT) of a planar straight-line graph consisting of points and segments. All existing methods compute the Delaunay triangulation of the given point set, insert all the segments, and then finally transform the resulting triangulation into the CDT. To the contrary, our novel approach simultaneously inserts points and segments into the triangulation, taking special care to avoid conflicts during retriangulations due to concurrent insertion of points or concurrent edge flips. Our implementation using the Compute Unified Device Architecture programming model on NVIDIA GPUs improves, in terms of running time, the best known GPU-based approach to the CDT problem.
Error-aware Construction and Rendering of Multi-scan Panoramas from Massive Point Clouds
Comino, Marc; Andújar, Carlos; Chica, Antoni; Brunet, Pere
Computer Vision and Image Understanding, Vol. 157, pp 43--54, 2017.
DOI: http://dx.doi.org/10.1016/j.cviu.2016.09.011
Obtaining 3D realistic models of urban scenes from accurate range data is nowadays an important research topic, with applications in a variety of fields ranging from Cultural Heritage and digital 3D archiving to monitoring of public works. Processing massive point clouds acquired from laser scanners involves a number of challenges, from data management to noise removal, model compression and interactive visualization and inspection. In this paper, we present a new methodology for the reconstruction of 3D scenes from massive point clouds coming from range lidar sensors. Our proposal includes a panorama-based compact reconstruction where colors and normals are estimated robustly through an error-aware algorithm that takes into account the variance of expected errors in depth measurements. Our representation supports efficient, GPU-based visualization with advanced lighting effects. We discuss the proposed algorithms in a practical application on urban and historical preservation, described by a massive point cloud of 3.5 billion points. We show that we can achieve compression rates higher than 97% with good visual quality during interactive inspections.
Díaz, Jose; Ropinski, Timo; Navazo, Isabel; Gobbetti, Enrico; Vázquez, Pere-Pau
The Visual Computer, Vol. 33, Num. 1, pp 47-61, 2017.
DOI: http://dx.doi.org/10.1007/s00371-015-1151-6
Throughout the years, many shading techniques have been developed to improve the conveying of information in volume visualization. Some of these methods, usually referred to as realistic, are supposed to provide better cues for the understanding of volume data sets. While shading approaches are heavily exploited in traditional monoscopic setups, no previous study has analyzed the effect of these techniques in virtual reality. To further explore the influence of shading on the understanding of volume data in such environments, we carried out a user study in a desktop-based stereoscopic setup. The goals of the study were to investigate the impact of well-known shading approaches and the influence of real illumination on depth perception. Participants had to perform three different perceptual tasks when exposed to static visual stimuli. 45 participants took part in the study, giving us 1152 trials for each task. Results show that advanced shading techniques improve depth perception in stereoscopic volume visualization. As well, external lighting does not affect depth perception when these shading methods are applied. As a result, we derive some guidelines that may help the researchers when selecting illumination models for stereoscopic rendering.
Ferre, Josep; Peña, Marta; Susin, Antonio
International Journal of Bifurcation and Chaos, Vol. 27, Num. 1, pp 1-13, 2017.
DOI: http://dx.doi.org/10.1142/S0218127417500055
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We complete the study of the bifurcations of saddle/spiral bimodal linear systems, depending on the respective traces T and τ: one 2-codimensional bifurcation; four kinds of 1-codimensional bifurcations. We stratify the bifurcation set in the (T, τ)-plane and we describe the qualitative changes of the dynamical behavior at each kind of bifurcation point
A Perspective on procedural modeling based on structural analysis
Fita, Josep Lluis; Besuievsky, Gonzalo; Patow, Gustavo A.
Virtual Archaeology Review, Vol. 8, Num. 16, pp 44--50, 2017.
DOI: http://dx.doi.org/10.4995/var.2017.5765.
With the rise of available computing capabilities, structural analysis has recently become a key tool for building assessment usually managed by art historians, curators, and other specialist related to the study and preservation of ancient buildings. On the other hand, the flourishing field of procedural modeling has provided some exciting breakthroughs for the recreation of lost buildings and urban structures. However, there is a surprising lack of literature to enable the production of procedural-based buildings taking into account structural analysis, which has proven to be a crucial element for the recreation of faithful masonry structures. In order to perform an in-depth study of the advances in this type of analysis for cultural heritage buildings, we performed a study focused on procedural modeling that make use of structural analysis methods, especially in its application to historic masonry buildings such as churches and cathedrals. Moreover, with the aim of improving the knowledge about structural analysis of procedurally-recreated historical buildings, we have taken a geometric structure, added a set of procedural walls structured in masonry bricks, and studied its behavior in a generic, freely-available simulation tool, thus showing the feasibility of its analysis with non-specialized tools. This not only has allowed us to understand and learn how the different parameter values of a masonry structure can affect the results of the simulation, but also has proven that this kind of simulations can be easily integrated in an off-the-shelf procedural modeling tool, enabling this kind of analysis for a wide variety of historical studies, or restoration and preservation actions.
Intersecting two families of sets on the GPU
Fort, Marta; Sellarès, J. Antoni; Valladares, Ignacio
Journal of Parallel and Distributed Computing, 2017.
DOI: http://dx.doi.org/https://doi.org/10.1016/j.jpdc.2017.01.026
The computation of the intersection family of two large families of unsorted sets is an interesting problem from the mathematical point of view which also appears as a subproblem in decision making applications related to market research or temporal evolution analysis problems. The problem of intersecting two families of sets F and F′ is to find the family I of all the sets which are the intersection of some set of F and some other set of F′. In this paper, we present an efficient parallel GPU-based approach, designed under CUDA architecture, to solve the problem. We also provide an efficient parallel GPU strategy to summarize the output by removing the empty and duplicated sets of the obtained intersection family, maintaining, if necessary, the sets frequency. The complexity analysis of the presented algorithm together with experimental results obtained with its implementation is also presented.
Hermosilla, Pedro; Jorge Estrada; Víctor Guallar; Timo Ropinsky; Vinacua, Àlvar; Vázquez, Pere-Pau
IEEE Transactions on Visualization and Computer Graphics, Vol. 23, Num. 1, pp 731--740, 2017.
DOI: http://dx.doi.org/10.1109/TVCG.2016.2598825
Molecular simulations are used in many areas of biotechnology, such as drug design and enzyme engineering. Despite the development of automatic computational protocols, analysis of molecular interactions is still a major aspect where human comprehension and intuition are key to accelerate, analyze, and propose modifications to the molecule of interest. Most visualization algorithms help the users by providing an accurate depiction of the spatial arrangement: the atoms involved in inter-molecular contacts. There are few tools that provide visual information on the forces governing molecular docking. However, these tools, commonly restricted to close interaction between atoms, do not consider whole simulation paths, long-range distances and, importantly, do not provide visual cues for a quick and intuitive comprehension of the energy functions (modeling intermolecular interactions) involved. In this paper, we propose visualizations designed to enable the characterization of interaction forces by taking into account several relevant variables such as molecule-ligand distance and the energy function, which is essential to understand binding affinities. We put emphasis on mapping molecular docking paths obtained from Molecular Dynamics or Monte Carlo simulations, and provide time-dependent visualizations for different energy components and particle resolutions: atoms, groups or residues. The presented visualizations have the potential to support domain experts in a more efficient drug or enzyme design process.
Hermosilla, Pedro; Michael Krone; Víctor Guallar; Vázquez, Pere-Pau; Vinacua, Àlvar; Timo Ropinsky
The Visual Computer, Vol. 33, Num. 6, pp 869--881, 2017.
DOI: http://dx.doi.org/10.1007/s00371-017-1397-2
The Solvent Excluded Surface (SES) is a popular molecular representation that gives the boundary of the molecular volume with respect to a specific solvent. SESs depict which areas of a molecule are accessible by a specific solvent, which is represented as a spherical probe. Despite the popularity of SESs, their generation is still a compute-intensive process, which is often performed in a preprocessing stage prior to the actual rendering (except for small models). For dynamic data or varying probe radii, however, such a preprocessing is not feasible as it prevents interactive visual analysis. Thus, we present a novel approach for the on-the-fly generation of SESs, a highly parallelizable, grid-based algorithm where the SES is rendered using ray-marching. By exploiting modern GPUs, we are able to rapidly generate SESs directly within the mapping stage of the visualization pipeline. Our algorithm can be applied to large time-varying molecules and is scalable, as it can progressively refine the SES if GPU capabilities are insufficient. In this paper, we show how our algorithm is realized and how smooth transitions are achieved during progressive refinement. We further show visual results obtained from real world data and discuss the performance obtained, which improves upon previous techniques in both the size of the molecules that can be handled and the resulting frame rate.
Real-Time Solar Exposure Simulation in Complex Cities
Muñoz-Pandiella, Imanol; Bosch, Carles; Mérillou, Nicolas; Pueyo, Xavier; Mérillou, Stephane
Computer Graphics Forum, 2017.
DOI: http://dx.doi.org/10.1111/cgf.13152
In urban design, estimating solar exposure on complex city models is crucial but existing solutions typically focus on simplified building models and are too demanding in terms of memory and computational time. In this paper, we propose an interactive technique that estimates solar exposure on detailed urban scenes. Given a directional exposure map computed over a given time period, we estimate the sky visibility factor that serves to evaluate the final exposure at each visible point. This is done using a screen-space method based on a two-scale approach, which is geometry independent and has low storage costs. Our method performs at interactive rates and is designer-oriented. The proposed technique is relevant in architecture and sustainable building design as it provides tools to estimate the energy performance of buildings as well as weathering effects in urban environments.
Argudo, Oscar; Besora, Isaac; Brunet, Pere; Creus, Carles; Hermosilla, Pedro; Navazo, Isabel; Vinacua, Àlvar
Computer-Aided Design, Vol. 79, pp 48--59, 2016.
DOI: http://dx.doi.org/10.1016/j.cad.2016.06.005
The use of virtual prototypes and digital models containing thousands of individual objects is commonplace in complex industrial applications like the cooperative design of huge ships. Designers are interested in selecting and editing specific sets of objects during the interactive inspection sessions. This is however not supported by standard visualization systems for huge models. In this paper we discuss in detail the concept of rendering front in multiresolution trees, their properties and the algorithms that construct the hierarchy and efficiently render it, applied to very complex CAD models, so that the model structure and the identities of objects are preserved. We also propose an algorithm for the interactive inspection of huge models which uses a rendering budget and supports selection of individual objects and sets of objects, displacement of the selected objects and real-time collision detection during these displacements. Our solution ---based on the analysis of several existing view-dependent visualization schemes--- uses a Hybrid Multiresolution Tree that mixes layers of exact geometry, simplified models and impostors, together with a time-critical, view-dependent algorithm and a Constrained Front. The algorithm has been successfully tested in real industrial environments; the models involved are presented and discussed in the paper.
Single-picture reconstruction and rendering of trees for plausible vegetation synthesis
Argudo, Oscar; Chica, Antoni; Andújar, Carlos
Computers & Graphics, Vol. 57, pp 55--67, 2016.
DOI: http://dx.doi.org/10.1016/j.cag.2016.03.005
State-of-the-art approaches for tree reconstruction either put limiting constraints on the input side (requiring multiple photographs, a scanned point cloud or intensive user input) or provide a representation only suitable for front views of the tree. In this paper we present a complete pipeline for synthesizing and rendering detailed trees from a single photograph with minimal user effort. Since the overall shape and appearance of each tree is recovered from a single photograph of the tree crown, artists can benefit from georeferenced images to populate landscapes with native tree species. A key element of our approach is a compact representation of dense tree crowns through a radial distance map. Our first contribution is an automatic algorithm for generating such representations from a single exemplar image of a tree. We create a rough estimate of the crown shape by solving a thin-plate energy minimization problem, and then add detail through a simplified shape-from-shading approach. The use of seamless texture synthesis results in an image-based representation that can be rendered from arbitrary view directions at different levels of detail. Distant trees benefit from an output-sensitive algorithm inspired on relief mapping. For close-up trees we use a billboard cloud where leaflets are distributed inside the crown shape through a space colonization algorithm. In both cases our representation ensures efficient preservation of the crown shape. Major benefits of our approach include: it recovers the overall shape from a single tree image, involves no tree modeling knowledge and minimal authoring effort, and the associated image-based representation is easy to compress and thus suitable for network streaming.
Beacco, Alejandro; Pelechano, Nuria; Andújar, Carlos
Computer Graphics Forum, Vol. 35, Num. 8, pp 32--50, 2016.
DOI: http://dx.doi.org/10.1111/cgf.12774
In this survey we review, classify and compare existing approaches for real-time crowd rendering. We first overview character animation techniques, as they are highly tied to crowd rendering performance, and then we analyze the state of the art in crowd rendering. We discuss different representations for level-of-detail (LoD) rendering of animated characters, including polygon-based, point-based, and image-based techniques, and review different criteria for runtime LoD selection. Besides LoD approaches, we review classic acceleration schemes, such as frustum culling and occlusion culling, and describe how they can be adapted to handle crowds of animated characters. We also discuss specific acceleration techniques for crowd rendering, such as primitive pseudo-instancing, palette skinning, and dynamic key-pose caching, which benefit from current graphics hardware. We also address other factors affecting performance and realism of crowds such as lighting, shadowing, clothing and variability. Finally we provide an exhaustive comparison of the most relevant approaches in the field.
Colonic content in health and its relation to functional gut symptoms
Bendezú, Alvaro; Barba, Elizabeth; Burri, Emanuel; Cisternas, Daniel; Accarino, Ana; Quiroga, Sergi; Monclús, Eva; Navazo, Isabel; Malagelada, Juan Ramon; Azpiroz, Fernando
Neurogastroenterology and Motility , Vol. 28, Num. 6, pp 849--854, 2016.
DOI: http://dx.doi.org/10.1111/nmo.12782
Background: Gut content may be determinant in the generation of digestive symptoms, particularly in patients with impaired gut function and hypersensitivity. Since the relation of intraluminal gas to symptoms is only partial, we hypothesized that non-gaseous component may play a decisive role. Methods: Abdominal computed tomography scans were evaluated in healthy subjects during fasting and after a meal (n = 15) and in patients with functional gut disorders during basal conditions (when they were feeling well) and during an episode of abdominal distension (n = 15). Colonic content and distribution were measured by an original analysis program. Key results: In healthy subjects both gaseous (87 ± 24 mL) and non-gaseous colonic content (714 ± 34 mL) were uniformly distributed along the colon. In the early postprandial period gas volume increased (by 46 ± 23 mL), but non-gaseous content did not, although a partial caudad displacement from the descending to the pelvic colon was observed. No differences in colonic content were detected between patients and healthy subjects. Symptoms were associated with discrete increments in gas volume. However, no consistent differences in non-gaseous content were detected in patients between asymptomatic periods and during episodes of abdominal distension. Conclusions & inferences: In patients with functional gut disorders, abdominal distension is not related to changes in non-gaseous colonic content. Hence, other factors, such as intestinal hypersensitivity and poor tolerance of small increases in luminal gas may be involved.
3D Model deformations with arbitrary control points
Cerveró, M.Àngels; Brunet, Pere; Vinacua, Àlvar
Computer & Graphics, Vol. 57, pp 92-101, 2016.
DOI: http://dx.doi.org/10.1016/j.cag.2016.03.010
Cage-based space deformations are often used to edit and animate images and geometric models. The deformations of the cage are easily transferred to the model by recomputing fixed convex combinations of the vertices of the cage, the control points. In current cage-based schemes the configuration of edges and facets between these control points affects the resulting deformations. In this paper we present a family of similar schemes that includes some of the current techniques, but also new schemes that depend only on the positions of the control points. We prove that these methods afford a solution under fairly general conditions and result in an easy and flexible way to deform objects using freely placed control points, with the necessary conditions of positivity and continuity.
Díaz-García, Jesús; Brunet, Pere; Navazo, Isabel; Perez, Frederic; Vázquez, Pere-Pau
Computer Graphics International , Vol. 32, Num. 6, pp 835--845, 2016.
DOI: http://dx.doi.org/10.1007/s00371-016-1253-9
Medical datasets are continuously increasing in size. Although larger models may be available for certain research purposes, in the common clinical practice the models are usually of up to 512×512×2000 voxels. These resolutions exceed the capabilities of conventional GPUs, the ones usually found in the medical doctors’ desktop PCs. Commercial solutions typically reduce the data by downsampling the dataset iteratively until it fits the available target specifications. The data loss reduces the visualization quality and this is not commonly compensated with other actions that might alleviate its effects. In this paper, we propose adaptive transfer functions, an algorithm that improves the transfer function in downsampled multiresolution models so that the quality of renderings is highly improved. The technique is simple and lightweight, and it is suitable, not only to visualize huge models that would not fit in a GPU, but also to render not-so-large models in mobile GPUs, which are less capable than their desktop counterparts. Moreover, it can also be used to accelerate rendering frame rates using lower levels of the multiresolution hierarchy while still maintaining high-quality results in a focus and context approach. We also show an evaluation of these results based on perceptual metrics.
A Fast Daylighting Method to Optimize Opening Configurations in Building Design
Fernandez, Eduardo; Beckers, Benoit; Besuievsky, Gonzalo
Energy and Buildings, Vol. 125, Num. 1, pp 205--218, 2016.
DOI: http://dx.doi.org/doi:10.1016/j.enbuild.2016.05.012
Daylighting plays a very important role for energy saving in sustainable building, therefore, setting the optimal shapes and positions of the openings is crucial for daylighting availability. On the other hand, computing daylighting for climate-based data is a time-consuming task involving large data set and is not well suited for optimization approaches. In this paper we propose a new and fast daylighting method that allows to perform opening shape optimizations. The base of our method is to model each element of an opening surface as a pinhole and then formulate a compact irradiance-based representation to ease global illumination calculations. We use the UDI metric to evaluate our method, on an office-based model, for different orientations and different geographical locations, showing that optimal windows shapes can be obtained in short times. Our method also provides an efficient way to analyze the impact of climate-based data on the shape of the openings, as they could be modified interactively.
Solving multiple kth smallest dissimilarity queries for non-metric dissimilarities with the GPU
Fort, Marta; Sellarès, J. Antoni
Information Sciences, Vol. 361, pp 66-83, 2016.
DOI: http://dx.doi.org/doi:10.1016/j.ins.2016.03.054
The kth smallest dissimilarity of a query point with respect to a given set is the dissimilarity that ranks number k when we sort, in increasing order, the dissimilarity value of the points in the set with respect to the query point. A multiple kth smallest dissimilarity query determines the kth smallest dissimilarity for several query points simultaneously. Although the problem of solving multiple kth smallest dissimilarity queries is an important primitive operation used in many areas, such as spatial data analysis, facility location, text classification and content-based image retrieval, it has not been previously addressed explicitly in the literature. In this paper we present three parallel strategies, to be run on a Graphics Processing Unit, for computing multiple kth smallest dissimilarity queries when non-metric dissimilarities, that do not satisfy the triangular inequality, are used. The strategies are theoretically and experimentally analyzed and compared among them and with an efficient sequential strategy to solve the problem.
Fort, Marta; Sellarès, J. Antoni
Information Systems, Vol. 62, pp 136-154, 2016.
DOI: http://dx.doi.org/https://doi.org/10.1016/j.is.2016.07.003
In this paper we propose, motivate and solve multiple bichromatic mutual nearest neighbor queries in the plane considering multiplicative weighted Euclidean distances. Given two sets of facilities of different types, a multiple bichromatic mutual (k,k′)-nearest neighbor query finds pairs of points, one of each set, such that the point of the first set is a k -nearest neighbor of the point of the second set and, at the same time, the point of the second set is a k′-nearest neighbor of the point of the first set. These queries find applications in collaborative marketing and prospective data analysis, where facilities of one type cooperate with facilities of the other type to obtain reciprocal benefits. We present a sequential and a parallel algorithm, to be run on the CPU and on a Graphics Processing Unit, respectively, for solving multiple bichromatic mutual nearest neighbor queries. We also present the time and space complexity analysis of both algorithms, together with their theoretical comparison. Finally, we provide and discuss experimental results obtained with the implementation of the proposed sequential and a parallel algorithm.
Continuity and Interpolation Techniques for Computer Graphics
Gonzalez Garcia, Francisco; Patow, Gustavo A.
Computer Graphics Forum, 2016.
DOI: http://dx.doi.org/DOI:10.1111/cgf.12727
Continuity and interpolation have been crucial topics for computer graphics since its very beginnings. Every time we want to interpolate values across some area, we need to take a set of samples over that interpolating region. However, interpolating samples faithfully allowing the results to closely match the underlying functions can be a tricky task as the functions to sample could not be smooth and, in the worst case, it could be even impossible when they are not continuous. In those situations bringing the required continuity is not an easy task, and much work has been done to solve this problem. In this paper, we focus on the state of the art in continuity and interpolation in three stages of the real-time rendering pipeline. We study these problems and their current solutions in texture space (2D), object space (3D) and screen space. With this review of the literature in these areas, we hope to bring new light and foster research in these fundamental, yet not completely solved problems in computer graphics.
Pelechano, Nuria; Fuentes, Carlos
Computers & Graphics, Vol. 59, pp 68--78, 2016.
DOI: http://dx.doi.org/10.1016/j.cag.2016.05.023
Path-finding can become an important bottleneck as both the size of the virtual environments and the number of agents navigating them increase. It is important to develop techniques that can be efficiently applied to any environment independently of its abstract representation. In this paper we present a hierarchical NavMesh representation to speed up path-finding. Hierarchical path-finding (HPA*) has been successfully applied to regular grids, but there is a need to extend the benefits of this method to polygonal navigation meshes. As opposed to regular grids, navigation meshes offer representations with higher accuracy regarding the underlying geometry, while containing a smaller number of cells. Therefore, we present a bottom-up method to create a hierarchical representation based on a multilevel k-way partitioning algorithm (MLkP), annotated with sub-paths that can be accessed online by our Hierarchical NavMesh Path-finding algorithm (HNA*). The algorithm benefits from searching in graphs with a much smaller number of cells, thus performing up to 7.7 times faster than traditional A⁎ over the initial NavMesh. We present results of HPA* over a variety of scenarios and discuss the benefits of the algorithm together with areas for improvement.
Sunet, Marc; Comino, Marc; Karatzas, Dimosthenis; Chica, Antoni; Vázquez, Pere-Pau
IADIS International Journal on Computer Science and Information Systems, Vol. 11, Num. 2, pp 1--18, 2016.
Despite the large amount of methods and applications of augmented reality, there is little homogenization on the software platforms that support them. An exception may be the low level control software that is provided by some high profile vendors such as Qualcomm and Metaio. However, these provide fine grain modules for e.g. element tracking. We are more concerned on the application framework, that includes the control of the devices working together for the development of the AR experience. In this paper we describe the development of a software framework for AR setups. We concentrate on the modular design of the framework, but also on some hard problems such as the calibration stage, crucial for projection-based AR. The developed framework is suitable and has been tested in AR applications using camera-projector pairs, for both fixed and nomadic setups.
Argudo, Oscar; Brunet, Pere; Chica, Antoni; Vinacua, Àlvar
Graphical Models, Vol. 82, pp 137–148, 2015.
DOI: http://dx.doi.org/10.1016/j.gmod.2015.06.010
We discuss bi-harmonic fields which approximate signed distance fields. We conclude that the biharmonic field approximation can be a powerful tool for mesh completion in general and complex cases. We present an adaptive, multigrid algorithm to extrapolate signed distance fields. By defining a volume mask in a closed region bounding the area that must be repaired, the algorithm computes a signed distance field in well-defined regions and uses it as an over-determined boundary condition constraint for the biharmonic field computation in the remaining regions. The algorithm operates locally, within an expanded bounding box of each hole, and therefore scales well with the number of holes in a single, complex model. We discuss this approximation in practical examples in the case of triangular meshes resulting from laser scan acquisitions which require massive hole repair. We conclude that the proposed algorithm is robust and general, and is able to deal with complex topological cases.
Barba, E.; Burri, E.; Accarino, A.; Cisterna, D.; Quiroga, S.; Monclús, Eva; Navazo, Isabel; Malagelada, J; Azpiroz, F.
Gastroenterology, Vol. 148, Num. 4, pp 732--739, 2015.
DOI: http://dx.doi.org/10.1053/j.gastro.2014.12.006
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Beacco, Alejandro; Pelechano, Nuria; Kapadia, M; Badler, N.I.
Computer & Graphics, Vol. 47, pp 105-112, 2015.
DOI: http://dx.doi.org/10.1016/j.cag.2014.12.004
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This paper presents a real-time animation system for fully-embodied virtual humans that satisfies accurate foot placement constraints for different human walking and running styles. Our method offers a fine balance between motion fidelity and character control, and can efficiently animate over sixty agents in real time (25 FPS) and over a hundred characters at 13 FPS. Given a point cloud of reachable support foot configurations extracted from the set of available animation clips, we compute the Delaunay triangulation. At runtime, the triangulation is queried to obtain the simplex containing the next footstep, which is used to compute the barycentric blending weights of the animation clips. Our method synthesizes animations to accurately follow footsteps, and a simple IK solver adjusts small offsets, foot orientation, and handles uneven terrain. To incorporate root velocity fidelity, the method is further extended to include the parametric space of root movement and combine it with footstep based interpolation. The presented method is evaluated on a variety of test cases and error measurements are calculated to offer a quantitative analysis of the results achieved.
Bendezú, A.; Barba, E.; Burri, E.; Cisternas, D.; Malagelada, C.; Segui, S.; Accarino, A.; Quiroga, S.; Monclús, Eva; Navazo, Isabel; Malagelada, J.; Azpiroz, F.
Neurogastroenterology and motility, Vol. 27, Num. 9, pp 1249--1257, 2015.
DOI: http://dx.doi.org/10.1111/nmo.12618
Inverse Opening Design with Anisotropic Lighting Incidence
Besuievsky, Gonzalo
Computers and Graphics, Vol. 47, Num. 1, pp 113--122, 2015.
DOI: http://dx.doi.org/10.1016/j.cag.2015.01.003
In architecturaldesign, configuring opening shapes is a crucial element of daylight analysis. In this paper we present a new method which optimizes opening shapes to meet specified lighting design purposes. This novel approach treats the problem as an inverse lighting problem considering global illumination contributions and anisotropic lighting incidence, therefore any kind of sky distribution can be used as an external source of light. The key to our technique is in exploiting coherence to formulate a compact representation that can be tailored to optimization processes. The resulting reduction in processing time and efficiency in achieving optimal shapes along with the feasibility of dealing with an isotropic light sources are our key contributions.
Immersive data comprehension: visualizing uncertainty in measurable models
Brunet, Pere; Andújar, Carlos
Frontiers in Robotics and AI, Virtual Environments, pp 2-22, 2015.
DOI: http://dx.doi.org/ http://dx.doi.org/10.3389/frobt.2015.00022
Recent advances in 3D scanning technologies have opened new possibilities in a broad range of applications including cultural heritage, medicine, civil engineering, and urban planning. Virtual Reality systems can provide new tools to professionals that want to understand acquired 3D models. In this review paper, we analyze the concept of data comprehension with an emphasis on visualization and inspection tools on immersive setups. We claim that in most application fields, data comprehension requires model measurements, which in turn should be based on the explicit visualization of uncertainty. As 3D digital representations are not faithful, information on their fidelity at local level should be included in the model itself as uncertainty bounds. We propose the concept of Measurable 3D Models as digital models that explicitly encode such local uncertainty bounds. We claim that professionals and experts can strongly benefit from immersive interaction through new specific, fidelity-aware measurement tools, which can facilitate 3D data comprehension. Since noise and processing errors are ubiquitous in acquired datasets, we discuss the estimation, representation, and visualization of data uncertainty. We show that, based on typical user requirements in Cultural Heritage and other domains, application-oriented measuring tools in 3D models must consider uncertainty and local error bounds. We also discuss the requirements of immersive interaction tools for the comprehension of huge 3D and nD datasets acquired from real objects.
Common influence region problems
Fort, Marta; Sellarès, J. Antoni
Information Sciences, Vol. 231, pp 116--135, 2015.
Hermosilla, Pedro; Guallar, Víctor; Vinacua, Àlvar; Vázquez, Pere-Pau
Computers & Graphics, Vol. 54, pp 113-120, 2015.
DOI: http://dx.doi.org/10.1016/j.cag.2015.07.017
All-atom simulations are crucial in biotechnology. In Pharmacology, for example, molecular knowledge of protein-drug interactions is essential in the understanding of certain pathologies and in the development of improved drugs. To achieve this detailed information, fast and enhanced molecular visualization is critical. Moreover, hardware and software developments quickly deliver extensive data, providing intermediate results that can be analyzed by scientists in order to interact with the simulation process and direct it to a more promising configuration. In this paper we present a GPU-friendly data structure for real-time illustrative visualization of all-atom simulations. Our system generates both ambient occlusion and halos using an occupancy pyramid that needs no precalculation and that is updated on the fly during simulation, allowing the real time rendering of simulation results at sustained high framerates.
Oliva, Ramón; Pelechano, Nuria
Computers & graphics, Vol. 47, pp 48--58, 2015.
DOI: http://dx.doi.org/10.1016/j.cag.2014.11.004
There are two frequent artifacts in crowd simulation caused by navigation mesh design. The first appears when all agents attempt to traverse the navigation mesh and share the same way points through portals, thus increasing the probability of collisions with other agents or queues forming around portals. The second is caused by way points being assigned at locations where clearance is not guaranteed, which causes the agents to either walk too close to the static geometry, slide along walls or get stuck. To overcome this we use the full length of the portal and propose a novel method for dynamically calculating way points based on current trajectory, destination, and clearance, therefore guaranteeing that agents in a crowd will have different way points assigned. To achieve collision free paths we propose two novel techniques: the first provides the computation of paths with clearance for cells of any shape (even with concavities) and the second presents a new method for calculating portals with clearance, so that the dynamically assigned way points will always guarantee collision free paths relative to the static geometry. In this paper, we extend our previous work by describing a new version of the algorithm that is suitable for a larger number of navigation meshes, while further improving performance. Our results show how the combination of portals with exact clearance and dynamic way points improve local movement by reducing the number of collision between agents and the static geometry. We evaluate our algorithm with a variety of scenarios and compare our results with traditional way points to show that our technique also offers better use of the space by the agents.
Ramirez-Flores, J.E.; Susin, Antonio
Computer Animation and Virtual Worlds, 2015.
DOI: http://dx.doi.org/10.1002/cav.1687
Skeleton-driven animation is popular by its simplicity and intuitive control of the limbs of a character. Linear blend skinning (LBS) is up to date the most efficient and simple deformation method; however, painting influence skinning weights is not intuitive, and it suffers the candy-wrapper artifact. In this paper, we propose an approach based on mesh segmentation for skinning and skeleton-driven computer animation. We propose a novel and fast method, based in watershed segmentation to deal with characters in T-Pose and arbitrary poses, a simple weight assign algorithm based in the rigid skinning obtained with the segmentation algorithm for the LBS deformation method, and finally, a modified version of the LBS that avoids the loss of volume in twist rotations using the segmentation stage output values.
Real-Time Molecular Visualization Supporting Diffuse Illumination and Ambient Occlusion
Skanberg, Robin; Vázquez, Pere-Pau; Guallar, Victor; Ropinski, Timo
IEEE Transactions on Visualization and Computer Graphics, Vol. 22, Num. 1, pp 718-27, 2015.
DOI: http://dx.doi.org/10.1109/TVCG.2015.2467293
Today molecular simulations produce complex data sets capturing the interactions of molecules in detail. Due to the complexity of this time-varying data, advanced visualization techniques are required to support its visual analysis. Current molecular visualization techniques utilize ambient occlusion as a global illumination approximation to improve spatial comprehension. Besides these shadow-like effects, interreflections are also known to improve the spatial comprehension of complex geometric structures. Unfortunately, the inherent computational complexity of interreflections would forbid interactive exploration, which is mandatory in many scenarios dealing with static and time-varying data. In this paper, we introduce a novel analytic approach for capturing interreflections of molecular structures in real-time. By exploiting the knowledge of the underlying space filling representations, we are able to reduce the required parameters and can thus apply symbolic regression to obtain an analytic expression for interreflections. We show how to obtain the data required for the symbolic regression analysis, and how to exploit our analytic solution to enhance interactive molecular visualizations.
Andújar, Carlos; Chica, Antoni; Vico, Miguel Angel; Moya, Sergio; Brunet, Pere
Computer Graphics Forum, Vol. 33, Num. 6, pp 101--117, 2014.
DOI: http://dx.doi.org/10.1111/cgf.12281
In this paper, we present an inexpensive approach to create highly detailed reconstructions of the landscape surrounding a road. Our method is based on a space-efficient semi-procedural representation of the terrain and vegetation supporting high-quality real-time rendering not only for aerial views but also at road level. We can integrate photographs along selected road stretches. We merge the point clouds extracted from these photographs with a low-resolution digital terrain model through a novel algorithm which is robust against noise and missing data. We pre-compute plausible locations for trees through an algorithm which takes into account perceptual cues. At runtime we render the reconstructed terrain along with plants generated procedurally according to pre-computed parameters. Our rendering algorithm ensures visual consistency with aerial imagery and thus it can be integrated seamlessly with current virtual globes.
Procedural bread making
Baravalle, Rodrigo; Patow, Gustavo A.; Delrieux, Claudio
Computers & Graphics, Vol. 50, pp 13-24, 2014.
DOI: http://dx.doi.org/doi:10.1016/j.cag.2015.05.003
Accurate modeling and rendering of food, and in particular of bread and other baked edible stuff, have not received as much attention as other materials in the photorealistic rendering literature. In particular, bread turns out to be a structurally complex material, and the eye is very precise in spotting improper models, making adequate bread modeling a difficult task. In this paper we present an accurate computational bread making model that allows us to faithfully represent the geometrical structure and the appearance of bread through its making process. This is achieved by a careful simulation of the conditions during proving and baking to get realistically looking bread. Our results are successfully compared to real bread by both visual inspection and by a multifractal-based error metric.
Civit, Oscar; Susin, Antonio
Computer Graphics Forum, Vol. 33, Num. 6, pp 298--309, 2014.
DOI: http://dx.doi.org/10.1111/cgf.12351
We address the problem of robust and efficient treatment of element collapse and inversion in corotational FEM simulations of deformable objects in two and three dimensions, and show that existing degeneration treatment methods have previously unreported flaws that seriously threaten robustness and physical plausibility in interactive applications. We propose a new method that avoids such flaws, yields faster and smoother degeneration recovery and extends the range of well-behaved degenerate configurations without adding significant complexity or computational cost to standard explicit and quasi-implicit solvers.
A Sample-Based Method for Computing the Radiosity Inverse Matrix
Eduardo Fernández; Besuievsky, Gonzalo
Computers & Graphics, 2014.
DOI: http://dx.doi.org/10.1016/j.cag.2014.02.001
The radiosity {problem} can be expressed as a linear system, where the {light transport} interactions of all patches of the scene are considered. Due to the amount of computation required to solve the system, the whole matrix is rarely computed and iterative methods are used instead. In this paper we introduce a new algorithm to obtain an approximation of the radiosity inverse matrix. The method is based on the calculation of a random sample of rows of the form factor matrix. The availability of this matrix allows {us} to reduce the radiosity calculation costs, speeding up the radiosity process. This is useful in applications where the radiosity equation must be solved thousands of times for different light configurations. We apply it to solve inverse lighting problems, in scenes up to 170K patches. The optimization process used finds optimal solutions in nearly interactive times, which improves {on} previous work.
Eduardo Fernández; Besuievsky, Gonzalo
Automation in Construction, Vol. 37, Num. 1, pp 48--57, 2014.
DOI: http://dx.doi.org/10.1016/j.autcon.2013.09.004
Given a scene to illuminate satisfying a specific set of lighting intentions, the inverse lighting techniques allows to obtain the unknown light sources parameters, such as light position or flux emission. This paper introduces a new inverse lighting approach that uses the radiosity mean and variance to define lighting intentions of a scene. It is shown that these statistical parameters can be obtained without the previous calculation of the radiosity of the scene. Avoiding the explicit computation of the illumination of the scene results in a drastic reduction of the time required for the inverse process. This approach also provides a methodology that transforms a current set of lighting intentions into a single lighting intention with statistical parameters. The tests show that the processing time for solving the inverse problem can be reduced to a few seconds in most cases, improving previous work.
Ferrer, J.; Peña, M.; Susin, Antonio
Mathematical Problems in Engineering, Vol. 2014, pp 8, 2014.
DOI: http://dx.doi.org/10.1155/2014/892948
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Structural stability ensures that the qualitative behavior of a system is preserved under small perturbations. We study it for planar bimodal linear dynamical systems, that is, systems consisting of two linear dynamics acting on each side of a given hyperplane and assuming continuity along the separating hyperplane. We describe which one of these systems is structurally stable when (real) spiral does not appear and when it does we give necessary and sufficient conditions concerning finite periodic orbits and saddle connections. In particular, we study the finite periodic orbits and the homoclinic orbits in the saddle/spiral case.
Finding extremal sets on the GPU
Fort, Marta; Sellarès, J. Antoni
Journal of Parallel and Distributed Computing, Vol. 74, Num. 1, pp 1891-1899, 2014.
DOI: http://dx.doi.org/10.1016/j.jpdc.2013.07.004
The extremal sets of a family FF of sets consist of all sets of FF that are maximal or minimal with respect to the partial order induced by the subset relation in FF. In this paper we present efficient parallel GPU-based algorithms, designed under CUDA architecture, for finding the extremal sets of a family FF of sets. The complexity analysis of the presented algorithms together with experimental results showing the efficiency and scalability of the approach is provided.
Solving the k-influence region problem with the GPU
Fort, Marta; Sellarès, J. Antoni
Information Sciences, 2014.
DOI: http://dx.doi.org/10.1016/j.ins.2013.12.002
In this paper we study a problem that arises in the competitive facility location field. Facilities and customers are represented by points of a planar Euclidean domain.We associate a weighted distance to each facility to reflect that customers select facilities depending on distance and importance. We define, by considering weighted distances, the k-influence region of a facility as the set of points of the domain that has the given facility among their k-nearest/farthest neighbors. On the other hand, we partition the domain into subregions so that each subregion has a non-negative weight associated to it which measures a characteristic related to the area of the subregion. Given a weighted partition of the domain, the k-influence region problem finds the points of the domain where are new facility should be opened. This is done considering the known weight associated to the new facility and ensuring a minimum weighted area of its k-influence region. We present a GPU parallel approach, designed under CUDA architecture, for approximately solving the k-influence region problem. In addition, we describe how to visualize the solutions, which improves the understanding of the problem and reveals complicated structures that would be hard to capture otherwise. Integration of computation and visualization facilitates decision makers with an iterative what-if analysis process, to acquire more information to obtain an approximate optimal location. Finally, we provide and discuss experimental results showing the efficiency and scalability of our approach.
Fort, Marta; Sellarès, J. Antoni; Valladares, Ignacio
Knowledge and Information Systems, 2014.
DOI: http://dx.doi.org/DOI 10.1007/s10115-013-0639-5
Data analysis and knowledge discovery in trajectory databases is an emerging ?eld with a growing number of applications such as managing traf?c, planning tourism infrastructures, analyzing professional sport matches or better understanding wildlife. A well-known collection of patterns which can occur for a subset of trajectories of moving objects exists. In this paper, we study the popular places pattern, that is, locations that are visited by many moving objects. We consider two criteria, strong and weak, to establish either the exact number of times that an object has visited a place during its complete trajectory or whether it has visited the place, or not. To solve the problem of reporting popular places, we introduce the popularity map. The popularity of a point is a measure of how many times the moving objects of a set have visited that point. The popularity map is the subdivision, into regions, of a plane where all the points have the same popularity. We propose different algorithms to ef?ciently compute and visualize popular places, the so-called popular regions and their schematization, by taking advantage of the parallel computing capabilities of the graphics processing units. Finally, we provide and discuss the experimental results obtained with the implementation of our algorithms.
A parallel GPU-based approach for reporting flock patterns
Fort, Marta; Sellarès, J. Antoni; Valladares, Ignacio
International Journal of Geographical Information Science, Vol. 28, Num. 9, pp 1877--1903, 2014.
DOI: http://dx.doi.org/doi:10.1080/13658816.2014.902949
Fracture Modeling in Computer Graphics
Muguercia, Lien; Bosch, Carles; Patow, Gustavo A.
Computers & Graphics, Vol. 45, pp 86-100, 2014.
DOI: http://dx.doi.org/doi:10.1016/j.cag.2014.08.006
While object deformation has received a lot of attention in Computer Graphics in recent years, with several good surveys that summarize the state-of-the-art in the field, a comparable comprehensive literature review is still needed for the related problem of crack and fracture modeling. In this paper we present such a review, with a special focus on the latest advances in this area, and a careful analysis of the open issues along with the avenues for further research. With this survey, we hope to provide the community not only a fresh view of the topic, but also an incentive to delve into and explore these unsolved problems further.
Ojeda, Jesús; Susin, Antonio
Communications in computer and information science, Vol. 458, pp 3--18, 2014.
DOI: http://dx.doi.org/10.1007/978-3-662-44911-0_1
We present a new approach for the simulation of surface-based fluids based in a hybrid formulation of Lattice Boltzmann Method for Shallow Waters and particle systems. The modified LBM can handle arbitrary underlying terrain conditions and arbitrary fluid depth. It also introduces a novel method for tracking dry-wet regions and moving boundaries. Dynamic rigid bodies are also included in our simulations using a two-way coupling. Certain features of the simulation that the LBM can not handle because of its heightfield nature, as breaking waves, are detected and automatically turned into splash particles. Here we use a ballistic particle system, but our hybrid method can handle more complex systems as SPH. Both the LBM and particle systems are implemented in CUDA, although dynamic rigid bodies are simulated in CPU. We show the effectiveness of our method with various examples which achieve real-time on consumer-level hardware.
Pueyo, Oriol; Patow, Gustavo A.
The Visual Computer, Vol. 30, Num. 2, pp 159-172, 2014.
DOI: http://dx.doi.org/10.1007/s00371-013-0791-7
Geometric city modeling is an open problem without standard solutions. Within this problem, there appear several sub-problems that must be faced, like the accurate modeling of streets, buildings and other architectonic structures. One important source of geographical information is (measured) cadastral urban data. However, this information is not always well structured, and sometimes it is even simply corrupted GIS data. In this paper we present a robust and generic solution for the generation of block and building layouts based on a repairing process applied when this data is not correct. Our input data is a top projection map of a city which usually has been created by a mixture of photogrammetric restitution and, in a second stage, hand-drawn using any GIS application. Moreover, these maps are under continuous modifications, like in the case of public administrations. This process sometimes results in the introduction of mistakes and anomalies, which are hard to correct without the appropriate tools. Our solution is based on a novel semiautomatic 2D restructuring algorithm, which uniformly corrects errors and ambiguities that are commonly present in corrupted cadastral data. This problem is complex because it is necessary to identify not just simple elements from the input file, but also their connectivity and structure in the real world. The output of our algorithm is the urban data restructured into a hierarchy of blocks and buildings, from which we can get a realistic 3D model by extruding each building using the floor number for each building within the cadastral data.
Pueyo, Xavier; Bosch, Carles; Patow, Gustavo A.
Frontiers in Robotics and A.I., Vol. 1, Num. 17, 2014.
DOI: http://dx.doi.org/10.3389/frobt.2014.00017
Computer Graphics has evolved into a mature and powerful field that offers many opportunities to enhance different disciplines, adapting to the specific needs of each. One of these important fields is the design and analysis of Urban Environments. In this article we try to offer a perspective of one of the sectors identified in Urban Environment studies: Urbanization. More precisely we focus on geometric and appearance modeling, rendering and simulation tools to help stakeholders in key decision stages of the process.
Continuous surveillance of points by rotating floodlights
S.Bereg; J.M. Díaz-Bañez; Fort, Marta; M.A.López; P. Pérez-Lantero; J.Urrutia
International Journal of Computational Geometry, Vol. 24, Num. 3, pp 183--196, 2014.
DOI: http://dx.doi.org/10.1142/S0218195914600024
Argelaguet, Ferran; Andújar, Carlos
Computers & Graphics, Vol. 37, Num. 3, pp 121-136, 2013.
DOI: http://dx.doi.org/10.1016/j.cag.2012.12.003
Computer graphics applications controlled through natural gestures are gaining increasing popularity these days due to recent developments in low-cost tracking systems and gesture recognition technologies. Although interaction techniques through natural gestures have already demonstrated their benefits in manipulation, navigation and avatar-control tasks, effective selection with pointing gestures remains an open problem. In this paper we survey the state-of-the-art in 3D object selection techniques. We review important findings in human control models, analyze major factors influencing selection performance, and classify existing techniques according to a number of criteria. Unlike other components of the application's user interface, pointing techniques need a close coupling with the rendering pipeline, introducing new elements to be drawn, and potentially modifying the object layout and the way the scene is rendered. Conversely, selection performance is affected by rendering issues such as visual feedback, depth perception, and occlusion management. We thus review existing literature paying special attention to those aspects in the boundary between computer graphics and human–computer interaction.
Barba, Elisabeth; Quiroga, Sergi; Accarino, Anna; Monclús, Eva; Malagelada, C.; Burri, E; Navazo, Isabel; Malagelada, JR; Azpiroz, Fernando
Neurogastroenterology and motility, Vol. 25, Num. 6, pp e389--e394, 2013.
DOI: http://dx.doi.org/10.1111/nmo.12128
We previously showed that abdominal distension in patients with functional gut disorders is due to a paradoxical diaphragmatic contraction without major increment in intraabdominal volume. Our aim was to characterize the pattern of gas retention and the abdomino-thoracic mechanics associated with abdominal distension in patients with intestinal dysmotility.
Barroso, Santiago; Besuievsky, Gonzalo; Patow, Gustavo A.
Computers & Graphics, Vol. 37, pp 238--246, 2013.
DOI: http://dx.doi.org/10.1016/j.cag.2013.01.003
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With the increase in popularity of procedural urban modeling for film, TV, and interactive entertainment, an urgent need for editing tools to support procedural content creation has become apparent. In this paper we present an end-to-end system for procedural copy & paste in a rule-based setting to address this need. As we show, no trivial extension exists to perform this action in a way such that the resulting ruleset is ready for production. For procedural copy & paste we need to handle the rulesets in both the source and target graphs to obtain a final consistent ruleset. As one of the main contributions of our system, we introduce a graph-rewriting procedure for seamlessly gluing both graphs and obtaining a consistent new procedural building ruleset. Hence, we focus on intuitive and minimal user interaction, and our editing operations perform interactively to provide immediate feedback.
Besuievsky, Gonzalo; Patow, Gustavo A.
Computer Graphics Forum, Vol. 32, Num. 8, pp 1467-8659, 2013.
DOI: http://dx.doi.org/10.1111/cgf.12141
This paper presents a new semantic and procedural level-of-Detail (LoD) method applicable to any rule-based procedural building definition. This new LoD system allows the customizable and flexible selection of the archi- tectural assets to simplify, doing it in an efficient and artist-transparent way. The method, based on an extension of traditional grammars, uses LoD-oriented commands. A graph-rewriting process introduces these new commands in the artist-provided ruleset, which allows to select different simplification criteria (distance, screen-size projec- tion, semantic selection, or any arbitrary method) through a scripting interface, according to user needs. This way we define a flexible, customizable and efficient procedural LoD system, which generates buildings directly with the correct LoD for a given set of viewing and semantic conditions.
Besuievsky, Gonzalo; Patow, Gustavo A.
Virtual Archaeology Review, Vol. 4, Num. 9, pp 160--166, 2013.
In this paper we target the goal of obtaining detailed historical virtual buildings, like a castle or a city old town, through a methodology that facilitates their reconstruction. We allow having in a short time an approximation model that is flexible for being explored, analyzed and eventually modified. This is crucial for serious game development pipelines, whose objective is focused not only on accuracy and realism, but also on transmitting a sense of immersion to the player.
Campoalegre, Lázaro; Brunet, Pere; Navazo, Isabel
Personal and Ubiquitous Computing, Vol. 17, Num. 7, pp 1503-1514, 2013.
DOI: http://dx.doi.org/10.1007/s00779-012-0596-0
Interactive visualization of volume models in standard mobile devices is a challenging present problem with increasing interest from new application fields like telemedicine. The complexity of present volume models in medical applications is continuously increasing, therefore increasing the gap between the available models and the rendering capabilities in low-end mobile clients. New and efficient rendering algorithms and interaction paradigms are required for these small platforms. In this paper, we propose a transfer function-aware compression and interaction scheme, for client-server architectures with visualization on standard mobile devices. The scheme is block-based, supporting adaptive ray-casting in the client. Our two-level ray-casting allows focusing on small details on targeted regions while keeping bounded memory requirements in the GPU of the client. Our approach includes a transfer function-aware compression scheme based on a local wavelet transformation, together with a bricking scheme that supports interactive inspection and levels of detail in the mobile device client. We also use a quantization technique that takes into account a perceptive metrics of the visual error. Our results show that we can have full interaction with high compression rates and with transmitted model sizes that can be of the order of a single photographic image.
R4: Realistic Rain Rendering in Realtime
Carles Creus; Patow, Gustavo A.
Computers & Graphics, Vol. 37, Num. 2, pp 33--40, 2013.
DOI: http://dx.doi.org/10.1016/j.cag.2012.12.002
Realistic rain simulation is a challenging problem due to the variety of different phenomena to consider. In this paper we propose a new rain rendering algorithm that extends present state of the art in the field, achieving real-time rendering of rain streaks and splashes with complex illumination effects, along with fog, halos and light glows as hints of the participating media. Our algorithm creates particles in the scene using an artist-defined storm distribution (e.g., provided as a 2D cloud distribution). Unlike previous algorithms, no restrictions are imposed on the rain area dimension or shape. Our technique adaptively samples the storm area to simulate rain particles only in the relevant regions and only around the observer. Particle simulation is executed entirely in the graphics hardware, by placing the particles at their updated coordinates at each time-step, also checking for collisions with the scene. To render the rain streaks, we use precomputed images and combine them to achieve complex illumination effects. Several optimizations are introduced to render realistic rain with virtually millions of falling rain droplets.
Fort, Marta; Sellarès, J. Antoni
Knowledge-Based Systems, 2013.
DOI: http://dx.doi.org/10.1016/j.knosys.2013.03.013
In this paper we introduce and solve several problems that arise in the single facility location field. A reverse k-influential location problem finds a region such that the location of a new facility, desirable or obnoxious, in the region guarantees a minimum k-influential value associated to the importance, attractiveness or repulsiveness, of the facility as a solution to a reverse k-nearest or farthest neighbor query. Solving reverse k-influential location problems help decision makers to progress towards suitable locations for a new facility. We present a parallel approach, to be ran on a graphics processing unit, for approximately solving reverse k-influential location problems, and also provide and discuss experimental results showing the efficiency and scalability of our approach.
Interactive applications for sketch-based editable polycube-map
Garcia Fernández, Ismael; Jiazhi Xia; Ying He; Shi-Qing Xin; Patow, Gustavo A.
IEEE Transactions on Visualization and Computer Graphics, Vol. 19, Num. 7, pp 1158–-1171, 2013.
DOI: http://dx.doi.org/10.1109/TVCG.2012.308
In this paper we propose a sketch-based editable polycube mapping method that, given a general mesh and a simple polycube that coarsely resembles the shape of the object, plus sketched features indicating relevant correspondences between the two, provides a uniform, regular and user-controllable quads-only mesh that can be used as a basis structure for subdivision. Large scale models with complex geometry and topology can be processed efficiently with simple, intuitive operations. We show that the simple, intuitive nature of the polycube map is a substantial advantage from the point of view of the interface by demonstrating a series of applications, including kit-basing, shape morphing, painting over the parameterization domain, and GPU-friendly tessellated subdivision displacement, where the user is also able to control the number of patches in the base mesh by the construction of the base polycube.
Gonzalez Garcia, Francisco; Paradinas, Teresa; Coll, Narcís; Patow, Gustavo A.
ACM Transactions on Graphics, Vol. 32, Num. 3, pp 13, 2013.
DOI: http://dx.doi.org/10.1145/2487228.2487232
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Cage-based deformation has been one of the main approaches for mesh deformation in recent years, with a lot of interesting and active research. The main advantages of cage-based deformation techniques are their simplicity, relative flexibility and speed. However, to date there has been no widely accepted solution that provides both user control at different levels of detail and high quality deformations. We present *Cages (star-cages), a significant step forward with respect to traditional single-cage coordinate systems, and which allows the usage of multiple cages enclosing the model for easier manipulation while still preserving the smoothness of the mesh in the transitions between them. The proposed deformation scheme is extremely flexible and versatile, allowing the usage of heterogeneous sets of coordinates and different levels of deformation, ranging from a whole- model deformation to a very localized one. That locality allows faster evaluation and a reduced memory footprint, and as a result outperforms single-cage approaches in flexibility, speed and memory requirements for complex editing operations.
Ramon Oliva; Pelechano, Nuria
Computer & Graphics, Vol. 37, Num. 5, pp 403--412, 2013.
DOI: http://dx.doi.org/10.1016/j.cag.2013.03.004
In this paper we introduce a novel automatic method for generating near optimal navigation meshes from a 3D multi-layered virtual environment. Firstly, a GPU voxelization of the entire scene is calculated in order to identify and extract the different walkable layers. Secondly, a high resolution render is performed with a fragment shader to obtain the 2D floor plan of each layer. Finally, a convex decomposition of each layer is calculated and layers are linked in order to create a Navigation Mesh of the scene. Results show that our method is not only faster than the previous work, but also creates more accurate NavMeshes since it respects the original shape of the static geometry. It also provides a significantly lower number of cells and avoids ill-conditioned cells and T-Joints between portals that could lead to unnatural character navigation.
Andújar, Carlos
Computer Graphics Forum, Vol. 31, Num. 6, pp 1973–1983, 2012.
High-quality texture minification techniques, including trilinear and anisotropic filtering, require texture data to be arranged into a collection of pre-filtered texture maps called mipmaps. In this paper, we present a compression scheme for mipmapped textures which achieves much higher quality than current native schemes by exploiting image coherence across mipmap levels. The basic idea is to use a high-quality native compressed format for the upper levels of the mipmap pyramid (to retain efficient minification filtering) together with a novel compact representation of the detail provided by the highest-resolution mipmap. Key elements of our approach include delta-encoding of the luminance signal, efficient encoding of coherent regions through texel runs following a Hilbert scan, a scheme for run encoding supporting fast random-access, and a predictive approach for encoding indices of variable-length blocks. We show that our scheme clearly outperforms native 6:1 compressed texture formats in terms of image quality while still providing real-time rendering of trilinearly filtered textures.
Andújar, Carlos; Chica, Antoni; Brunet, Pere
Computer & Graphics, Vol. 36, Num. 1, pp 28--37, 2012.
DOI: http://dx.doi.org/10.1016/j.cag.2011.10.005
Computer Graphics and Virtual Reality technologies provide powerful tools for visualizing, documenting and disseminating cultural heritage. Virtual inspection tools have been used proficiently to show cultural artifacts either through the web or in museum exhibits. The usability of the user interface has been recognized to play a crucial role in overcoming the typical fearful attitude of the cultural heritage community towards 3D graphics. In this paper we discuss the design of the user interface for the virtual inspection of the impressive entrance of the Ripoll Monastery in Spain. The system was exhibited in the National Art Museum of Catalonia (MNAC) during 2008 and since June 2011 it is part of its Romanesque exhibition. The MNAC is the third most visited art museum in Spain, and features the world?s largest collection on Romanesque Art. We analyze the requirements from museum curators and discuss the main interface design decisions. The user interface combines (a) focus-plus-context visualization, with focus (detail view) and context (overview) being shown at separate displays, (b) touch-based camera control techniques, and (c) continuous feedback about the exact location of the detail area within the entrance. The interface allows users to aim the camera at any point of the entrance with centimeter accuracy using a single tap. We provide the results of a user study comparing our user interface with alternative approaches. We also discuss the benefits the exhibition had to the cultural heritage community.
Beacco, Alejandro; Andújar, Carlos; Pelechano, Nuria; Bernhard Spanlang
Journal of Computer Animation and Virtual Worlds, Vol. 23, Num. 2, pp 33-47, 2012.
DOI: http://dx.doi.org/10.1002/cav.1422
In this paper we present a new impostor-based representation for 3D animated characters supporting real-time rendering of thousands of agents. We maximize rendering performance by using a collection of pre-computed impostors sampled from a discrete set of view directions. Our approach differs from previous work on view-dependent impostors in that we use per-joint rather than per-character impostors. Our characters are animated by applying the joint rotations directly to the impostors, instead of choosing a single impostor for the whole character from a set of pre-de?ned poses. This offers more ?exibility in terms of animation clips, as our representation supports any arbitrary pose and thus the agent behavior is not constrained to a small collection of pre-de?ned clips. Since our impostors are intended to be valid for any pose, a key issue is to de?ne 1a proper boundary for each impostor to minimize image artifacts while animating the agents. We pose this problem as a variational optimization problem, and provide an ef?cient algorithm for computing a discrete solution as a preprocess. To the best of our knowledge, this is the ?rst time a crowd rendering algorithm encompassing image based performance, small GPU footprint and animation-independence is proposed.
Chica, Antoni; Monclús, Eva; Brunet, Pere; Navazo, Isabel; Vinacua, Àlvar
Graphical Models, Vol. 74, Num. 6, pp 302--310, 2012.
DOI: http://dx.doi.org/10.1016/j.gmod.2012.03.002
In this paper, we propose a novel strategy to automatically segment volume data using a high-quality mesh segmentation of an "example" model as a guiding example. The example mesh is deformed until it matches the relevant volume features. The algorithm starts from a medical volume model (scalar field of densities) to be segmented, together with an already existing segmentation (polygonal mesh) of the same organ, usually from a different person. The pre-process step computes a suitable atracting scalar field in the volume model. After an approximate 3D registration between the example mesh and the volume (this is the only step requiring user intervention), the algorithm works by minimizing an energy and adapts the shape of the polygonal mesh to the volume features in order to segment the target organ. The resulting mesh adapts to the volume features in the areas which can be unambiguously segmented, while taking the shape of the example mesh in regions which lack relevant volume information. The paper discusses several examples involving human foot bones, with results that clearly outperform present segmentation schemes.
Díaz, Jose; Monclús, Eva; Navazo, Isabel; Vázquez, Pere-Pau
Computer Graphics Forum, Vol. 31, Num. 7, pp 2155--2164, 2012.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2012.03208.x
Medical illustrations have been used for a long time for teaching and communicating information for diagnosis or surgery planning. Illustrative visualization systems create methods and tools that adapt traditional illustration techniques to enhance the result of renderings. Clipping the volume is a popular operation in volume rendering for inspecting the inner parts, though it may remove some information of the context that is worth preserving. In this paper we present a new editing technique based on the use of clipping planes, direct structure extrusion, and illustrative methods, which preserves the context by adapting the extruded region to the structures of interest of the volumetric model. We will show that users may interactively modify the clipping plane and edit the structures to highlight, in order to easily create the desired result. Our approach works with segmented volume models and nonsegmented ones. In the last case, a local segmentation is performed on-the-fly. We will demonstrate the efficiency and utility of our method
Díaz-García, Jesús; Vázquez, Pere-Pau
International Symposium on Visual Computing , Vol. 7431, pp 698-707, 2012.
DOI: http://dx.doi.org/10.1007/978-3-642-33179-4_66
The visualization of human brain fibers is becoming a new challenge in the computer graphics field. Nowadays, with the aid of DTI and fiber tracking algorithms, complex geometric models consisting of massive sets of polygonal lines can be extracted. However, rendering such massive models often results in non-detailed, cluttered visualizations. In this paper we propose two methods (one object-space and another image-space) for the fast rendering of fiber tracts by including illustrative effects such as halos and ambient occlusion. We will show how our approaches provide extra visible cues that enhance the final result by removing clutter, thus revealing fibers’ shapes and orientations. Moreover, the use of ambient-occlusion based techniques improves the perception of their absolute and relative positions in space.
Eduardo Fernández; Besuievsky, Gonzalo
Computers & Graphics, Vol. 36, Num. 8, pp 1096--1108, 2012.
DOI: http://dx.doi.org/10.1016/j.cag.2012.09.003
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In this paper we propose a new method for solving inverse lighting design problems that can include diverse sources as roof skylights or artificial light sources. Given a user specification of illumination requirements, our approach provides optimal light sources positions as well as optimal shapes for skylight installations in interior architectural models. The well known huge computational e ort that involves searching for an optimal solution is tackled combining two concepts: exploiting the scene coherence to compute global illumination and using a metaheuristic technique for optimization. Results and analysis show that our methodology presents fast and accurate results and that it can be applied for lighting design in indoors environments with interactive global illumination visualization support.
Fort, Marta; Sellarès, J. Antoni
Journal of Computational and Applied Mathematics, Vol. 236, Num. 14, pp 3461--3477, 2012.
DOI: http://dx.doi.org/10.1016/j.cam.2012.03.028
Given P, a simple connected, possibly non-convex, polyhedral surface composed of positively weighted triangular faces, we consider paths from generalized sources (points, segments, polygonal chains or polygonal regions) to points on P that stay on P and avoid obstacles (segments, polygonal chains or polygonal regions). The distance function defined by a generalized source is a function that assigns to each point of P the cost of the shortest path from the source to the point. In this paper we present an algorithm for computing approximate generalized distance functions. We also provide an algorithm that computes a discrete representation of the approximate distance function and, as applications, algorithms for computing discrete order-k Voronoi diagrams and for approximately solving facility location problems. Finally, we present experimental results obtained with our implementation of the provided algorithms.
Glondu, Loeiz; Muguercia, Lien; Marchal, Maud; Bosch, Carles; Rushmeier, Holly; Dumont, Georges; Drettakis, George
Computer Graphics Forum, Vol. 31, Num. 4, pp 1547--1556, 2012.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2012.03151.x
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A common weathering effect is the appearance of cracks due to material fractures. Previous exemplar-based aging and weathering methods have either reused images or sought to replicate observed patterns exactly. We introduce a new approach to exemplar-based modeling that creates weathered patterns on synthetic objects by matching the statistics of fracture patterns in a photograph. We present a user study to determine which statistics are correlated to visual similarity and how they are perceived by the user. We then describe a revised physically-based fracture model capable of producing a wide range of crack patterns at interactive rates. We demonstrate how a Bayesian optimization method can determine the parameters of this model so it can produce a pattern with the same key statistics as an exemplar. Finally, we present results using our approach and various exemplars to produce a variety of fracture effects in synthetic renderings of complex environments. The speed of the fracture simulation allows interactive previews of the fractured results and its application on large scale environments.
User-Friendly Graph Editing for Procedural Buildings
Patow, Gustavo A.
IEEE Computer Graphics and Applications, Vol. 32, Num. 2, pp 66--75, 2012.
DOI: http://dx.doi.org/10.1109/MCG.2010.104
A proposed rule-based editing metaphor intuitively lets artists create buildings without changing their workflow. It's based on the realization that the rule base represents a directed acyclic graph and on a shift in the development paradigm from product-based to rule-based representations. Users can visually add or edit rules, connect them to control the workflow, and easily create commands that expand the artist's toolbox (for example, Boolean operations or local controlling operators). This approach opens new possibilities, from model verification to model editing through graph rewriting.
Tim Reiner; Sylvain Lefebvre; Lorenz Diener; Garcia Fernández, Ismael; Bruno Jobard; Carsten Dachsbacher
Computers & Graphics, Vol. 36, Num. 5, pp 366--375, 2012.
DOI: http://dx.doi.org/10.1016/j.cag.2012.03.031
We present an efficient runtime cache to accelerate the display of procedurally displaced and textured implicit surfaces, exploiting spatio-temporal coherence between consecutive frames. We cache evaluations of implicit textures covering a conceptually infinite space. Rotating objects, zooming onto surfaces, and locally deforming shapes now requires minor cache updates per frame and benefits from mostly cached values, avoiding expensive re-evaluations. A novel parallel hashing scheme supports arbitrarily large data records and allows for an automated deletion policy: new information may evict information no longer required from the cache, resulting in an efficient usage. This sets our solution apart from previous caching techniques, which do not dynamically adapt to view changes and interactive shape modifications. We provide a thorough analysis on cache behavior for different procedural noise functions to displace implicit base shapes, during typical modeling operations.
Multi-Modal Medical Image Registration Using Normalized Compression Distance
Vázquez, Pere-Pau; Marco, Jordi
IADIS International Journal on Computer Science and Information Systems, Vol. 7, Num. 1, pp 47-63, 2012.
Image registration is an important task in medicine, especially when images have been acquired by different scanner/sensor types, since they provide information on different body structures (bones, muscles, vessels...). Several techniques have been proposed in the past, and among those, Normalized Mutual Information has been proven as successful in many cases. Normalized Compression Distance has been proposed as a simple yet effective technique for image registration. It is especially suitable for the case of CT-MRI registration. However, other image modalities such as PET pose some problems and do not achieve accurate registration. In this paper we analyse and propose a valid approach for image registration using compression that works properly for different combinations of CT, MRI and PET images.
The ViRVIG Institute
Andújar, Carlos; Navazo, Isabel; Vázquez, Pere-Pau; Patow, Gustavo A.; Pueyo, Xavier
SBC Journal on 3D Interactive Systems, Vol. 2, Num. 2, 2011.
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—In this paper we present the ViRVIG Institute, a recently created institution that joins two well-known research groups: MOVING in Barcelona, and GGG in Girona. Our main research topics are Virtual Reality devices and interaction techniques, complex data models, realistic materials and lighting, geometry processing, and medical image visualization. We briefly introduce the history of both research groups and present some representative projects. Finally, we sketch our lines for future research.
Beacco, Alejandro; Andújar, Carlos; Pelechano, Nuria
Computer Graphics Forum, Vol. 30, Num. 8, pp 2328--2340, 2011.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2011.02065.x
Rendering detailed animated characters is a major limiting factor in crowd simulation. In this paper we present a new representation for 3D animated characters which supports output-sensitive rendering. Our approach is flexible in the sense that it does not require us to pre-define the animation sequences beforehand, nor to pre-compute a dense set of pre-rendered views for each animation frame. Each character is encoded through a small collection of textured boxes storing colour and depth values. At runtime, each box is animated according to the rigid transformation of its associated bone and a fragment shader is used to recover the original geometry using a dual-depth version of relief mapping. Unlike competing output-sensitive approaches, our compact representation is able to recover high-frequency surface details and reproduces view-motion parallax effectively. Our approach drastically reduces both the number of primitives being drawn and the number of bones influencing each primitive, at the expense of a very slight per-fragment overhead. We show that, beyond a certain distance threshold, our compact representation is much faster to render than traditional level-of-detail triangle meshes. Our user study demonstrates that replacing polygonal geometry by our impostors produces negligible visual artefacts.
Bosch, Carles; Laffont, Pierre-Yves; Rushmeier, Holly; Dorsey, Julie; Drettakis, George
ACM Transactions on Graphics, Vol. 30, Num. 3, pp 20:1--20:13, 2011.
DOI: http://dx.doi.org/10.1145/1966394.1966399
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The simulation of weathered appearance is essential in the realistic modeling of urban environments. A representative and particularly difficult effect to produce on a large scale is the effect of fluid flow. Changes in appearance due to flow are the result of both the global effect of large-scale shape, and local effects, such as the detailed roughness of a surface. With digital photography and Internet image collections, visual examples of flow effects are readily available. These images, however, mix the appearance of flows with the specific local context. We present a methodology to extract parameters and detail maps from existing imagery in a form that allows new target-specific flow effects to be produced, with natural variations in the effects as they are applied in different locations in a new scene. In this paper, we focus on producing a library of parameters and detail maps for generating flow patterns – and this methodology can be used to extend the library with additional image exemplars. To illustrate our methodology, we show a rich collection of patterns applied to urban models.
Callieri, Marco; Chica, Antoni; Dellepiane, Matteo; Besora, Isaac; Corsini, Massimiliano; Moyés, Jordi; Ranzuglia, Guido; Scopigno, Roberto; Brunet, Pere
ACM Journal on Computing and Cultural Heritage, Vol. 3, Num. 4, pp 14:1 -- 14:20, 2011.
DOI: http://dx.doi.org/10.1145/1957825.1957827
The dichotomy between full detail representation and the efficient management of data digitization is still a big issue in the context of the acquisition and visualization of 3D objects, especially in the field of the Cultural Heritage. Modern scanning devices enable very detailed geometry to be acquired, but it is usually quite hard to apply these technologies to large artifacts. In this paper we present a project aimed at virtually reconstructing the impressive (7x11 m.) portal of the Ripoll Monastery, Spain. The monument was acquired using triangulation laser scanning technology, producing a dataset of 2212 range maps for a total of more than 1 billion triangles. All the steps of the entire project are described, from the acquisition planning to the final setup for dissemination to the public. We show how time-of-flight laser scanning data can be used to speed-up the alignment process. In addition we show how, after creating a model and repairing imperfections, an interactive and immersive setup enables the public to navigate and display a fully detailed representation of the portal. This paper shows that, after careful planning and with the aid of state-of-the-art algorithms, it is now possible to preserve and visualize highly detailed information, even for very large surfaces.
Coll, Narcís; Guerrieri, Marité Ethel; Rivara, María Cecilia; Sellarès, J. Antoni
Journal of Computational and Applied Mathematics, Vol. 236, Num. 6, pp 1410-1422, 2011.
DOI: http://dx.doi.org/10.1016/j.cam.2011.09.005
We propose and discuss a new Lepp-surface method able to produce a small triangular approximation of huge sets of terrain grid data by using a two-goal strategy that assures both small approximation error and well-shaped 3D triangles. This is a refinement method which starts with a coarse initial triangulation of the input data, and incrementally selects and adds data points into the mesh as follows: for the edge e having the highest error in the mesh, one or two points close to (one or two) terminal edges associated with e are inserted in the mesh. The edge error is computed by adding the triangle approximation errors of the two triangles that share e, while each L(2)-norm triangle error is computed by using a curvature tensor (a good approximation of the surface) at a representative point associated with both triangles. The method produces triangular approximations that capture well the relevant features of the terrain surface by naturally producing well-shaped triangles. We compare our method with a pure L(2)-norm optimization method.
Coll, Narcís; Paradinas, Teresa
Computer Graphics Forum, Vol. 30, Num. 1, pp 187-198, 2011.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2010.01842.x
Development of approximation techniques for highly detailed surfaces is one of the challenges faced today. We introduce a new mesh structure that allows dense triangular meshes of arbitrary topology to be approximated. The structure is constructed from the information gathered during a simplification process. Each vertex of the simplified model collects a neighbourhood of input vertices. Then, each neighbourhood is fitted by a set of local surfaces taking into account the sharp features detected. The simplified model plus the parameters of these local surfaces, conveniently stored in a file, is what we call Compact Model (CM). The input model can be approximated from its CM by refining each triangle of the simplified model. The main feature of our approach is that each triangle is refined by blending the local surfaces at its vertices, which can be done independently of the others. Consequently, adaptive reconstructions are possible, local shape deformations can be incorporated and the whole approximation process can be completely parallelized.
Durupinar, Funda; Pelechano, Nuria; Allbeck, Jan; Gudukbay, Ugur; Badler, Norman
IEEE Computer Graphics and Applications, Vol. 31, Num. 3, pp 22--31, 2011.
DOI: http://doi.ieeecomputersociety.org/10.1109/MCG.2009.105
Most crowd simulators animate homogeneous crowds but include underlying parameters that users can tune to create variations in the crowd. However, these parameters are specific to the crowd models and might be difficult for animators or naïve users to use. A proposed approach maps these parameters to personality traits. It extends the HiDAC (High-Density Autonomous Crowds) system by providing each agent with a personality model based on the Ocean (openness, conscientiousness, extroversion, agreeableness, and neuroticism) personality model. Each trait has an associated nominal behavior. Specifying an agent's personality leads to automation of low-level parameter tuning. User studies validated the mapping by assessing users' perception of the traits in animations that illustrate such behaviors.
Fortuny, G.; López-Cano, M.; Susin, Antonio; Herrera, B.
Computer Methods in Biomechanics and Biomedical Engineering, Vol. 15, Num. 2, pp 195-201, 2011.
DOI: http://dx.doi.org/10.1080/10255842.2010.522182
We are interested in studying the genesis of a very common pathology: the human inguinal hernia. How the human inguinal hernia appears is not definitively clear, but it is accepted that it is caused by a combination of mechanical and biochemical alterations, and that muscular simulation plays an important role in this. This study proposes a model to explain how some physical parameters affect the ability to simulate the region dynamically and how these parameters are involved in generating inguinal hernias. We are particularly interested in understanding the mechanical alterations in the inguinal region because little is known about them or how they behave dynamically. Our model corroborates the most important theories regarding the generation of inguinal hernias and is an initial approach to numerically evaluating this affection.
Garcia Fernández, Ismael; Sylvain Lefebvre; Samuel Hornus; Anass Lasram
ACM Transactions on Graphics, 2011.
DOI: http://dx.doi.org/10.1145/2070781.2024195
Recent spatial hashing schemes hash millions of keys in parallel, compacting sparse spatial data in small hash tables while still allowing for fast access from the GPU. Unfortunately, available schemes suffer from two drawbacks: Multiple runs of the construction process are often required before success, and the random nature of the hash functions decreases access performance. We introduce a new parallel hashing scheme which reaches high load factor with a very low failure rate. In addition our scheme has the unique advantage to exploit coherence in the data and the access patterns for faster performance. Compared to existing approaches, it exhibits much greater locality of memory accesses and consistent execution paths within groups of threads. This is especially well suited to Computer Graphics applications, where spatial coherence is common. In absence of coherence our scheme performs similarly to previous methods, but does not suffer from construction failures. Our scheme is based on the Robin Hood scheme modified to quickly abort queries of keys that are not in the table, and to preserve coherence. We demonstrate our scheme on a variety of data sets. We analyze construction and access performance, as well as cache and threads behavior.
Monclús, Eva; Vázquez, Pere-Pau; Navazo, Isabel
Visualization in Medicine and Life Sciences II, pp 133-151, 2011.
DOI: http://dx.doi.org/10.1007/978-3-642-21608-4_8
The visualization of volumetric datasets, quite common in medical image processing, has started to receive attention fromother communities such as scientific and engineering. The main reason is that it allows the scientists to gain important insights into the data. While the datasets are becoming larger and larger, the computational power does not always go hand to hand, because the requirements of using low-end PCs or mobile phones increase. As a consequence, the selection of an optimal viewpoint that improves user comprehension of the datasets is challenged with time consuming trial and error tasks. In order to facilitate the exploration process, informative viewpoints together with camera paths showing representative information on the model can be determined. In this paper we present amethod for representative viewselection and path construction, togetherwith some accelerations that make this process extremely fast on a modern GPU.
Pelechano, Nuria; Spanlang, Bernhard; Beacco, Alejandro
The International Journal of Virtual Reality, Vol. 10, Num. 1, pp 13-19, 2011.
This paper presents an Animation Planning Mediator (APM) designed to synthesize animations efficiently for virtual characters in real time crowd simulation. From a set of animation clips, the APM selects the most appropriate and modifies the skeletal configuration of each character to satisfy desired constraints (e.g. eliminating foot-sliding or restricting upper body torsion), while still providing natural looking animations. We use a hardware accelerated character animation library to blend animations increasing the number of possible locomotion types. The APM allows the crowd simulation module to maintain control of path planning, collision avoidance and response. A key advantage of our approach is that the APM can be integrated with any crowd simulator working in continuous space. We show visual results achieved in real time for several hundreds of agents, as well as the quantitative accuracy.
Rossignac, Jarek; Vinacua, Àlvar
ACM TOG, Vol. 30, Num. 5, pp 116:1--116-16, 2011.
DOI: http://dx.doi.org/10.1145/2019627.2019635
We propose to measure the quality of an affine motion by its steadiness, which we formulate as the inverse of its Average Relative Acceleration (ARA). Steady affine motions, for which ARA=0, include translations, rotations, screws, and the golden spiral. To facilitate the design of pleasing in-betweening motions that interpolate between an initial and a final pose (affine transformation), B and C, we propose the Steady Affine Morph (SAM), defined as A^t? B with A=C ? B^{?1}. A SAM is affine-invariant and reversible. It preserves isometries (i.e., rigidity), similarities, and volume. Its velocity field is stationary both in the global and the local (moving) frames. Given a copy count, n, the series of uniformly sampled poses, A^{i/n}? B, of a SAM form a regular pattern which may be easily controlled by changing B, C, or n, and where consecutive poses are related by the same affinity A^{1/n}. Although a real matrix A^t does not always exist, we show that it does for a convex and large subset of orientation-preserving affinities A. Our fast and accurate Extraction of Affinity Roots (EAR) algorithm computes A^t, when it exists, using closed-form expressions in two or in three dimensions. We discuss SAM applications to pattern design and animation and to key-frame interpolation.
Vázquez, Pere-Pau; Marco, Jordi
The Visual Computer, Vol. 28, Num. 11, pp 1063--1084, 2011.
DOI: http://dx.doi.org/10.1007/s00371-011-0651-2
Similarity metrics are widely used in computer graphics. In this paper, we will concentrate on a new, algorithmic complexity-based metric called Normalized Compression Distance. It is a universal distance used to compare strings. This measure has also been used in computer graphics for image registration or viewpoint selection. However, there is no previous study on how the measure should be used: which compressor and image format are the most suitable. This paper presents a practical study of the Normalized Compression Distance (NCD) applied to color images. The questions we try to answer are: Is NCD a suitable metric for image comparison? How robust is it to rotation, translation, and scaling? Which are the most adequate image formats and compression algorithms? The results of our study show that NCD can be used to address some of the selected image comparison problems, but care must be taken on the compressor and image format selected.
Andújar, Carlos; Brunet, Pere; Chica, Antoni; Navazo, Isabel
Computer Graphics Forum, Vol. 29, Num. 8, pp 2456--2468, 2010.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2010.01757.x
In this paper, we present an efficient approach for the interactive rendering of large-scale urban models, which can be integrated seamlessly with virtual globe applications. Our scheme fills the gap between standard approaches for distant views of digital terrains and the polygonal models required for close-up views. Our work is oriented towards city models with real photographic textures of the building facades. At the heart of our approach is a multi-resolution tree of the scene defining multi-level relief impostors. Key ingredients of our approach include the pre-computation of a small set of zenithal and oblique relief maps that capture the geometry and appearance of the buildings inside each node, a rendering algorithm combining relief mapping with projective texture mapping which uses only a small subset of the pre-computed relief maps, and the use of wavelet compression to simulate two additional levels of the tree. Our scheme runs considerably faster than polygonal-based approaches while producing images with higher quality than competing relief-mapping techniques. We show both analytically and empirically that multi-level relief impostors are suitable for interactive navigation through large urban models.
Argelaguet, Ferran; Andújar, Carlos
10th International Symposium on Smart Graphics, pp 115--126, 2010.
DOI: http://dx.doi.org/10.1007/978-3-642-13544-6_11
Predefined camera paths are a valuable tool for the exploration of complex virtual environments. The speed at which the virtual camera travels along different path segments is key for allowing users to perceive and understand the scene while maintaining their attention. Current tools for speed adjustment of camera motion along predefined paths, such as keyframing, interpolation types and speed curve editors provide the animators with a great deal of flexibility but offer little support for the animator to decide which speed is better for each point along the path. In this paper we address the problem of computing a suitable speed curve for a predefined camera path through an arbitrary scene. We strive at adapting speed along the path to provide non-fatiguing, informative, interestingness and concise animations. Key elements of our approach include a new metric based on optical flow for quantifying the amount of change between two consecutive frames, the use of perceptual metrics to disregard optical flow in areas with low image saliency, and the incorporation of habituation metrics to keep the user attention. We also present the results of a preliminary user-study comparing user response with alternative approaches for computing speed curves.
Argelaguet, Ferran; Kunert, André; Kulik, Alexander; Froehlich, Bernd
IEEE Symposium on 3D User Interfaces, pp 55--62, 2010.
DOI: http://dx.doi.org/10.1109/3DUI.2010.5444719
Multi-user virtual reality systems enable natural interaction with shared virtual worlds. Users can talk to each other, gesture and point into the virtual scenery as if it were real. As in reality, referring to objects by pointing, results often in a situation whereon objects are occluded from the other users' viewpoints. While in reality this problem can only be solved by adapting the viewing position, specialized individual views of the shared virtual scene enable various other solutions. As one such solution we propose show-through techniques to make sure that the objects one is pointing to can be seen by others. We analyzed the influence of such augmented viewing techniques on the spatial understanding of the scene, the rapidity of mutual information exchange as well as the social behavior of users. The results of our user study revealed that show-through techniques support spatial understanding on a similar level as walking around to achieve a non-occluded view of specified objects. However, advantages in terms of comfort, user acceptance and compliance to social protocols could be shown, which suggest that virtual reality techniques can in fact be better than 3D reality.
Real-Time Path-Based Surface Detail
Bosch, Carles; Patow, Gustavo A.
Computers & Graphics, Vol. 34, Num. 4, pp 430--440, 2010.
DOI: http://dx.doi.org/10.1016/j.cag.2010.04.001
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We present a GPU algorithm to render path-based 3D surface detail in real-time. Our method models these features using a vector representation that is efficiently stored in two textures. First texture is used to specify the position of the features, while the second texture contains their paths, profi les and material information. A fragment shader is then proposed to evaluate this data on the GPU by performing an accurate and fast rendering of the details, including visibility computations and antialiasing. Some of our main contributions include a CSG approach to efficiently deal with intersections and similar cases, and an efficient antialiasing method for the GPU. This technique allows application of path-based features such as grooves and similar details just like traditional textures, thus can be used onto general surfaces.
Coll, Narcís; Madern, Narcís; Sellarès, J. Antoni
Visual Computer, Vol. 26, Num. 2, pp 109-120, 2010.
DOI: http://dx.doi.org/10.1007/s00371-009-0380-y
Given a set V of viewpoints and a set S of obstacles in an environmental space, the good-visibility depth of a point q in relation to V and S is a measure of how deep or central q is with respect to the points in V that see q while minding the obstacles of S. The good-visibility map determined by V and S is the subdivision of the environmental space in good-visibility regions where all points have the same fixed good-visibility depth. In this paper we present algorithms for computing and efficiently visualizing, using graphics hardware capabilities, good-visibility maps in the plane as well as on triangulated terrains, where the obstacles are the terrain faces. Finally, we present experimental results obtained with the implementation of our algorithms.
Coll, Narcís; Paradinas, Teresa
Computer Graphics Forum, Vol. 29, Num. 6, pp 1842-1853, 2010.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2010.01652.x
Scanning and acquisition methods produce highly detailed surface meshes that need multi-chart parameterizations to reduce stretching and distortion. From these complex shape surfaces, high-quality approximations are automatically generated by using surface simplification techniques. Multi-chart textures hinder the quality of the simplification of these techniques for two reasons: either the chart boundaries cannot be simplified leading to a lack of geometric fidelity; or texture distortions and artefacts appear near the simplified boundaries. In this paper, we present an edge-collapse based simplification method that provides an accurate, low-resolution approximation from a multi-chart textured model. For each collapse, the model is reparameterized by local bijective mappings to avoid texture distortions and chart boundary artefacts on the simplified mesh due to the geometry changes. To better apply the appearance attributes and to guarantee geometric fidelity, we drive the simplification process with the quadric error metrics weighted by a local area distortion measure.
Díaz, Jose; Vázquez, Pere-Pau; Duguet, Florent; Navazo, Isabel
Computers & Graphics, Vol. 34, Num. 4, pp 337--350, 2010.
DOI: http://dx.doi.org/10.1016/j.cag.2010.03.005
Volume models often show high depth complexity. This poses difficulties to the observer in judging the spatial relationships accurately. Illustrators usually use certain techniques such as improving the shading through shadows, halos, or edge darkening in order to enhance depth perception of certain structures. Both effects are difficult to generate in real-time for volumetric models. Either they may have an important impact in rendering time, or they require precomputation that prevents changing the transfer function interactively, as it determines the occlusions. In this paper we present two methods for the fast generation of ambient occlusion on volumetric models. The first is a screen-space approach that does not require any precomputed data structure. The second is a view independent method that stores volumetric information in the form of a Summed Area Table of the density values, and thus, allows the interactive change of transfer functions on demand, although at the expense of memory space. Despite the fact that similar quality results are obtained with both approaches, the 3D version is more suitable for objects with discontinuous structures such as a vessels tree or the intestines, and it yields better framerates. The screen-space version is more suitable in limited GPU memory environments because it does not need extra 3D texture storage. As an extra result, our screen-space technique also allows for the computation of view dependent, interactively configurable halos using the same data structure. We have also implemented both methods using CUDA and have analyzed their efficiency.
Díaz, Jose; Vázquez, Pere-Pau; Navazo, Isabel; Duguet, Florent
Computers & Graphics, Vol. 34, Num. 4, pp 337--350, 2010.
DOI: http://dx.doi.org/10.1016/j.cag.2010.03.005
Volume models often show high depth complexity. This poses difficulties to the observer in judging the spatial relationships accurately. Illustrators usually use certain techniques such as improving the shading through shadows, halos, or edge darkening in order to enhance depth perception of certain structures. Both effects are difficult to generate in real-time for volumetric models. Either they may have an important impact in rendering time, or they require precomputation that prevents changing the transfer function interactively, as it determines the occlusions. In this paper we present two methods for the fast generation of ambient occlusion on volumetric models. The first is a screen-space approach that does not require any precomputed data structure. The second is a view independent method that stores volumetric information in the form of a Summed Area Table of the density values, and thus, allows the interactive change of transfer functions on demand, although at the expense of memory space. Despite the fact that similar quality results are obtained with both approaches, the 3D version is more suitable for objects with discontinuous structures such as a vessels tree or the intestines, and it yields better framerates. The screen-space version is more suitable in limited GPU memory environments because it does not need extra 3D texture storage. As an extra result, our screen-space technique also allows for the computation of view dependent, interactively configurable halos using the same data structure. We have also implemented both methods using CUDA and have analyzed their efficiency.
Hétroy, Frank; Rey, Stéphanie; Andújar, Carlos; Brunet, Pere; Vinacua, Àlvar
Computer-Aided Design, Vol. 43, Num. 1, pp 101--113, 2010.
DOI: http://dx.doi.org/10.1016/j.cad.2010.09.012
Limitations of current 3D acquisition technology often lead to polygonal meshes exhibiting a number of geometrical and topological defects which prevent them from widespread use. In this paper we present a new method for model repair which takes as input an arbitrary polygonal mesh and outputs a valid 2-manifold triangle mesh. Unlike previous work, our method allows users to quickly identify areas with potential topological errors and to choose how to fix them in a user-friendly manner. Key steps of our algorithm include the conversion of the input model into a set of voxels, the use of morphological operators to allow the user to modify the topology of the discrete model, and the conversion of the corrected voxel set back into a 2-manifold triangle mesh. Our experiments demonstrate that the proposed algorithm is suitable for repairing meshes of a large class of shapes.
Brunet, Pere; Chica, Antoni; Navazo, Isabel; Vinacua, Àlvar
Computing, Vol. 86, Num. 2, pp 101--115, 2009.
DOI: http://dx.doi.org/10.1007/s00607-009-0052-9
In constructing a model of a large twelfth century monument, we face the repair of a huge amount of small to medium-sized defects in the mesh. The total size of the mesh after registration was in the vicinity of 173M-triangles, and presented 14,622 holes of different sizes. Although other algorithms have been presented in the literature to fix these defects, in this case a fully automatic algorithm able to fix most of the defects is needed. In this paper we present the algorithms developed for this purpose, together with examples and results to measure the final surface quality. The algorithm is based on the iteration of smoothing and fitting steps on a uniform B-Spline defined on a 3D box domain bounding the hole. Tricubic and trilinear B-Splines are compared and the respective effectiveness is discussed.
El-Hajjar, Jean-François; Jolivet, Vincent; Ghazanfarpour, Djamchid; Pueyo, Xavier
The Visual Computer , Vol. 25, Num. 2, pp 87--100, 2009.
DOI: http://dx.doi.org/10.1007/s00371-007-0207-7
We present a novel empirical method for the animation of liquid droplets lying on a flat surface, the core of our technique being a simulation operating on a 2D grid which is implementable on GPU. The wetted surface can freely be oriented in space and is not limited to translucent materials, the liquid flow being governed by external forces, the viscosity parameter and the presence of obstacles. Furthermore, we show how to simply incorporate in our simulation scheme two enriching visual effects, namely absorption and ink transport. Rendering can be achieved from an arbitrary view point using a GPU image based ray-casting approach and takes into account the refraction and reflection of light. Even though our method doesn’t benefit from the literature of fluid mechanics, we show that convincing animations in terms of realism can be achieved in real-time.
Fort, Marta; Sellarès, J. Antoni
Applied Mathematics and Computation, Vol. 215, Num. 1, pp 235 -- 250, 2009.
DOI: http://dx.doi.org/10.1016/j.amc.2009.04.075
We present an algorithm for computing exact shortest paths, and consequently distance functions, from a generalized source (point, segment, polygonal chain or polygonal region) on a possibly non-convex triangulated polyhedral surface. The algorithm is generalized to the case when a set of generalized sites is considered, providing their distance field that implicitly represents the Voronoi diagram of the sites. Next, we present an algorithm to compute a discrete representation of the distance function and the distance field. Then, by using the discrete distance field, we obtain the Voronoi diagram of a set of generalized sites (points, segments, polygonal chains or polygons) and visualize it on the triangulated surface. We also provide algorithms that, by using the discrete distance functions, provide the closest, furthest and k-order Voronoi diagrams and an approximate 1-Center and 1-Median.
Fort, Marta; Sellarès, J. Antoni; Cabello, Sergio
Information Processing Letters, Vol. 109, Num. 9, pp 440--445, 2009.
DOI: http://dx.doi.org/10.1016/j.ipl.2009.01.001
We study the complexity of higher-order Voronoi diagrams on triangulated surfaces under the geodesic distance, when the sites may be polygonal domains of constant complexity. More precisely, we show that on a surface defined by n triangles the sum of the combinatorial complexities of the order-j Voronoi diagrams of m sites, for j=1,…,k, is O(k2n2+k2m+knm), which is asymptotically tight in the worst case.
Gonzalez Garcia, Francisco; Patow, Gustavo A.
ACM Transactions on Graphics, Vol. 28, Num. 5, pp 1--8, 2009.
DOI: http://dx.doi.org/10.1145/1618452.1618455
It is well known that multi-chart parameterizations introduce seams over meshes, causing serious problems for applications like texture filtering, relief mapping and simulations in the texture domain. Here we present two techniques, collectively known as Continuity Mapping, that together make any multi-chart parameterization seamless: Traveler’s Map is used for solving the spatial discontinuities of multi-chart parameterizations in texture space thanks to a bidirectional mapping between areas outside the charts and the corresponding areas inside; and Sewing the Seams addresses the sampling mismatch at chart boundaries using a set of stitching triangles that are not true geometry, but merely evaluated on a perfragment basis to perform consistent linear interpolation between non-adjacent texel values. Continuity Mapping does not require any modification of the artist-provided textures or models, it is fully automatic, and achieves continuity with small memory and computational costs.
László Szirmay-Kalos; Tamás Umenhoffer; Patow, Gustavo A.; László Szécsi; Mateu Sbert
Computer Graphics Forum, Vol. 28, Num. 6, pp 1586--1617, 2009.
DOI: http://dx.doi.org/ 10.1111/j.1467-8659.2009.01350.x
This survey reviews algorithms that can render specular, i.e. mirror reflections, refractions, and caustics on the GPU. We establish a taxonomy of methods based on the three main different ways of representing the scene and computing ray intersections with the aid of the GPU, including ray tracing in the original geometry, ray tracing in the sampled geometry, and geometry transformation. Having discussed the possibilities of implementing ray tracing, we consider the generation of single reflections/refractions, inter-object multiple reflections/refractions, and the general case which also includes self reflections or refractions. Moving the focus from the eye to the light sources, caustic effect generation approaches are also examined.
Mas, Albert; Patow, Gustavo A.; Martín, Ignacio
Computer Graphics Forum, Vol. 28, Num. 8, pp 2046--2056, 2009.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2009.01430.x
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This paper presents a new inverse reflector design method using a GPU-based computation of outgoing light distribution from reflectors. We propose a fast method to obtain the outgoing light distribution of a parameterized reflector, and then compare it with the desired illumination. The new method works completely in the GPU. We trace millions of rays using a hierarchical height-field representation of the reflector. Multiple reflections are taken into account. The parameters that define the reflector shape are optimized in an iterative procedure in order for the resulting light distribution to be as close as possible to the desired, user-provided one. We show that our method can calculate reflector lighting at least one order of magnitude faster than previous methods, even with millions of rays, complex geometries and light sources.
Vázquez, Pere-Pau
The Visual Computer, Vol. 25, Num. 5, pp 441--449, 2009.
DOI: http://dx.doi.org/10.1007/s00371-009-0326-4
Although the real world is composed of threedimensional objects, we communicate information using two-dimensional media. The initial 2D view we see of an object has great importance on how we perceive it. Deciding which of the all possible 2D representations of 3D objects communicates the maximum information to the user is still challenging, and it may be highly dependent on the addressed task. Psychophysical experiments have shown that three-quarter views (oblique views between frontal view and profile view) are often preferred as representative views for 3D objects; however, for most models, no knowledge of its proper orientation is provided. Our goal is the selection of informative views without any user intervention. In order to do so, we analyze some stability-based view descriptors and present a new one that computes view stability through the use of depth maps, without prior knowledge on the geometry or orientation of the object.We will show that it produces good views that, in most of the analyzed cases, are close to three-quarter views.
Booada, Imma; Coll, Narcís; Sellarès, J. Antoni
International Journal of Computer Mathematics, Vol. 85, Num. 7, pp 1003--1022, 2008.
DOI: http://dx.doi.org/10.1080/00207160701466362
We propose a new approach for computing in an efficient way polygonal approximations of generalized 2D/3D Voronoi diagrams. The method supports distinct site shapes (points, line-segments, curved-arc segments, polygons, spheres, lines, polyhedra, etc.), different distance functions (Euclidean distance, convex distance functions, etc.) and is restricted to diagrams with connected Voronoi regions. The presented approach constructs a tree (a quadtree in 2D/an octree in 3D) which encodes in its nodes and in a compact way all the information required for generating an explicit representation of the boundaries of the Voronoi diagram approximation. Then, by using this hierarchical data structure a reconstruction strategy creates the diagram approximation. We also present the algorithms required for dynamically maintaining under the insertion or deletion of sites the Voronoi diagram approximation. The main features of our approach are its generality, efficiency, robustness and easy implementation.
A Resolution Independent Approach for the Accurate Rendering of Grooved Surfaces
Bosch, Carles; Pueyo, Xavier; Mérillou, Stéphane; Ghazanfarpour, Djamchid
Computer Graphics Forum, Vol. 27, Num. 7, pp 1937--1944, 2008.
DOI: http://dx.doi.org/
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This paper presents a method for the accurate rendering of path-based surface details such as grooves, scratches and similar features. The method is based on a continuous representation of the features in texture space, and the rendering is performed by means of two approaches: one for isolated or non-intersecting grooves and another for special situations like intersections or ends. The proposed solutions perform correct antialiasing and take into account visibility and inter-re?ections with little computational effort and memory requirements. Compared to anisotropic BRDFs and scratch models, we have no limitations on the distribution of grooves over the surface or their geometry, thus allowing more general patterns. Compared to displacement mapping techniques, we can efficiently simulate features of all sizes without requiring additional geometry or multiple representations
Chica, Antoni; Williams, Jason; Andújar, Carlos; Brunet, Pere; Navazo, Isabel; Rossignac, Jarek; Vinacua, Àlvar
Computer Graphics Forum, Vol. 27, Num. 1, pp 36--46, 2008.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2007.01039.x
We present "Pressing", an algorithm for smoothing isosurfaces extracted from binary volumes while recovering their large planar regions (flats). Pressing yields a surface that is guaranteed to contain the samples of the volume classified as interior and exclude those classified as exterior. It uses global optimization to identify flats and constrained bilaplacian smoothing to eliminate sharp features and high-frequencies from the rest of the isosurface. It recovers sharp edges between flat regions and between flat and smooth regions. Hence, the resulting isosurface is usually a much more accurate approximation of the original solid than isosurfaces produced by previously proposed approaches. Furthermore, the segmentation of the isosurface into flat and curved faces and the sharp/smooth labelling of their edges may be valuable for shape recognition, simplification, compression, and various reverse engineering and manufacturing applications.
Coll, Narcís; Guerrieri, Marité Ethel; Sellarès, J. Antoni
Applied Mathematics and Computation, Vol. 201, pp 527--546, 2008.
DOI: http://dx.doi.org/10.1016/j.amc.2007.12.040
We propose a framework that combines improvement and Delaunay refinement techniques for incrementally adapting a refined mesh by interactively inserting and removing domain elements. Our algorithms achieve quality mesh by deleting, moving or inserting Steiner points from or into the mesh. The modifications applied to the mesh are local and the number of Steiner points added during the mesh adaptation process remains low. Moreover, since a mesh generation process can be viewed as an adaptation mesh process when domain elements are inserted one by one, our approach can also be applied to the generation of refined Delaunay quality meshes by incorporating our framework in the main body of Delaunay refinement mesh generation algorithms.
Garcia Fernández, Ismael; Patow, Gustavo A.
ACM Transactions on Graphics, Vol. 27, Num. 5, pp 1--9, 2008.
DOI: http://dx.doi.org/10.1145/1409060.1409090
Preserving details from a high resolution reference model onto lower resolution models is a complex, and sometimes daunting, task as manual intervention is required to correct texture misplacements. Inverse Geometric Textures (IGT) is a parameterization independent texturing technique that allows preservation of texture details from a high resolution reference model onto lower resolutions, generated with a given simplification method. IGT uses a parameterization defined on the reference model to generate an inversely parameterized texture that stores, for each texel, a list of all triangles that mapped onto it. This way, for any valid texture coordinate, IGT can know the point and the triangle of the detailed model that was projected, allowing application of details from the reference model onto the fragment from the low-resolution model. IGT is encoded in compact data structures and can be evaluated quickly. Furthermore, the high resolution model can have its own independent, secondary parameterization, so that no additional effort is required to directly use artist-designed content.
Mas, Albert; Martín, Ignacio; Patow, Gustavo A.
Computer Graphics Forum, Vol. 27, Num. 8, pp 2013--2027, 2008.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2008.01180.x
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This paper presents a method for compressing measured datasets of the near-field emission of physical light sources (represented by raysets). We create a mesh on the bounding surface of the light source that stores illumination information. The mesh is augmented with information about directional distribution and energy density. We have developed a new approach to smoothly generate random samples on the illumination distribution represented by the mesh, and to eficiently handle importance sampling of points and directions. We will show that our representation can compress a 10 million particle rayset into a mesh of a few hundred triangles. We also show that the error of this representation is low, even for very close objects.
Bose, Prosenjit; Coll, Narcís; Hurtado, Ferran; Sellarès, J. Antoni
International Journal of Computational Geometry and Applications, Vol. 17, pp 529--554, 2007.
DOI: http://dx.doi.org/10.1142/S0218195907002471
Given an unknown target planar map, we present an algorithm for constructing an approximation of the unknown target based on information gathered from linear probes of the target. Our algorithm is a general purpose reconstruction algorithm that can be applied in many settings. Our algorithm is particularly suited for the setting where computing the intersection of a line with an unknown target is much simpler than computing the unknown target itself. The algorithm maintains a triangulation from which the approximation of the unknown target can be extracted. We evaluate the quality of the approximation with respect to the target both in the topological sense and the metric sense. The correctness of the algorithm and the evaluation of its time complexity are also presented. Finally, we present some experimental results. For example, since generalized Voronoi diagrams are planar maps, our algorithm presents a simpler alternative method for constructing approximations of generalized Voronoi diagrams, which are notoriously difficult to compute.
Coll, Narcís; Fort, Marta; Madern, Narcís; Sellarès, J. Antoni
International Journal of Geographical Information Science, Vol. 21, Num. 10, pp 1115--1134, 2007.
DOI: http://dx.doi.org/10.1080/13658810701300097
Visibility computation on terrain models is an important research topic with many applications in Geographical Information Systems. A multi‐visibility map is the subdivision of the domain of a terrain into regions that, according to different criteria, encodes the visibility with respect to a set of view elements. We present an approach for visualising approximated multi‐visibility maps of a triangulated terrain corresponding to a set of view elements by using graphics hardware. Our method supports heterogeneous sets of view elements containing points, segments, polygonal chains and polygons and works for weak and strong visibility. Moreover, we are also able to efficiently solve approximated point and polygonal region multi‐visibility queries. To illustrate the usefulness of our approach we present results obtained with an implementation of the proposed algorithms.
Optimizing the topological and combinatorial complexity of isosurfaces
Andújar, Carlos; Brunet, Pere; Chica, Antoni; Navazo, Isabel; Rossignac, Jarek; Vinacua, Àlvar
Computer Aided Design, Vol. 37, Num. 8, pp 847--857, 2005.
DOI: http://dx.doi.org/10.1016/j.cad.2004.09.013
Since the publication of the original Marching Cubes algorithm, numerous variations have been proposed for guaranteeing water-tight constructions of triangulated approximations of isosurfaces. Most approaches divide the 3D space into cubes that each occupy the space between eight neighboring samples of a regular lattice. The portion of the isosurface inside a cube may be computed independently of what happens in the other cubes, provided that the constructions for each pair of neighboring cubes agree along their common face. The portion of the isosurface associated with a cube may consist of one or more connected components, which we call sheets. The topology and combinatorial complexity of the isosurface is influenced by three types of decisions made during its construction: (1) how to connect the four intersection points on each ambiguous face, (2) how to form interpolating sheets for cubes with more than one loop, and (3) how to triangulate each sheet. To determine topological properties, it is only relevant whether the samples are inside or outside the object, and not their precise value, if there is one. Previously reported techniques make these decisions based on local —per cube — criteria, often using precomputed look-up tables or simple construction rules. Instead, we propose global strategies for optimizing several topological and combinatorial measures of the isosurfaces: triangle count, genus, and number of shells. We describe efficient implementations of these optimizations and the auxiliary data structures developed to support them.
Computing maximal tiles and application to impostor-based simplification
Andújar, Carlos; Brunet, Pere; Chica, Antoni; Navazo, Isabel; Rossignac, Jarek; Vinacua, Àlvar
Computer Graphics Forum, Vol. 23, Num. 3, pp 401--410, 2004.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2004.00771.x
The computation of the largest planar region approximating a 3D object is an important problem with wide applications in modeling and rendering. Given a voxelization of the 3D object, we propose an efficient algorithm to solve a discrete version of this problem. The input of the algorithm is the set of grid edges connecting the interior and the exterior of the object (called sticks). Using a voting-based approach, we compute the plane that slices the largest number of sticks and is orientation-compatible with these sticks. The robustness and efficiency of our approach rests on the use of two different parameterizations of the planes with suitable properties. The first of these is exact and is used to retrieve precomputed local solutions of the problem. The second one is discrete and is used in a hierarchical voting scheme to compute the global maximum. This problem has diverse applications that range from finding object signatures to generating simplified models. Here we demonstrate the merits of the algorithm for efficiently computing an optimized set of textured impostors for a given polygonal model.
Bosch, Carles; Pueyo, Xavier; Stéphane Mérillou; Djamchid Ghazanfarpour
Computer Graphics Forum, Vol. 23, Num. 3, pp 361--370, 2004.
DOI: http://dx.doi.org/10.1111/j.1467-8659.2004.00767.x
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Individually visible scratches, also called isolated scratches, are very common in real world surfaces. Although their microgeometry is not visible, they are individually perceptible by the human eye, lying into a representation scale between BRDF and texture. In order to simulate this kind of scratches in synthetic images we need to know their position over the surface (texture scale), so we can determine where to use the specific scratch BRDF instead of the ordinary surface BRDF. Computing the BRDF of a scratch is difficult because it depends on the scratch’s invisible microgeometry. In this paper, we propose a new physically based model to derive this microgeometry by simulating the formation process of scratches. We allow specifying intuitively the parameters involved in the process such as the scratching tool, the penetration forces, and the material properties of the object. From these parameters, we derive the microgeometries of the scratches by taking into account the real behaviour of the process. This behaviour has been determined by analysing existing models in the field of materials engineering and some “scratch tests” that we performed on metals. Our method has the advantages of easily simulating scratches with a wide range of microgeometries and taking into account the variability of their microgeometry along the scratch path. Another contribution is related to the location of the scratches over the surface. Instead of using an image of the paths as in previous work, we present a new representation based on curves defining the paths. This offers an independence on the image resolution or the distance from the observer and accurately provides the scratch direction in order to compute scratch BRDFs
Conferences
GL-Socket: A CG Plugin-based Framework for Teaching and Assessment
Andújar, Carlos; Chica, Antoni; Fairén, Marta; Vinacua, Àlvar
EG 2018 - Education Papers, 2018.
DOI: http://dx.doi.org/10.2312/eged.20181003
In this paper we describe a plugin-based C++ framework for teaching OpenGL and GLSL in introductory Computer Graphics courses. The main strength of the framework architecture is that student assignments are mostly independent and thus can be completed, tested and evaluated in any order. When students complete a task, the plugin interface forces a clear separation of initialization, interaction and drawing code, which in turn facilitates code reusability. Plugin code can access scene, camera, and OpenGL window methods through a simple API. The plugin interface is flexible enough to allow students to complete tasks requiring shader development, object drawing, and multiple rendering passes. Students are provided with sample plugins with basic scene drawing and camera control features. One of the plugins that the students receive contains a shader development framework with self-assessment features. We describe the lessons learned after using the tool for four years in a Computer Graphics course involving more than one hundred Computer Science students per year.
Rendering and interacting with volume models in immersive environments
Fons, Joan; Monclús, Eva; Vázquez, Pere-Pau; Navazo, Isabel
In Proc. of Congreso Español de Informática Gráfica (CEIG), 2018.
The recent advances in VR headsets, such as the Oculus Rift or HTC Vive, at affordable prices offering a high resolution display, has empowered the development of immersive VR applications. data. In this paper we propose an immersive VR system that uses some well-known acceleration algorithms to achieve real-time rendering of volumetric datasets in an immersive VR system. Moreover, we have incorporated different basic interaction techniques to facilitate the inspection of the volume dataset. The interaction has been designed to be as natural as possible in order to achieve the most comfortable, user-friendly virtual experience. We have conducted an informal user study to evaluate the user preferences. Our evaluation shows that our application is perceived usable, easy of learn and very effective in terms of the high level of immersion achieved
A procedural approach for thermal visualization on buildings
Muñoz, David; Besuievsky, Gonzalo; Patow, Gustavo A.
CEIG, pp 109--117, 2018.
DOI: http://dx.doi.org/10.2312/ceig.20181164
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Agus, M.; Gobbetti, E.; Martona, F.; Pintore, G.; Vázquez, Pere-Pau
Proceedings in EuroGraphics Tutorials, 2017.
The increased availability and performance of mobile graphics terminals, including smartphones and tablets with high resolution screens and powerful GPUs, combined with the increased availability of high-speed mobile data connections, is opening the door to a variety of networked graphics applications. In this world, native apps or mobile sites coexist to reach the goal of providing us access to a wealth of multimedia information while we are on the move. This half-day tutorial provides a technical introduction to the mobile graphics world spanning the hardware-software spectrum, and explores the state of the art and key advances in specific application domains, including capture and acquisition, real-time high-quality 3D rendering and interactive exploration.
Agus, M.; Gobbetti, E.; Martona, F.; Pintore, G.; Vázquez, Pere-Pau
Siggraph Asia Courses, 2017.
This half-day tutorial provides a technical introduction to the mobile graphics world spanning the hardware-software spectrum, and explores the state of the art and key advances in specific application domains, including capture and acquisition, real-time high-quality 3D rendering, and interactive exploration.
Tree Variations
Argudo, Oscar; Andújar, Carlos; Chica, Antoni
CEIG - Spanish Computer Graphics Conference, pp 121--130, 2017.
DOI: http://dx.doi.org/10.2312/ceig.20171218
The cost-effective generation of realistic vegetation is still a challenging topic in computer graphics. The simplest representation of a tree consists of a single texture-mapped billboard. Although a tree billboard does not support top views, this is the most common representation for still image generation in areas such as architecture rendering. In this paper we present a new approach to generate new tree models from a small collection of RGBA images of trees. Key ingredients of our method are the representation of the tree contour space with a small set of basis vectors, the automatic crown/trunk segmentation, and the continuous transfer of RGBA color from the exemplar images to the synthetic target. Our algorithm allows the efficient generation of an arbitrary number of tree variations and thus provides a fast solution to add variety among trees in outdoor scenes
Díaz-García, Jesús; Brunet, Pere; Navazo, Isabel; Vázquez, Pere-Pau
CEIG 2017:XXVII Spanish Computer Graphics Conference, pp 51--60, 2017.
DOI: http://dx.doi.org/10.2312/ceig.20171208
The way in which gradients are computed in volume data-sets influences both the quality of the shading and the performance obtained in rendering algorithms. In particular, the visualization of coarse datasets in multi-resolution representations is affected when gradients are evaluated on-the-fly in the shader code by accessing neighbouring positions. We propose a downsampling filter for pre-computed gradients that provides improved gradients that better match the originals such that the aforementioned artifacts disappear. Secondly, to address the storage problem, we present a method for the efficient storage of gradient directions that is able to minimize the minimum angle achieved among all representable vectors in a space of 3 bytes.
Díaz-García, Jesús; Brunet, Pere; Navazo, Isabel; Vázquez, Pere-Pau
IADIS International Conference Computer Graphics, Visualization, Computer Vision and Image Processing-CGVCVIP, pp 12--20, 2017.
Volume visualization software usually has to deal with datasets that are larger than the GPUs may hold. This is especially true in one of the most popular application scenarios: medical visualization. In this paper we explore the quality of different downsampling methods and present a new approach that produces smooth lower-resolution representations, yet still preserves small features that are prone to disappear with other approaches.
Virtual Reality to teach anatomy
Fairén, Marta; Farrés, Mariona; Moyés, Jordi; Insa, Esther
Proceedings in Eurographics Education Papers, pp 51-58, 2017.
DOI: http://dx.doi.org/10.2312/eged.20171026
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Virtual Reality (VR) and Augmented Reality (AR) have been gradually introduced in the curriculum of schools given the benefits they bring to classical education. We present an experiment designed to expose students to a VR session where they can directly inspect 3D models of several human organs by using Virtual Reality systems. Our systems allow the students to see the models directly visualized in 3D and to interact with them as if they were real. The experiment has involved 254 students of a Nursing Degree, enrolled in the Human anatomy and physiology course during 2 years (2 consecutive courses). It includes 10 3D models representing different anatomical structures which have been enhanced with meta-data to help the students understand the structure. In order to evaluate the students’ satisfaction facing such a new teaching methodology, the students were asked to fill in a questionnaire with two categories. The first one measured whether or not, the teaching session using VR facilitates the understanding of the structures. The second one measured the student’s satisfaction with this VR session. From the results we can see that the items most valuated are the use of the activity as a learning tool, and the satisfaction of the students’ expectations. We can therefore conclude that VR session for teaching is a powerful learning tool that helps to understand the anatomical structures.
Julien Pettré; Pelechano, Nuria
EG2017 - Tutorials, 2017.
DOI: http://dx.doi.org/10.2312/egt.20171029
Lopez-Garcia, Axel; Susin, Antonio
CEIG 2017:XXVII Spanish Computer Graphics Conference, pp 19-22, 2017.
DOI: http://dx.doi.org/ http://dx.doi.org/10.2312/ceig.20171203
Skeleton tracking has multiple applications such as games, virtual reality, motion capture and more. One of the main challenges of pose detection is to be able to obtain the best possible quality with a cheap and easy-to-use device. In this work we propose a physically based method to detect errors and tracking issues which appear when we use low cost tracking devices such as Kinect. Therefore, we can correct the animation in order to obtain a smoother movement. We have implemented the Newton- Euler Algorithm, which allow us to compute the internal forces involved in a skeleton. In a common movement, forces are usually smooth without sudden variations. When the tracking yields poor results or invalid poses the internal forces become very large with a lot of variation. This allow us to detect when the tracking system fails and the animation needs to be inferred through different methods.
Occlusion aware hand pose recovery from sequences of depth images
Meysam Madadi; Sergio Escalera; Carruesco, Alex; Andújar, Carlos; Xavier Baró; Jordi González
12th IEEE International Conference on Automatic Face & Gesture Recognition (FG 2017), pp 230-237, 2017.
DOI: http://dx.doi.org/10.1109/FG.2017.37
State-of-the-art approaches on hand pose estimation from depth images have reported promising results under quite controlled considerations. In this paper we propose a two-step pipeline for recovering the hand pose from a sequence of depth images. The pipeline has been designed to deal with images taken from any viewpoint and exhibiting a high degree of finger occlusion. In a first step we initialize the hand pose using a part-based model, fitting a set of hand components in the depth images. In a second step we consider temporal data and estimate the parameters of a trained bilinear model consisting of shape and trajectory bases. Results on a synthetic, highly-occluded dataset demonstrate that the proposed method outperforms most recent pose recovering approaches, including those based on CNNs.
Muñoz-Pandiella, Imanol; Akoglu, Kiraz; Bosch, Carles; Rushmeier, Holly
EUROGRAPHICS Workshop on Graphics and Cultural Heritage, 2017.
DOI: http://dx.doi.org/10.2312/gch.20171291
In Cultural Heritage projects, it is very important to identify and track weathering effects on monuments in order to design and test conservation strategies. Currently, this mapping is manual work performed by experts based on what they observe and their experience. In this paper, we present a workflow to map the weathering effect known as “scaling” on monuments with very little user interaction. First, we generate a 3D model of the monuments using photogrammetry techniques. Then, we reduce the noise in the acquired data using an adaptive and anisotropic filter. After that, we estimate the original shape of the surface before the weathering effects using the RANSAC algorithm. With this information, we perform a geometrical analysis to detect the features affected by this weathering effect and compute their characteristics. Then, we map the regions that have suffered scaling using the detected features and a segmentation based on the distance between the mesh and the unweathered surface. Our technique results can be very useful to understand the level of weathering of a monument and to trace the weathered parts through time automatically.
Real-time solar exposure in complex cities
Muñoz-Pandiella, Imanol; Bosch, Carles; Mérillou, Nicolas; Pueyo, Xavier; Mérillou, Stephane
28th Eurographics Symposium on Rendering, 2017.
Invited presentation by CGF journal.
Rogla, Otger; Pelechano, Nuria
Spanish Computer Graphics Conference (CEIG), pp 113-120, 2017.
DOI: http://dx.doi.org/10.2312/ceig.20171217
Authoring Semantic Procedural Cities
Rogla, Otger; Pelechano, Nuria
10th ACM SIGGRAPH Conference on Motion in Games - Posters, pp 1, 2017.
Vahid Rahmani; Pelechano, Nuria
10th ACM SIGGRAPH Conference on Motion in Games, pp 8:1-8:6, 2017.
DOI: http://dx.doi.org/10.1145/3136457.3136465
Víctor Ceballos; Monclús, Eva; Vázquez, Pere-Pau; Bendezú, Álvaro; Mego, Marianela; Merino, Xavier; Navazo, Isabel; Azpiroz, Fernando
EuroVis 2017 - 19th EG/VGTC Conference on Visualization (Poster), 2017.
DOI: http://dx.doi.org/10.2312/eurp.20171184
The analysis of the morphology and content of the gut is necessary in order to understand metabolic and functional gut activity and for diagnostic purposes. Magnetic resonance imaging (MRI) has become an important modality technique since it is able to visualize soft tissues using no ionizing radiation, and hence removes the need for any contrast agents. In the last few years, MRI of gastrointestinal function has advanced substantially, although scarcely any publication has been devoted to the analysis of the colon content. This paper presents a semi-automatic segmentation tool for the quantitative assessment of the unprepared colon from MRI images. This application has allowed for the analysis of the colon content in various clinical experiments. The results of the assessment have contributed to a better understanding of the functionality of the colon under different diet conditions. The last experiment carried out by medical doctors showed a marked influence of diet on colonic content, accounting for about 30% of the volume variations.
Yağmur Güçlütürk; Umut Güçlü; Marc Pérez; Hugo Escalante; Xavier Baró; Isabelle Guyon; Andújar, Carlos; Julio Jacques Jr; Meysam Madadi; Sergio Escalera; M. A. J. van Gerven; R. van Lier
ICCVW, 2017.
DOI: http://dx.doi.org/10.1109/ICCVW.2017.367
Automatic prediction of personality traits is a subjective task that has recently received much attention. Specifically, automatic apparent personality trait prediction from multimodal data has emerged as a hot topic within the filed of computer vision and, more particularly, the so called "looking at people" sub-field. Considering "apparent" personality traits as opposed to real ones considerably reduces the subjectivity of the task. The real world applications are encountered in a wide range of domains, including entertainment, health, human computer interaction, recruitment and security. Predictive models of personality traits are useful for individuals in many scenarios (e.g., preparing for job interviews, preparing for public speaking). However, these predictions in and of themselves might be deemed to be untrustworthy without human understandable supportive evidence. Through a series of experiments on a recently released benchmark dataset for automatic apparent personality trait prediction, this paper characterizes the audio and visual information that is used by a state-of-the-art model while making its predictions, so as to provide such supportive evidence by explaining predictions made. Additionally, the paper describes a new web application, which gives feedback on apparent personality traits of its users by combining model predictions with their explanations.
Ordering Triangles in Triangulated Terrains Over Regular Grids
Alonso, Jesús; Joan Arinyo, Robert
CEIG'16, 2016.
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In this work we report on a set of rules to visit triangles in triangulated height fields defined over regular grids in a back-to-front order with respect to an arbitrary viewpoint. With the viewpoint, we associate an axis-alligned local reference framework. Projections on the $XY$ plane of the local axis and the bisector of the first and third quadrants define six sectors. Specific visiting rules for collections of triangles that project on each sector are then defined. The experiments conducted show that the implementation of a simple algorithm based on the set of visiting rules defined allows real time interaction when the viewing position moves along an arbitrary 3D path.
Computing the maximum overlap of a disk and a piecewise circular domain under translation
Coll, Narcís; Fort, Marta; Sellarès, J. Antoni
European Workshop on Computational Geometry (EuroCG), 2016.
Human-Document Interaction systems - a new frontier for document image analysis
Dimosthenis Karatzas; Vincent Poulain d’Andecy; Marçal Rusiñol; Chica, Antoni; Vázquez, Pere-Pau
Proceedings of the International Workshop on Document Analysis Systems (DAS), 2016.
All indications show that paper documents will not cede in favour of their digital counterparts, but will instead be used increasingly in conjunction with digital information. An open challenge is how to seamlessly link the physical with the digital -how to continue taking advantage of the important affordances of paper, without missing out on digital functionality. This paper presents the authors’ experience with developing systems for Human-Document Interaction based on augmented document interfaces and examines new challenges and opportunities arising for the document image analysis field in this area. The system presented combines state of the art camera-based document image analysis techniques with a range of complementary technologies to offer fluid Human-Document Interaction. Both fixed and nomadic setups are discussed that have gone through user testing in real-life environments, and use cases are presented that span the spectrum from business to educational applications.
Optimized Skin Rendering for Scanned Models
Hernando, Roger; Chica, Antoni; Vázquez, Pere-Pau
Proceedings of the WSCG International Conferences in Central Europe on Computer Graphics, Visualization and Computer Vision, pp 89--96, 2016.
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Skin is one of the most difficult materials to reproduce in computer graphics, mainly due to two major factors: First, the complexity of the light interactions happening at the subsurface layers of skin, and second, the high sensitivity of our perceptual system to the artificial imperfections commonly appearing in synthetic skin models. Many current approaches mix physically-based algorithms with image-based improvements to achieve realistic skin rendering in realtime. Unfortunately, those algorithms still suffer from artifacts such as halos or incorrect diffusion. Some of these artifacts (e.g. incorrect diffusion) are especially noticeable if the models have not been previously segmented. In this paper we present some extensions to the Separable Subsurface Scattering (SSSS) framework that reduce those artifacts while still maintaining a high framerate. The result is an improved algorithm that achieves high quality rendering for models directly obtained from scanners, not requiring further processing.
Knopf-Lenoir, Catherine; Muñoz, David; Nahon, Raphaël; Beckers, Benoit
FICUP, 2016.
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The objective of the present work is to illustrate and discuss how numerical models can be used to simulate different physical phenomena that have a great impact on the comfort of buildings: acoustics and solar radiation, in order to: 1) get estimations of the noise level, temperature and natural light on any facade as functions of the noise sources in the street and the local climate, 2) evaluate the sensitivity of these quantities with respect to some basic shape features, 3) and propose an approach to optimize the shape considering different requirements on the levels of noise (to minimize), temperature and/or natural light at the ground floor (to maximize).
Enhanced Magnification for Reduced Movement in Virtual Reality Environments
Monclús, Eva; Navazo, Isabel; Vázquez, Pere-Pau
International Conference on Computer Graphics, Visualization, Computer Vision and image Processing (IADIS'16), pp 149--156, 2016.
Although Virtual Reality (VR) environments have seen slower adoption rates than expected among the medical community, throughout the last years, several new techniques and devices (e.g. Oculus Rift) have been developed that might boost the popularity of VR. However, the use of 3D environments is not exempt of some shortcomings, such as the difficulties of adaption to 3D, that sometimes produces motion sickness, or the increase in effort the user has to undergo when interacting with a 3D system, as compared to using a simple mouse on a desktop. Our proposal is tailored to focus on the second problem. More concretely, we propose a magnification technique that is able to reduce the movements required to explore a concrete part of a volumetric dataset as compared to other zooming techniques. Our technique, builds the zooming result in the same virtual position of the initial interaction while still maintaining a contextual view on the region of interest and its surroundings. This way, the user reduces the amount of movements required to explore a model and efficiency is increased. An informal user study was carried out that confirmed our hypotheses.
Muñoz-Pandiella, Imanol; Bosch, Carles; Mérillou, Nicolas; Pueyo, Xavier; Mérillou, Stephane; Patow, Gustavo A.
Workshop on Material Appearance Modeling, 2016.
DOI: http://dx.doi.org/10.2312/mam.20161248
In this paper, we propose to discuss on one of the main challenges in realistic rendering of urban scenes: changes in appearance over time within a urban context. After studying the previous work on weathering techniques, we have found that there is a lack of estimation for some important environmental parameters (such as sun radiation) that have a wrong impact on weathering phenomena simulation and, thus, on the appearance of virtual objects. We also think that such a problem needs to be addressed on large urban models. Here, we discuss some possible solutions we have studied in our research. These solutions are focused on screen-space techniques, in order to efficiently compute those factors and use them to interactively generate weathering effects.
Pelechano, Nuria; Jan Allbeck
IEEE VR Workshop on Virtual Humans and Crowds for Immersive Environment. VHCIE, 2016.
A software framework for the development of projection-based Augmented Reality Systems
Sunet, Marc; Comino, Marc; Karatzas, Dimosthenis; Chica, Antoni; Vázquez, Pere-Pau
IADIS Computer Graphics, Visualization, Computer Vision and Image Processing, 2016.
Despite the large amount of methods and applications of augmented reality, there is little homogenization on the software platforms that support them. An exception may be the low level control software that is provided by some high profile vendors such as Qualcomm and Metaio. However, these provide fine grain modules for e.g. element tracking. We are more concerned on the application framework, that includes the control of the devices working together for the development of the AR experience. In this paper we present a soft- ware framework that can be used for the development of AR applications based on camera-projector pairs, that is suitable for both fixed, and nomadic setups.
Sunet, Marc; Vázquez, Pere-Pau
Web3D, pp 127-135, 2016.
DOI: http://dx.doi.org/10.1145/2945292.2945300
Screen space ambient occlusion is a popular dynamic global illumination technique that has seen widespread adoption in PC computer games and other applications due to its simplicity, scalability, and ability to be integrated with other techniques. Mobile platforms, however, have traditionally been unable to run screen space ambient occlusion and other global illumination techniques in real-time, forcing developers to bake most of the illumination as a consequence. In this paper, we analyze the technical aspects involved in porting Ambient Occlusion to mobile devices and propose an optimized pipeline that is able to improve Ambient Occlusion calculation in OpenGL-based pipelines. We demonstrate our optimizations by applying them to four popular techniques: Crytek, Starcraft, Alchemy, and Horizon-Based Ambient Occlusion.
Parallel GPU Collision Computation in Conformal Warped Space For Surface Detail Haptic
Theoktisto, Víctor; Fairén, Marta; Navazo, Isabel
13th International Congress on Numerical Methods in Engineering and Applied Sciences, 2016.
GPU for rendering accurate touch sensation in highly complex geometric models. An object’s geometry is covered completely by a connected mesh of edge-joined prismoids, while surface detail is encoded by an image-based approach in a special tangent space flat texture containing relief and surface normals values. Heights and normals within each prism are warped from object volume space to orthogonal tangent space, by means fast method for computing barycentric coordinates, and stored in a per-face sorted RGBA texture. Parallel computation is performed in GPU for many prisms at once. Applying a unified treatment of collision detection in conformal space from euclidean R 3 (3D) to a higher dimensional conformal space R 4 ; 1 (5D), CUDA core kernels perform parallel real-time collision detection in GPU, with a haptic probe’s position vector without any bounding volume prefiltering, to identify which prisms of the object’s surface are being touched, and then map coordinates to tangent space, sampling relative surface height and normal from texture, for a final decision on whether the surface was h it. Results show throughput is increased between one and two orders of magnitude in collision benchmarks among known mesh models, computed in blind all vs. all manner, accurately colliding against 3D surface detail at high sampling rates without degrading touch sensation.
Van Toll, Wouter; Triesscheijn, Roy; Kallmann, Marcelo; Oliva, Ramon; Pelechano, Nuria; Pettré, Julien; Geraerts, Roland
Proceedings of the 9th International Conference on Motion in Games, pp 91--100, 2016.
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A navigation mesh is a representation of a 2D or 3D virtual environment that enables path planning and crowd simulation for walking characters. Various state-of-the-art navigation meshes exist, but there is no standardized way of evaluating or comparing them. Each implementation is in a different state of maturity, has been tested on different hardware, uses different example environments, and may have been designed with a different application in mind. In this paper, we conduct the first comparative study of navigation meshes. First, we give general definitions of 2D and 3D environments and navigation meshes. Second, we propose theoretical properties by which navigation meshes can be classified. Third, we introduce metrics by which the quality of a navigation mesh implementation can be measured objectively. Finally, we use these metrics to compare various state-of-the-art navigation meshes in a range of 2D and 3D environments. We expect that this work will set a new standard for the evaluation of navigation meshes, that it will help developers choose an appropriate navigation mesh for their application, and that it will steer future research on navigation meshes in interesting directions.
From One to Many:Simulating Groups of Agents with Reinforcement Learning Controllers
Casadiego, Luiselena; Pelechano, Nuria
15th International Conference on Intelligence Virtual Agents ( IVA'16), pp 119-123, 2015.
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This poster present a Reinforcement Learning (RL) approach for crowd simulation . We train an agent to move towards a goal while avoiding obstacles . Once one agent has learned, its knowledge is transferred to the rest of the members of the group by sharing the resulting Q - Table . This results in individual behavior leading to emergent group behavior . We present a framework with states, actions and reward functions general enough to easily adapt to different environment configurations .
DAPS for Degenerate Elements in FEM Simulations
Civit, Oscar; Susin, Antonio
Symposium Geometry Processing, 2015.
Fast Contact Determination for Intersecting Deformable Solids
Civit-Flores, Oscar; Susin, Antonio
In Proc. of Motion in Games (MIG'15), pp 205-214, 2015.
We present a fast contact determination scheme for intersecting deformable solids with detailed surface geometry. Given a high resolution closed surface mesh we automatically build a coarse embedding tetrahedralization and a partitioned representation of the surface in a preprocess. During simulation, the contact determination algorithm finds all intersecting pairs of deformed triangles using a memory-efficient barycentric bounding volume hierarchy, connects them into potentially disjoint intersection curves and performs a topological flood process on the exact intersection surfaces to discover a minimal set of contact points. A unique contact normal is computed for each contact volume, based on a continuous definition of surface normals, and used to find contact point correspondences suitable for contact treatment. The algorithm is strongly output-sensitive and we demonstrate robust contact determination at 60 frames per second for a pair of objects with 100K triangles in shallow intersecting contact.
Computing the maximum overlap of a disk and a polygon with holes under translation
Coll, Narcís; Fort, Marta; Sellarès, J. Antoni
Actas XVI Encuentros de Geometría Computacional, Barcelona, 2015.
Perceptual effects of volumetric shading models in stereoscopic desktop-based environments
Díaz, Jose; Ropinski, Timo; Navazo, Isabel; Gobetti, Enrico; Vázquez, Pere-Pau
In Proc. of Computer Graphics International, pp 1-10, 2015.
Throughout the years, many shading techniques have been developed to improve the conveying of informa- tion in Volume Visualization. Some of these methods, usually referred to as realistic, are supposed to provide better cues for the understanding of volume data sets. While shading approaches are heavily exploited in traditional monoscopic setups, no previous study has analyzed the effect of these techniques in Virtual Reality. To further explore the influence of shading on the understanding of volume data in such en- vironments, we carried out a user study in a desktop-based stereoscopic setup. The goals of the study were to investi- gate the impact of well-known shading approaches and the influence of real illumination on depth perception. Partici- pants had to perform three different perceptual tasks when exposed to static visual stimuli. 45 participants took part in the study, giving us 1152 trials for each task. Results show that advanced shading techniques improve depth perception in stereoscopic volume visualization. As well, external light- ing does not affect depth perception when these shading methods are applied. As a result, we derive some guidelines that may help the researchers when selecting illumination models for stereoscopic rendering.
Feature-Preserving Downsampling for Medical Images
Díaz-García, Jesús; Brunet, Pere; Navazo, Isabel; Pérez, Frederic; Vázquez, Pere-Pau
EuroVis, 2015.
InclineType: An Accelerometer-based Typing Approach for Smartwatches
Götzelmann, Timo; Vázquez, Pere-Pau
Proc. of XVI International Conference on Human Computer Interaction, pp 59:1-59:4, 2015.
Instant Visualization of Secondary Structures of Molecular Models
Hermosilla, Pedro; Guallar, Víctor; Vinacua, Àlvar; Vázquez, Pere-Pau
In Proc. of Eurographics Workshop on Visual Computing for Biology and Medicine, pp 51-60, 2015.
DOI: http://dx.doi.org/10.2312/vcbm.20151208
Molecular Dynamics simulations are of key importance in the drug design field. Among all possible representations commonly used to inspect these simulations, Ribbons has the advantage of giving the expert a good overview of the conformation of the molecule. Although several techniques have been previously proposed to render ribbons, all of them have limitations in terms of space or calculation time, making them not suitable for real-time interaction with simulation software. In this paper we present a novel adaptive method that generates ribbons in real-time, taking advantage of the tessellation shader. The result is a fast method that requires no precomputation, and that generates high quality shapes and shading.
Adaptive on-the-fly molecular ribbons generation
Hermosilla, Pedro; Guallar, Víctor; Vázquez, Pere-Pau; Vinacua, Àlvar
EuroVis, 2015.
Molecular Dynamics simulations are of key importance in the drug design field. Among all possible representations commonly used to inspect these simulations, Ribbons has the advantage of giving the expert a good overview of the conformation of the molecule. Although several techniques have been previously proposed to render ribbons, all of them have limitations in terms of space or calculation time, making them not suitable for real-time interaction with simulation software. In this paper we present a novel adaptive method that generates ribbons in real-time, taking advantage of the tessellation shader. The result is a fast method that requires no precomputation, and that generates high quality shapes and shading.
Muñoz, David; Beckers, Benoit; Besuievsky, Gonzalo; Patow, Gustavo A.
UDMV 2015 - TU Delft, 2015.
DOI: http://dx.doi.org/10.2312/udmv.20151341
In many applications, such as in urban physical simulations or in the study of the effect of the solar impact at different scales, models with different levels of detail are required. In this paper we propose an efficient system for quickly computing the Sky View Factor (SVF) for any point inside a large city. To do that, we embed the city into a regular grid, and for each cell we select a subset of the geometry consisting of a square area centered on the cell and including it. Then, we remove the selected geometry from the city model and we project the rest onto a panoramic image (in our case, the sides of a box). Later, when several SVF evaluations are required, we only need to determine the cell that the evaluation point belongs to, and compute the SVF with the cell's geometry plus the environment map. To test our system, we perform several evaluations inside a cell's area, and compare the results with the ground truth SVF evaluation. Our results show the feasibility of the method and its advantages when used for a large set of computations. We show that our tool provides a way to handle the complexity of urban scale models, and specifically to study the sensitivity of the geometry.
Surinyac, Jordi; Brunet, Pere
XXV Spanish Computer Graphics Conference, pp 61-70, 2015.
DOI: http://dx.doi.org/10.2312/ceig.20151201
Nowadays, an increasing interest on tele-medicine and tele-diagnostic solutions can be observed, with client/server architectures for remote inspection of volume image-based medical data which are becoming more and more popular. The use of portable devices is gradually spreading due to their portability and easy maintenance. In this paper, we present an efficient data model for segmented volume models based on a hierarchical data structure of surfels per anatomical structure. Surfel Octrees are compact enough for transmission through networks with limited bandwidth, and provide good visual quality in the client devices at a limited footprint. Anatomy atlases are represented as octree forests, supporting local interaction in the client device and selection of groups of medical organs. After presenting the octree generation and interaction algorithms, we present several examples and discuss the interest of the proposed approach in low-end devices such as mobiles and tablets
AdaptiveCave: A new high-resolution, multi-projector VR system
Andújar, Carlos; Brunet, Pere; Díaz-García, Jesús; Vico, Miguel Angel; Vinacua, Àlvar
In Proc. of XXIV Congreso Español de Informática Gráfica (CEIG), pp 11-20, 2014.
In this paper, a novel four wall, passive stereo multi-projector CAVE architecture is presented. It is powered by 40 ¿possibly different¿ off the shelf DLP projectors controlled by 12 PCs. We have achieved high resolution while significantly reducing the overall cost, resulting on a high brigthness, 2000 x 2000 pixel resolution on each of the 4 walls. The AdaptiveCave VR System has an increased versatility both in terms of projectors and screen architecture. First, the system works with any mix of a wide range of projector models that can be substituted ¿one by one¿ at any moment, for more modern or cheaper ones. Second, the self-calibration software, which guarantees a uniform final image with concordance and continuity, can be adapted to many other wall and screen configurations. The AdaptiveCave project includes the set-up and all related software components: geometric and chromatic calibration, simultaneous rendering on 40 projected viewports, synchronization and interaction. The interaction is based on a cableless, kinect-based gesture interface with natural interaction paradigms.
Beacco, Alejandro; Pelechano, Nuria
In Proc. of XXIV Congreso Español de Informática Gráfica (CEIG), pp 1--10, 2014.
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Simulation, animation and rendering of crowds has become an important part of real-time applications such as videogames. Virtual environments achieve higher realism when being populated by virtual crowds as opposed to appearing uninhabited. There has been a large amount of research on simulation, animation and rendering of crowds, but in most cases they seem to be treated separately as if the limitations in one area did not affect the others. At the end of the day the goal is to populate environments with as many characters as possible in real time, and it is of little use if one can for instance render thousands of characters in real time, but you cannot move more than a hundred due to a simulation bottleneck. The goal of our work is to provide a framework that lets the researcher focus on each of these topics at a time (simulation, animation, or rendering) and be able to explore and push the boundaries on one topic without being strongly limited by the other related issues. This paper presents therefore a new prototyping testbed for crowds that lets the researcher focus on one of these areas of research at a time without loosing sight of the others. We offer default representations, animation and simulation controllers for real time crowd simulation, that can easily be replaced or extended. Fully configurable level-of-detail for both rendering and simulation is also available.
Hybrid ROI-based visualization of medical models
Campoalegre, Lázaro; Navazo, Isabel; Brunet, Pere
In Proc. of XXIV Congreso Español de Informática Gráfica (CEIG), pp 7, 2014.
Handling three-dimensional information during the remote visualization of medical images in tele-medicine requires efficient systems to achieve fast data transmission and interactive visualization. Client-server architectures meet these functionalities. The use of mobile devices is sometimes required due to the portability and easy maintenance, but issues like transmission time for the volumetric information and low performance hardware properties must be addressed. We present a hybrid visualization approach which is based on two transfer-function aware models. Given a user defined region of interest (ROI), the client stores a low resolution wavelet-based model of the volume data, and a ROI-dependent high resolution model based on gradient-octrees. This last model is only sent to the client when the ROI is changed. Interaction in the client is autonomous (without any data transmission) while the ROI is inspected. The results show that our hybrid approach is compact, efficient and scalable, with compression rates that decrease when the size of the volume model increases.
Robust Force Differentials for Implicit Corotational FEM.
Civit, Oscar; Susin, Antonio
Symposium on Computer Animation, pp 4, 2014.
Poster at Symposium on Computer Animation
Ferrer, J.; Peña, M.; Susin, Antonio
In Proc. of International Conference on Mathematical Models and Methoks in Applied Sciences, pp 90--92, 2014.
We continue the study of the bifurcations and the structural stability of planar bimodal linear dynamical systems (that is, systems consisting of two linear dynamics acting on each side of a straight line, assuming continuity along the separating line). Here we determine the tangency-saddle singularities in the saddle/spiral case, the only where they can appear
Ferrer, Josep; Peña, Marta; Susin, Antonio
In Proc. of 19th Conference of the International Linear Algebra Society, 2014.
It is well known that algebraic invariants lead to the topological and dierentiable typology of planar linear systems. However these systems cannot model complex dynamics such as hyperbolic orbits or homoclinic orbits. Here we show that they do appear in planar bimodal linear systems and that their typology is again determined by the algebraic invariants of both subsystems.
Solving multiple bichromatic mutual nearest neighbor queries with the GPU
Fort, Marta; Sellarès, J. Antoni
DASFAA 2014: SIM3, Database Systems for Advanced Applications, Lecture Notes in Computer Science, 2014.
DOI: http://dx.doi.org/308-316
Kappadia, Mubbasir; Pelechano, Nuria; Guy, Stephen; Allbeck, Jan; Chrysanthou, Yiorgos
Eurographics, pp 1--191, 2014.
Over the last decade there has been a large amount of work towards trying to simulate crowds for different applications, such as movies, video games, training, and evacuations. This course focuses on heterogeneous crowd simulation for interactive applications and will describe state of the art methods to simulate large groups of agents exhibiting a variety of behaviors, appearances and animations. We will present different techniques including psychological models and data-driven approaches that attempt to imitate real humans. We also present different systems to speed up both navigation, through multi-domain planners, and rendering, using per-joint impostors on fully animated 3D characters. Finally we provide quantitative and qualitative techniques to evaluate the quality of the simulated crowds, and include an overview of future research directions in the field.
Minimizing user movement with zoom in place
Monclús, Eva; Navazo, Isabel; Vázquez, Pere-Pau
In Proc. of 2nd International Workshop on Immersive Volumetric Interaction, 2014.
Although Virtual Reality environments have seen slower adoption rates than expected among the medical community, throughout the last years, several new techniques have been developed that might boost the increase of popularity of these techniques and tools. However, the use of 3D environments is not exempt of some shortcomings, such as the difficulties of adaption to 3D, that sometimes produce motion sickness, or the increase in effort the user has to develop when interacting with a 3D system, as compared to using a simple mouse on a desktop. Our proposal is tailored to focus on the second problem. More concretely, we propose a zooming technique that is able to reduce the movements required to explore a concrete part of a volumetric dataset as compared to other similar techniques. Our technique, Zoom-in-Place, builds the zooming result in the same virtual position of the initial interaction while still maintaining a contextual view on the region of interest and its surroundings. This way, the user reduces the amount of movements required to explore a model.
Rivalcobas Rivas, Jorge Ivan; De Gyves, Oriam; Rudomin, Isaac; Pelechano, Nuria
In Proc. of International Conference on Computer Graphics and Applications, 2014.
Our objective with this paper is to show how we can couple a group of real people and a simulated crowd of virtual humans. We attach group behaviors to the simulated humans to get a plausible reaction to real people. We use a two stage system: in the first stage, a group of people are segmented from a live video,then a human detector algorithm extracts the positions of the people in the video, which are finally used to feed the second stage, the simulation system. The positions obtained by this process allow the secondmodule to render the real humans as avatars in the scene, while the behavior of additional virtual humansis determined by using a simulation based on a social forces model. Developing the method required threespecific contributions: a GPU implementation of the codebook algorithm that includes an auxiliary codebook to improve the background subtraction against illumination changes; the division of the human classification task into local binary patterns; the parallelization of a social forces model, in which we solve a case of agents merging with each other. The experimental results show how a large virtual crowd reacts to over a dozen humans in a real environment.
Rivalcobas Rivas, Jorge Ivan; De Gyves, Oriam; Rudomin, Isaac; Pelechano, Nuria
In Proc. of 9th International Conference on Computer Graphics Theory and Applications: Lisbon, Portugal, pp 312--321, 2014.
DOI: http://dx.doi.org/978-989-758-002-4
Our objective with this paper is to show how we can couple a group of real people and a simulated crowd of virtual humans. We attach group behaviors to the simulated humans to get a plausible reaction to real people. We use a two stage system: in the first stage, a group of people are segmented from a live video, then a human detector algorithm extracts the positions of the people in the video, which are finally used to feed the second stage, the simulation system. The positions obtained by this process allow the second module to render the real humans as avatars in the scene, while the behavior of additional virtual humans is determined by using a simulation based on a social forces model. Developing the method required three specific contributions: a GPU implementation of the codebook algorithm that includes an auxiliary codebook to improve the background subtraction against illumination changes; the use of semantic local binary patterns as a human descriptor; the parallelization of a social forces model, in which we solve a case of agents merging with each other. The experimental results show how a large virtual crowdreacts to over a dozen humans in a real environment.
Theoktisto, Víctor; Fairén, Marta; Navazo, Isabel
XXIV Spanish Computer Graphics Conference: Zaragoza, Spain, pp 1, 2014.
A haptic rendering procedure for surface relief detail of large models relying on an image-based Hybrid Rugosity Mesostructure Atlas (HyRMA) shell is presented, allowing real-time accurate perception of meshes with millions of triangles. The procedure grows as a generalization of a haptic rendering acceleration method based on rugosity mesostructures, modified for synced visualization of relief detail [TFN13]. The method builds a mesostructure-based reconstruction atlas from depth/normal textures, computed from surface differences of the same mesh object at different resolutions: one with thousands/millions of triangles (Figure 1.a), and the other a highly decimated version with irregular prisms enclosing faces along the vertices’s normals (Figure 1.b). The depth distances and surfaces normals within each prism are warped from object volume space to orthogonal tangent space by means of a novel and fast method for computing barycentric coordinates, and then encoded in a sorted normalmap/relief atlas (Figure 1.c). While the user is visualizing a high quality mesh (Figure 1.e), the Haptic Rendering algorithm checks probe hits against the prisms (Figure 1.d), warping the probe into the tangent space of the corresponding normalmap/relief texture. An actual contact occurs if the probe is below the sampled relief height, and a haptic response is effected along the sampled normal. The main contributions of this paper are (i) a faster way to compute barycentric coordinates needed for volume warping; and (ii) an accurate correspondence between the surface detail rendering and fine features’ perception with a qualification of some loss of detail in mesostructures. The method is validated threefold: numerically, by showing no surface differences between pure geometric and HyRMA-rebuilt models; statistically, by obtaining optimal haptic/visual rendering framerates; and perceptually, in user testing controlled trials measuring accurate haptic sensation of large meshes’ fine features at interactive rendering rates.
Beacco, Alejandro; Andújar, Carlos; Pelechano, Nuria; Spanlang, Bernhard
Eurographics Symposium on Rendering, pp 1, 2013.
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We presenttwo methods for rendering thousands of animated characters in real-time. We maximize rendering performance by using a collection of pre?computed impostors sampled from a discrete set of view directions. The first method is based on relief impostors and the second one in flat impostors. Our work differs from previous approaches on view-dependent impostors in that we use per?joint rather than per character impostors. Characters are animated by applying the joint rotations directly to the impostors, instead of choosing a single impostor for the whole character from a set of predefined poses. This representation supports any arbitrary pose and thus the agent behavior is not constrained to a small collection of predefined clips. To the best of our knowledge, this is the first time a crowd rendering algorithm encompassing image-based performance, small GPU footprint and animation?independence is proposed.
Beacco, Alejandro; Pelechano, Nuria; Mubbasir Kapadia
In Proc. of Congreso Español de Informática Gráfica (CEIG), pp 147--160, 2013.
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Animating multiple interacting characters in real-time dynamic scenarios is a challenging task that requires not only positioning the root of the character, but also placing the feet in the right spatio-temporal state. Prior work either controls agents as cylinders by ignoring feet constraints, thus introducing visual artifacts, or use a small set of animations which limits the granularity of agent control. In this work we present a planner that given any set of animation clips outputs a sequence of footsteps to follow from an initial position to a goal such that it guarantees obstacle avoidance and correct spatio-temporal foot placement. We use a best-?rst search technique that dynamically repairs the output footstep trajectory based on changes in the environment. We show results of how the planner works in different dynamic scenarios with trade-offs between accuracy of the resulting paths and computational speed, which can be used to adjust the search parameters accordingly.
City-Level Level-of-Detail
Besuievsky, Gonzalo; Patow, Gustavo A.
Congreso Español de Inforática Grafica, pp 29--36, 2013.
Modeling large, detailed cities with complex buildings is now feasible with current procedural modeling techniques, which allow their use in large game and movie productions. However, this possibility of generating almost infinite amounts of detailed geometry can become a serious problem when generating a large urban model. In this paper we propose a new LoD technique that precisely selects the detail of the geometry to generate, reducing the geometric quality of those areas that accept simpler representations, according to a user-defined criteria. Our technique operates at all urban levels: at the block level, the building level, and it smoothly combines with previous asset-level efforts [BP13].
Gradient Octrees: A new scheme for remote interactive exploration of volume models
Campoalegre, Lázaro; Navazo, Isabel; Brunet, Pere
In Proc. of 13th International Conference on Computer-Aided Design and Computer Graphics(CAD/Graphics), pp 306-313, 2013.
DOI: http://dx.doi.org/10.1109/CADGraphics.2013.47
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Remote exploration of medical volume models is nowadays a challenging problem. Interactive visualization algorithms must be able to send and render in real-time the volume model at the maximum possible visual quality, while adapting to network bandwidth limitations and to hardware constraints in the client device. In this paper we present a transfer function-aware scheme for the remote interactive inspection of volume models in client-server architectures with the objectives of supporting multi-resolution, avoiding gradient computations in the client device and sending a very limited amount of information through the network. Gradient Octrees can be progressively transmitted to the clients in a strongly compact way while achieving a minimum loss of visual quality as compared to state of the art ray-casting renderings. Visual volume understanding can be complemented by showing 2D sections of the original volume data on demand.
Cage-free Spatial Deformations
Cerveró, M.Àngels; Vinacua, Àlvar; Brunet, Pere
In Proc. of VISIGRAPP - International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications, 2013., pp 111-114, 2013.
We propose a new deformation scheme for polygonal meshes through generalized barycentric coordinates that does not require any explicit cage definition. Our system infers the connectivity of the control points defined by the user and computes the coordinates using this structure. This allows the user to incrementally position the control points (or delete them) wherever he considers more suitable. This freedom gives more control, precision and locality to the deformation process.
Parallel constrained Delaunay triangulation
Coll, Narcís; Guerrieri, Marité Ethel
XV Spanish Meeting on Computational Geometry, 2013.
Statistical inverse lighting
Fernández, Eduardo; Besuievsky, Gonzalo
GRAPP - INTERNATIONAL CONFERENCE ON COMPUTER GRAPHICS THEORY AND APPLICATIONS, pp 185--191, 2013.
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Inverse lighting techniques allows to obtain the unknown light sources parameters, such as light position or flux emission, from desired lighting intentions. In this paper we present a new inverse lighting technique that uses the statistical mean and variance of the illuminated scene to obtain optimal solutions for a given lighting intention. This technique allows to explore a huge number of full radiosity solutions in a short time, reducing in this way drastically the optimization time required.
Fort, Marta; S. Bereg; J.M. Díaz-Bañez; M.A. López; P. Pérez-Lantero; J.Urrutia
XV Spanish Meeting on Computational Geometry, 2013.
Let P and F be sets of n  2 and m  2 points in the plane, respectively, so that P[F is in general position. We study the problem of finding the minimum angle in [2Pi/m,2Pi] such that one can install at each point of F a stationary rotating floodlight with illumination angle , initially oriented in a suitable direction, in such a way that, at all times, every target point of P is illuminated by at least one light. All floodlights rotate at unit speed and clockwise. We give an upper bound for the 1-dimensional problem and present results for some instances of the general problem. Specifically, we solve the problem for the case in which we have two floodlights and many points, and give an upper bound for the case in which there are many floodlights and only two target points.
Fort, Marta; Sellarès, J. Antoni
International Symposium on Voronoi Diagrams (ISVD 2013), pp 53-62, 2013.
Abstract—In this paper, we propose and solve several queries called common influence region queries. They are related to the simultaneous influence, i.e. capacity of attracting customers, of two sets of facilities of different type. In them a facility of the first type competes with the other facilities of the first type and cooperates with several facilities of the second type. The studied queries find applications, for example, in decision making support systems. We present GPU parallel algorithms, designed under CUDA architecture, for approximately solving the studied queries and provide and discuss experimental results showing the efficiency and scalability of our approach.
Fort, Marta; Sellarès, J. Antoni
XV Spanish Meeting on computational Geometry (EGC 2013), pp 15-19, 2013.
In this paper we propose and solve common inu- ence region queries. We present GPU parallel algorithms, designed under CUDA architecture, for approximately solving the studied queries. We also provide and discuss experimental results showing the efficiency of our approach.
Fort, Marta; Sellarès, J. Antoni; Valladares, Ignacio
XV Spanish Meeting on computational Geometry (EGC 2013), pp 19-23, 2013.
In this paper we study the problem of finding flock patterns in a set of trajectories of moving entities. A flock refers to a large enough subset of entities that move close to each other for a given time interval. We present a parallel approach, to be run on a Graphics Processing Unit, for reporting maximal flocks. We also provide experimetnal results that show the efficiency and scalability of oru approach.
Kappadia, Mubbasir; Beacco, Alejandro; Garcia, Francisco; Reddy, Vivek C.; Pelechano, Nuria; Badler, Norman
In Proc. of 12th ACM SIGGRAPH/Eurographics Symposium on Computer Animation (SCA), pp 115--124, 2013.
DOI: http://dx.doi.org/10.1145/2485895.2485909
This paper presents a real-time planning framework for multi character navigation that enables the use of multiple heterogeneous problem domains of differing complexities for navigation in large, complex, dynamic virtual environments. The original navigation problem is decomposed into a set of smaller problems that are distributed across planning tasks working in these different domains. An anytime dynamic planner is used to efficiently compute and repair plans for each of these tasks, while using plans in one domain to focus and accelerate searches in more complex domains. We demonstrate the benefits of our framework by solving many challenging multi-agent scenarios in complex dynamic environments requiring space-time precision and explicit coordination between interacting agents, by accounting for dynamic information at all stages of the decision-making process.
Morpho-volumetric measurement tools for abdominal distension diagnose
Monclús, Eva; Muñoz-Pandiella, Imanol; Navazo, Isabel; Vázquez, Pere-Pau; Accarino, Anna; Barba, Elisabeth; Quiroga, Sergi; Azpiroz, Fernando
In Proc. of Congreso Español de Informática Gráfica (CEIG), pp 39-48, 2013.
Digestive bloating is a very common disease, that, though tightly linked to other better known functional diseases such as functional dyspepsia and irritable intestine syndrome, often appears as isolated disfunction itself. Patients refer episodes of abdominal pressure that are difficult to explain. Through the analysis of CT captures of the patients, using a series of measuring tools developed ad-hoc, we have obtained a better comprehension of these functional digestive diseases that have lead to a proper diagnosis and treatment of such patients. In this paper we present the tools that have been developed to assist physicians in obtaining measures of different morpho-volumetric parameters of the abdominal and pulmonary structures and how these are used in the clinical practice to effectively diagnosing digestive bloating.
DAAPMed: A data-aware anchor point selection tool for medical Models in VR environments
Monclús, Eva; Vázquez, Pere-Pau; Navazo, Isabel
In Proc. of VISIGRAPP - International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications, pp 308--317, 2013.
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There is a number of problems where the analysis of medical datasets requires the selection of anchoring points in 3D space, such as the measurement of anatomical structures (i.e. lengths of bones), pathological structures (i.e. tumors), and the measurement of other elements such as the air contents in the lungs or the gut. Previous research indicates that measurement tasks can be usually carried out more efficiently in VR environments than in desktop-based systems. However, there is a lack of tools for measurement support for medical models in VR environments. This paper presents a new VR-based interaction technique, Data-Aware Anchor Points for Medical models (DAAPMed), specially focused on the efficient selection of 3D points in datasets rendered using methods with semi-transparency such as Direct Volume Rendering. We will show that our method is effective, precise, and reduces the time and amount of movements required to set the anchor points as compared with other classical techniques based on clipping planes.
Muñoz-Pandiella, Imanol; Andújar, Carlos; Patow, Gustavo A.
In Proc. of Eurographics Workshop on Urban Data Modelling and Visualisation, pp 13--16, 2013.
DOI: http://dx.doi.org/10.2312/UDMV/UDMV13/013-016
Real time rendering of cities with realistic global illumination is still an open problem. In this paper we propose a two-step algorithm to simulate the nocturnal illumination of a city. The first step computes an approximate aerial solution using simple textured quads for each street light. The second step uses photon mapping to locally compute the global illumination coming from light sources close to the viewer. Then, we transfer the local, highquality solution to the low resolution buffers used for aerial views, refining it with accurate information from the local simulation. Our approach achieves real time frame rates in commodity hardware.
Hybrid Particle Lattice Boltzmann Shallow Water for Interactive Fluid Simulations
Ojeda, Jesús; Susin, Antonio
In Proc. of VISIGRAPP - International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications, pp 217--226, 2013.
We introduce a hybrid approach for the simulation of fluids based in the Lattice Boltzmann Method for Shallow Waters and particle systems. Our modified LBM Shallow Waters can handle arbitrary underlying terrain and arbitrary fluid depth. It also introduces a novel and simplified method of tracking dry-wet regions. Dynamic rigid bodies are also included in our simulations using a two-way coupling. Certain features of the simulation that the LBM can not handle, as breaking waves, are detected and automatically turned into splash particles. Albeit we use a simple ballistic particle system, our hybrid method can handle more complex systems as SPH. Both the LBM and particle systems are implemented in CUDA, yet dynamic rigid bodies are simulated in CPU. We show the effectiveness of our method with various examples which achieve real-time on commodity hardware.
Enhanced Lattice Boltzmann Shallow Waters for real-time fluid simulations
Ojeda, Jesús; Susin, Antonio
In Proc. of Eurographics – Short Papers, pp 25-28, 2013.
DOI: http://dx.doi.org/10.2312/conf/EG2013/short/025-028
We present a novel approach at simulating fluids in real-time by coupling the Lattice Boltzmann Method for Shallow Waters with particle systems. The LBM can handle arbitrary underlying terrain and arbitrary fluid depth, which, in turn, allows us to extend it to track dry regions. The LBM is also two-way coupled with rigid bodies. The particle system adds more detail to the LBM; breaking waves are detected from the surface simulation and particles are generated to provide the effect, taking effectively certain amounts of fluid and reintegrating it back once they fall over again. Both the LBM and the particle simulation are implemented in CUDA, although rigid bodies are simulated in CPU. Finally, we show the effectiveness of the method on commodity hardware.
Oliva, Ramon; Beacco, Alejandro; Pelechano, Nuria
ACM SIGGRAPH/Eurographics Symposium on Computer Animation, pp 1, 2013.
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Character navigation in virtual environments is mostly handled by a combination of path planning with local movement algorithms. Clearance should be taken into consideration when choosing a path, but also when deciding the location of way points within portals. Previous work has considered clearance for path planning, but it is ignored when assigning way points within portals. Paths with clearance, do not necessarily guarantee that characters can walk through collision-free way points in portals. In this work we present a method for calculating clearance in navigation meshes (NavMeshes) consisting of convex cells of any type, as well as a novel method to calculate portals with clearance. We also introduce a new method to dynamically locate attractors over portals based on current trajectory, destination, and clearance.
A Generalized Exact Arbitrary Clearance Technique for Navigation Meshes
Oliva, Ramon; Pelechano, Nuria
In Proc. of ACM SIGGRAPH conference on Motion in Games, pp 103-110, 2013.
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There are two frequent artifacts in crowd simulation, the first one appears when all agents attempt to traverse the navigation mesh sharing the same way point over portals, increasing the probability of collision against other agents and lining up towards portals; the second one is caused by way points being assigned at locations where clearance is not guaranteed which causes the agents to walk too close to the static geometry, slide along walls or even get stuck. In this work we propose a novel method for dynamically calculating way points based on current trajectory, destination, and clearance while using the full length of the portal, thus guaranteeing that agents in a crowd will have different way points assigned. To guarantee collision free paths we propose two novel techniques: the first one provides the computation of paths with clearance for cells of any shape (even with concavities) and the second one presents a new method for calculating portals with clearance, so that the dynamically assigned way points will always guarantee collision free paths. We evaluate our results with a variety of scenarios, and compare our results against traditional way points at the center of portals to show that our technique offers a better use of the space by the agents, as well as a reduction in the number of collisions.
NavMeshes with Clearance for Different Character Sizes
Oliva, Ramon; Pelechano, Nuria
In Proc. of Congreso Español de Informática Gráfica (CEIG), pp 21--28, 2013.
Navigation in virtual environments for autonomous characters is typically handled by the combination of a path planning algorithm which decides the cells to walk through in the navigation mesh, and a local movement algorithm that carries out the frame to frame trajectory within each cell. Local movement is driven by intermediate goals (attractors) along the portals that connect cells in the navigation mesh. In both cases, clearance should be taken into consideration, since it is relevant when choosing the right sequence of cells that each character can walk by, and also when deciding the location of goals within portals. Previous work has considered clearance for path planning, but it has not been taken into account when assigning attractors within portals. We demonstrate in this work that although a path with clearance guarantees that the character can walk through, it does not guarantee a collision free trajectory. In this work we present three novelties: first a general method for calculating clearance in Navigation Meshes consisting of convex cells of any type which allows for a small degree of concavities, second a novel method for assigning attractors within portals that guarantee collision free paths, and third a new method to dynamically locate attractors over portals based on current trajectory, destination, and clearance.
Patow, Gustavo A.; Besuievsky, Gonzalo
Eurographics Workshop on Urban Data Modelling and Visualisation (UDMV 2013), pp 25--28, 2013.
DOI: http://dx.doi.org/10.2312/UDMV/UDMV13/025-028
The use of procedural modeling for building generation has risen dramatically over the last years, being an elegant and fast way to generate huge, complex and realistically looking urban sites. However, due to its generative nature there are still unsolved problems that limits they usage. In this paper we report on the challenges still pending on procedural modeling of buildings. We provide a state of the art on most recent solution and we draw possible research avenue that could be taken for spreading the use of procedural modeling in current applications.
Introductory Graphics for Very Diverse Audiences
Pelechano, Nuria; Fairén, Marta
In Proc. of Eurographics – Education Papers, pp 1-2, 2013.
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This paper presents our first experience teaching WebGL in a master’s degree for a class of students with very different backgrounds. The main challenge was to prepare a course that would be engaging for students with computer graphics experience, and yet interesting and non-frustrating for those students unfamiliar with OpenGL. In this paper we explain how we prepared this course, and the project assignment to achieve our goal. The results achieved by the students show that the course succeeded in keeping different kinds of students engaged and excited with the implementation of their final project.
Ramirez, Raissel; Celine Loscos; Martín, Ignacio; Alessandro Artusi
HDRi2013 - First International Conference and SME Workshop on HDR imaging, 2013.
Creating High Dynamic Range (HDR) images of static scenes is a common procedure nowadays that combines several Low Dynamic Range (LDR) images. However, HDR video and 3D content creation and management is an active, unsolved research topic. This work proposes a method to build HDR images from Low Dynamic Range (LDR) input images taken with multi-view cameras. We propose an image registration method to produce 3D HDR content for auto-stereoscopic displays. This method is based on the Patch Match algorithm which has been adapted to take advantage of epipolar geometry constraints of multi-view cameras. Different image similarity measures are used to improve the accuracy of the matching process. In our case we use an 8-view LDR camera from which we generate an 8-view HDR output. Partial results show accurate registration and HDR reconstruction for each LDR input image.
Theoktisto, Víctor; Fairén, Marta; Navazo, Isabel
In Proc. of The 8th International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications, pp 191-196, 2013.
A fast global approach that encodes haptic surface relief detail using an image-based Hybrid Rugosity Mesostructure Atlas (HyRMA) shell is presented. It is based on a depth/normal texture computed from surface differences of the same mesh object at different resolutions (a dense one with thousands/millions of triangles, and a highly decimated version). Per-face local depth differences are warped from volume space into tangent space, and stored in a sorted relief atlas. Next, the atlas is sampled by a vertex/fragment shader pair, unwarped, displacing the pixels at each face of the decimated mesh to render the original mesh detail with quite fewer triangles. We achieve accurate correspondence between visualization of surface detail and perception of its fine features without compromising rendering framerates, with some loss of detail at mesostructure “holes”.
From lighting intention to light filters
Vincent Tourré; Eduardo Fernández; Besuievsky, Gonzalo
CISBAT 2013 - INTERNATIONAL SCIENTIC CONFERENCE ON RENEWABLES IN A CHANGING CLIMATE, FROM NANO TO URBAN SCALE, pp 1181--1186, 2013.
Lighting intentions are the goals and constraints that designers would like to achieve in an illumination design process. Light lters can be used as an architectural element to obtain such intentions by inserting them into walls or roofs. De ning correctly the lter shape is a challenge. In this work, we present a novel method to design optimal lters from a given lighting intention. Our methodology, which could be completely integrated within a computer aided architectural design framework, it is based on a global illumination and optimization approach. Our test results show that optimal lter shapes can be obtained in a short time.
Interactive rendering of urban models with global illumination
Argudo, Oscar; Andújar, Carlos; Patow, Gustavo A.
In Proc. of Proceedings of Computer Graphics International, pp 1-10, 2012.
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We propose a photon mapping-based technique for the efficient rendering of urban landscapes. Unlike traditional photon mapping approaches, we accumulate the photon energy into a collection of 2D photon buffers encoding the incoming radiance for a superset of the surfaces contributing to the current image. We define an implicit parameterization to map surface points onto photon buffer locations. This is achieved through a cylindrical projection for the building blocks plus an orthogonal projection for the terrain. An adaptive scheme is used to adapt the resolution of the photon buffers to the viewing conditions. Our customized photon mapping algorithm combines multiple acceleration strategies to provide efficient rendering during walkthroughs and flythroughs with minimal temporal artifacts. To the best of our knowledge, the algorithm we present in this paper is the first one to address the problem of interactive global illumination for large urban landscapes.
Balsa, Marcos; Vázquez, Pere-Pau
International Symposium on Visual Computing , pp 708--718, 2012.
DOI: http://dx.doi.org/10.1007/978-3-642-33179-4_67
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Volume rendering has been a relevant topic in scientific visualization for the last two decades. A decade ago the exploration of reasonably big volume datasets required costly workstations due to the high processing cost of this kind of visualization. In the last years, a high end PC or laptop was enough to be able to handle medium-sized datasets thanks specially to the fast evolution of GPU hardware. New embedded CPUs that sport powerful graphics chipsets make complex 3D applications feasible in such devices. However, besides the much marketed presentations and all its hype, no real empirical data is usually available that makes comparing absolute and relative capabilities possible. In this paper we analyze current graphics hardware in most high-end Android mobile devices and perform a practical comparison of a well-known GPU-intensive task: volume rendering. We analyze different aspects by implementing three different classical algorithms and show how the current state-of-the art mobile GPUs behave in volume rendering.
Barroso, Santiago; Patow, Gustavo A.
Spanish Computer Graphics Conference, pp 57--66, 2012.
DOI: http://dx.doi.org/10.2312/LocalChapterEvents/CEIG/CEIG12/057-066
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Procedural modeling has become the accepted standard for the creation of detailed large scenes, in particular urban landscapes. With the introduction of visual languages there has been a huge leap forward in terms of usability, but there is still need of more sophisticated tools to simplify the development process. In this paper we present extensions to the visual modeling of procedural buildings, which adapt concepts from general purpose programming languages, with the objective of providing higher descriptive power. In particular, we present the concepts of visual modules, parameter linking and the possibility to seamlessly add abstract parameter templates to the designer visual toolbox. We base our demonstrations on a new visual language created for volume-based models like historic architectonic structures (aqueducts, churches, cathedrals, etc.), which cannot be modeled as 2D facades because of the intrinsic volumetric structure of these construction (e.g. vaults or arches).
Chica, Antoni; Fairén, Marta; Pelechano, Nuria
In Proc. of Proceeding Eurographics - Education Papers, pp 65-72, 2012.
Computer graphic courses are pretty established in most computer graphics degrees worldwide. Distance learning has been gaining popularity as fast internet access from home has been reaching wider areas around the world. If we combine this with the fact that the popularity of video games all over the world has made the technology required for rendering real time graphics widely available and affordable, we have the perfect environment for teaching computer graphics through distance learning universities. In this paper, we explain our experience in teaching Computer Graphics basics for the last 10 years at the distance teaching university Universitat Oberta de Catalunya (UOC). By using materials that include interactive applets which allow the student to play with basic Computer Graphics concepts, we observe that students can understand the course despite the teacher not being present in the learning process.
Cubero, Francisco; Mas, Albert; Patow, Gustavo A.
CEIG, pp 123--132, 2012.
DOI: http://dx.doi.org/10.2312/LocalChapterEvents/CEIG/CEIG12/123-132
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This paper presents a new general purpose procedural geometrical modeling system. It is focused on providing flexibility, modularity and scalability. Furthermore, it is taylored to manage huge geometric models, with millions of polygons. An out-of-core memory management system assures that any scene size can be generated during the modeling evolution. This generation is performed by a set of rules and operations on geometrical objects, organized as a directed acyclic graph.
Fernandez, Eduardo; Ezzatti, Pablo; Nesmachnow, Sergio; Besuievsky, Gonzalo
GRAPP - INTERNATIONAL CONFERENCE ON COMPUTER GRAPHICS THEORY AND APPLICATIONS, pp 260--267, 2012.
Radiosity methods are part of the global illumination techniques, which deal with the problem of generating photorealistic images in 3D scenes with Lambertian surfaces. Low-rank radiosity is a O(nk) method, where n is the number of polygons and k is the rank of the matrix used as a direct transport operator. This method allows calculating, in real-time and with infinite bounces, the illumination of a scene with static geometry and dynamic lighting. In this paper we present a new methodology for low-rank radiosity calculation based on the use of sparse matrices, which significantly reduces the memory storage required and achieves speedup improvements over the original low-rank method. Experimental analysis was performed in both traditional computers and new graphics processing unit architectures.
Biomechanical Validation of Upper-body and lower-body Joint Movements of Kinect Motion Capture Data
Fernández-Baena, A.; Susin, Antonio; Lligadas, Xavier
. In Proc. of 4th International Conference on Intelligent Networking and Collaborative Systems (INCoS), pp 656-661, 2012.
New and powerful hardware like Kinect introduces the possibility of changing biomechanics paradigm, usually based on expensive and complex equipment. Kinect is a markerless and cheap technology recently introduced from videogame industry. In this work we conduct a comparison study of the precision in the computation of joint angles between Kinect and an optical motion capture professional system. We obtain a range of disparity that guaranties enough precision for most of the clinical rehabilitation treatments prescribed nowadays for patients. This way, an easy and cheap validation of these treatments can be obtained automatically, ensuring a better quality control process for the patient’s rehabilitation.
Fort, Marta; Sellarès, J. Antoni
12th International Conference on Computational Science and Applications, pp 253-266, 2012.
In this paper we introduce an optimization problem, that arises in the competitive facility location area, which involves the maximization of the weighted area of the region where a new facility has influence. We consider a finite set of points S in a bounded polygonal region domain D subdivided into several non-negative weighted regions according to a weighted domain partition P . For each point in S we define its k-nearest/farthest neighbor influence region as the region containing all the points of D having the considered point as one of their k-nearest/farthest neighbors in S. We want to find a new point s in D whose k-influence region is maximal in terms of weighted area according to the weighted partition P . We present a GPU parallel approach, designed under CUDA architecture, for approximately solving the problem and we also provide experimental results showing the efficiency and scalability of the approach.
Fort, Marta; Sellarès, J. Antoni
28th European Workshop Computational Geometry (EuroCG 2012), pp 181-184, 2012.
Let S be a finite set of points included in a bounded polygonal domain D of the plane. The order- k near- est influence region of a point in S is the set of points of D having the given point between their k -nearest neighbors in S . Let P be a weighted partition of the domain D into polygonal regions with an associated non-negative weight per region. We want to solve the problem of finding a new point s of D maximizing the weighted area of its order- k nearest influence re- gion with respect to S ∪ { s } , which is the sum of the weighted areas of the regions obtained intersecting the order- k nearest influence region with the regions of P . We present a GPU parallel approach, designed under CUDA architecture, for approximately solving the problem and we also provide experimental results obtained with its implementation that show the effi- ciency and scalability of the approach.
He, Shuang; Besuievsky, Gonzalo; Tourré, Vincent; Patow, Gustavo A.; Moreau, Guillaume
3U3D - USAGE, USABILITY, AND UTILITY OF 3D CITY MODELS, pp 1--16, 2012.
DOI: http://dx.doi.org/10.1051/3u3d/201202006
3D City Models (3DCM) are key features into decision making of several urban related problems. Therefore 3DCM are needed by several applications, but the required level-of-detail (LoD) of the model depends on the application. Our goal is to propose a multi-scale 3DCM production and use method. Our approach consists of merging, procedural modeling, graph rewriting techniques, and a generalization technique to handle all different kinds of LoD of a 3DCM. In this way, it allows to handle various heterogeneous LoDs of a complete urban city model. We test our proposal with the 3DCM of the City of Nantes for a rendering application. Our results can also be applied to other LoDs criteria to match other 3DCM-based needs.
Hermosilla, Pedro; Brecheisen, Ralph; Vázquez, Pere-Pau; Vilanova, Anna
In Proc. of XXII Congreso Español de Informática Gráfica (CEIG), pp 31-40, 2012.
DOI: http://dx.doi.org/10.2312/LocalChapterEvents/CEIG/CEIG12/031-040
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Diffuse Tensor Imaging (DTI) is an acquisition method based on Magnetic Resonance (MR) that provides information on the white matter fiber pathways in the living human brain. Such knowledge is crucial for understanding the way different parts of the brain work and how they interact with each other. The reconstruction of fiber tracts, however, depends on a number of parameters that introduce a degree of uncertainty in the data. Together with the parameter setting, other elements such as noise, motion, partial volume effects, or image artifacts increase the uncertainty. Therefore, fiber tracking algorithms may produce misleading results. Visualizing such uncertainty is important to avoid taking wrong decisions in medical environments. In this paper we present a set of techniques that provide a better understanding on the visualization of brain fibers by means of textures, silhouettes, ambient occlusion, and animation.
Muñoz-Pandiella, Imanol; Monclús, Eva; Brunet, Pere; Conesa, Gerardo
In Proc. of XXII Congreso Español de Informática Gráfica (CEIG), pp 45--48, 2012.
DOI: http://dx.doi.org/10.2312/LocalChapterEvents/CEIG/CEIG12/045-048
The learning process in neurosurgery is a large and difficult task based on experimentation, being ventriculostomy not an exception. We have developed a virtual reality system to help training novel surgeons on this kind of operation. The system consists of the simulation of the surgery using a haptic device and a subsequent 3D visual inspection of the surgical trajectory. Our main objective was to proof that the tactile sensation produced by our system was enough realistic for the surgeons. We carried out a demonstration session in a medical workshop where all surgeons attending the workshop used the system with a very enthusiastic response about the perception experimented through the system.
Oliva, Ramon; Pelechano, Nuria
In Proc. of XXII Congreso Español de Informática Gráfica (CEIG), pp 49--56, 2012.
In this paper we present a novel, robust and efficient GPU based technique to automatically generate a Navigation Mesh for complex 3D scenes. Our method consists of two steps: firstly, starting with a 3D scene representing a complex environment of one floor with slopes, steps, and other obstacles, it automatically generates a 2D representation based on a single polygon (floor) with holes (obstacles).This step can handle degeneracies of the starting 3D scene model, such as interpenetrating geometry. Secondly, a novel method that exploits the GPU efficiency is used to automatically generate a near-optimal convex decomposition which will represent the cell and portal graph of the environment. Such convex decomposition is a 2D representation of the walkable areas of the environment with portals indicating the crossing borders. The results show that the presented technique not only is more robust than previous CPU methods, but also for the tested environments with up to 1000 vertices is five times faster.
Ramirez, Raissel; Martín, Ignacio; Celine Loscos; Vázquez, Pere-Pau
In Proc. of SPIE, 8436, Optics, Photonics and Digital Technologies for Multimedia Applications II, pp 9-16, 2012.
DOI: http://dx.doi.org/10.1117/12.922825
The limited dynamic range of digital images can be extended by composing photographs of the same scene taken with the same camera at the same view point at dierent exposure times. This is a standard procedure for static scenes but a challenging task for dynamic ones. Several methods have been presented but few recover high dynamic range within moving areas. We present a method to recover full high dynamic range (HDR) images from dynamic scenes, even in moving regions. Our method has 3 steps. Firstly, areas aected by motion are detected to generate a ghost mask. Secondly, we register dynamic objects over a reference image (the best exposed image in the input sequence). Thirdly, we combine the registered input photographs to recover HDR values in a whole image using a weighted average function. Once matching is found, the assembling step guarantees that all aligned pixels will contribute to the nal result, including dynamic content. Tests were made on more than 20 sets of sequences, with moving cars or pedestrians and dierent background. Our results show that Image Mapping Function approach detects best motion regions while Normalized Cross Correlation oers the best deal speed-accuracy for image registration. Results from our method oers better result when moving object are roughly rigid and their movement is mostly rigid. The nal composition is an HDR image with no ghosting and all dynamic content present in HDR values.
Procedural models to better compute solar flux at the neighbourhood scale
Rodriguez, Diana; Besuievsky, Gonzalo; Patow, Gustavo A.; Beckers, Benoit
Flow modelling for urban development, pp 112--122, 2012.
The aim of this paper is to define the optimal Level of Detail (LoD) of an urban 3D model for solar energy simulation at the neighbourhood scale. Procedural methods are used to build the geometry. They allow modifying easily the Level of Detail of the windows (first application) and of the roofs (second application). Simulations of direct solar irradiation and Sky View Factors are applied to the model, and the accuracy of the results is compared at different levels. The results show the good behaviour of intermediary LoDs, which should allow the handling of large urban models. Further steps of this research should conduce to establish dynamic LoD procedures, respecting the skyline and allowing an evaluation of the error. This method could be transposed to other fields of the urban physics.
Rossignac, Jarek; Luffel, Mark; Vinacua, Àlvar
In Proc. of 8th Annual Symposium on Computational Aesthetics in Graphics, pp 1-9, 2012.
DOI: http://dx.doi.org/10.2312/COMPAESTH/COMPAESTH12/001-009
Given the start positions of a group of dancers, a choreographer specifies their end positions and says: “Run!” Each dancer has the choice of his/her motion. These choices influence the perceived beauty (or grace) of the overall choreography. We report experiments with an automatic approach, SAMBA, that computes a pleasing choreography. Rossignac and Vinacua focused on affine motions, which, in the plane, correspond to choreographies for three independent dancers. They proposed the inverse of the Average Relative Acceleration (ARA) as a measure of grace and their Steady Affine Morph (SAM) as the most graceful interpolating motion. Here, we extend their approach to larger groups. We start with a discretized (uniformly time-sampled) choreography, where each dancer moves with constant speed. Each SAMBA iteration steadies the choreography by tweaking the positions of dancers at all intermediate frames towards corresponding predicted positions. The prediction for the position of dancer at a given frame is computed by using a novel combination of a distance weighted, least-squares registration between a previous and a subsequent frame and of a modified SAM interpolation. SAMBA is fully automatic, converges in a fraction of a second, and produces pleasing and interesting motions.
Valladares, Ignacio; Gudmundsson, Joachim
20th ACM SIGSPATIAL International Conference on Advances in Geographic Information Systems, pp 259-268, 2012.
DOI: http://doi.acm.org/10.1145/2424321.2424355
Given a trajectory T we study the problem of reporting all subtrajectory clusters of T. To measure similarity between curves we choose the Fréchet distance. We show how the existing sequential algorithm can be modified exploiting parallel algorithms together with the GPU computational power showing substantial speed-ups. This is to the best of our knowledge not only the first GPU implementation of a subtrajectory clustering algorithm but also the first implementation using the continuous Fréchet distance, instead of the discrete Fréchet distance.
A procedural modelling approach for automatic generation of LoD building models
Besuievsky, Gonzalo; Patow, Gustavo A.
CISBAT 2011 International Scientic Conference on Renewables in a Changing Climate, From Nano to Urban scale, pp 993--998, 2011.
In this paper we propose a procedural-based modeling system for building generation that can be automatically structured into different levels of detail (LoD). Starting from a ruleset-based model, the user can decide the level of specification to represent the model. This specification is described through a semantic combination using tags associated with the rules. As a result, we can obtain multiple representations of the same building model, each one having the appropriate accuracy for the required analysis task, in a flexible and automatic way. By giving meaning to the architectural element structures we allow the possibility to export the model to a standard format, as for example City Geography Markup Language (CityGML), appropriately designed to unify urban models at different levels of detail.
A volume approach to model repair and smoothing
Brunet, Pere; Chica, Antoni; Monclús, Eva; Navazo, Isabel; Vinacua, Àlvar
Geometric Modeling, Dagstuhl Reports, Vol. 1, Issue 5, pp 89--90, 2011.
Coll, Narcís; Guerrieri, Marité Ethel
XIV Spanish Meeting on Computational Geometry, pp 169-172, 2011.
Fort, Marta; Sellarès, J. Antoni
XIV Spanish Meeting on Computational Geometry, pp 75--78, 2011.
Multiple disk/ball range searching queries in 2d/3d Euclidean space are solved in parallel using a uniform grid under CUDA architecture. Experimental results of our implementation are presented.
Fort, Marta; Sellarès, J. Antoni; Valladares, Ignacio
27th European Workshop on Computational Geometry, pp 83-86, 2011.
The popularity of a point is a measure of how many of a set of moving objects have visited the point. The popularity map is the subdivision of the plane into regions where all points have the same popularity. In this paper we propose an algorithm to ef?ciently compute popularity maps that takes bene?t of the Graphics Processing Unit parallelism capabilities. We also present experimental results obtained with the implementation of our algorithm.
Fort, Marta; Sellarès, J. Antoni; Valladares, Ignacio
International Workshop on Spatial Spatiotemporal Data Mining (SSTDM'10) in Cooperation with IEEE , pp 233--240, 2011.
—Mobile devices provide the availability of tracking and collecting trajectories of moving objects such as vehicles, people or animals. There exists a well-known collection of patterns which can occur for a subset of trajectories. Specifically we study the so-called Popular Places, that is regions that are visited by many distinct moving objects. We propose algorithms to efficiently compute different forms of reporting Popular Places, that take benefit of the Graphics Processing Unit parallelism capabilities. We also describe how to visualize the reported solutions. Finally we present and discuss experimental results obtained with the implementation of our algorithms
Jiazhi Xia; Garcia Fernández, Ismael; Ying He; Shi-Qing Xin; Patow, Gustavo A.
Proc. of ACM SIGGRAPH Symposium on Interactive 3D Graphics and Games, pp 151--158, 2011.
DOI: http://dx.doi.org/0.1145/1944745.1944771
In this paper we propose an editable polycube mapping method that, given an arbitrary high-resolution polygonal mesh and a simple polycube representation plus optional sketched features indicating relevant correspondences between the two, provides a uniform, regular and artist-controllable quads-only mesh with a parameterized subdivision scheme. The method introduces a global parameterization, based on a divide and conquer strategy, which allows to create polycube-maps with a much smaller number of patches, and gives the user much more control over the quality of the induced subdivision surface. All this makes it a practical method for real-time rendering on modern hardware (e.g. OGL 4.1 and D3D11 tessellation hardware). By sketching these correspondence features, processing large-scale models with complex geometry and topology is now feasible. This is crucial for obtaining watertight displaced Catmull-Clark subdivision surfaces and high-quality texturing on real-time applications.
Oliva, Ramon; Pelechano, Nuria
The Fourth International Conference on Motion in Games 2011, 2011.
Most current games perform navigation in virtual environments through A* for path finding combined with a local movement algorithm. Navigation Meshes are the most popular approach to combine path finding with local movement. This paper presents a new Automatic Navigation Mesh Generator (ANavMG) that subdivides any polygon representing the environment, with or without holes, into a suboptimal number of convex cells where local movement algorithms can be applied without deadlocks. We introduce the concept of convex relaxation to further reduce the number of cells depending on the flexibility of the local movement algorithm. Finally we show results of the ANavMG and its application to a multi player game.
Procedural Modeling of Suspension Bridges
Patow, Gustavo A.
Proceedings of V Ibero-American Symposium in Computer Graphics , pp 135--140, 2011.
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In this paper we introduce a method for designing a class of engineering structures, namely suspension bridges. These bridges are ubiquitous in the industrialized countries, often appearing in known city landscapes, yet they are complex enough that hand-based modeling is tedious and time consuming. We present a method that finds the right proportions for such a structure through an optimization method that tries to distribute the tower positions while maintaining cable width to be a finite number. By simultaneously optimizing the span and sag of the cables of a bridge, we optimize the geometry and soundness of the structure. We present the details of our technique together with examples illustrating its use, including comparisons with real structures.
GPU Collision Detection in Conformal Geometric Space
Roa, Eduardo; Theoktisto, Víctor; Fairén, Marta; Navazo, Isabel
In Proc. of SIACG 2011: Iberoamerican Symposium on Computer Graphics, pp 153-156, 2011.
DOI: http://dx.doi.org/978-972-98464-6-5
We derive a conformal algebra treatment unifying all types of collisions among points, vectors, areas (defined by bivectors and trivectors) and 3D solid objects (defined by trivectors and quadvectors), based in a reformulation of collision queries from R3 to conformal R4,1 space. The algebraic formulation in this 5D space is then implemented in GPU to allow faster parallel computation queries. Results show expected orders of magnitude improvements computing collisions among known mesh models, allowing interactive rates without using optimizations and bounding volume hierarchies.
Accurate Multi-Modal Image Registration Using Compression
Vázquez, Pere-Pau; Marco, Jordi
In Proc. of IADIS Computer Graphics, Visualization, Computer Vision and Image Processing , pp 27--34, 2011.
Image registration is an important task in medicine, especially when images have been acquired by different scanner/sensor types, since they provide information on different body structures (bones, muscles, vessels...). Several techniques have been proposed in the past, and among those, Normalized Mutual Information has been proven as successful in many cases. Normalized Compression Distance has been proposed as a simple yet effective technique for image registration. It is especially suitable for the case of CT-MRI registration. However, other image modalities such as PET pose some problems and do not achieve accurate registration. In this paper we analyse and propose a valid approach for image registration using compression that works properly for different combinations of CT, MRI and PET images.
Output-Sensitive Rendering of Detailed Animated Characters for Crowd Simulation
Beacco, Alejandro; Spanlang, Bernhard; Andújar, Carlos; Pelechano, Nuria
Congreso Español de Informatica Grafica. CEIG'10, 2010.
Beacco, Alejandro; Spanlang, Bernhard; Pelechano, Nuria
The ACM SIGGRAPH / Eurographics Symposium on Computer Animation , 2010.
This poster presents an efficient Animation Planning Mediator (APM) designed to animate virtual characters in real time for crowd simulation. The APM selects the most appropriate animation clip available for each character and modifies the skeletal configuration to satisfy constraints given by the virtual environment and crowd simulation (CS) module, such as eliminating foot-sliding. Using a reduced number of animation clips, we blend within and between animation clips to increase the number of possible locomotion types and to adjust the animations to the velocity of each agent as indicated by the CS module. A key advantage of our approach is that it can be easily integrated with any existing real-time crowd simulation module working in continuous space
An Automatic modelling tool for sport buildings visualization
Bres, Albert; Besuievsky, Gonzalo; Mas, Albert; Montaner, Miquel
Congreso Español de Informática Grafica 2010, pp 187--196, 2010.
Accurate 3D models of sport buildings are required for several application involving visualization and ticketing management processes. In addition to the hard modeling task, the resulting model must also be categorized by specied zones as well as correctly indexed for all seats and blocks in order to solve any visualization query. In this work we present a geometric modeling and visualization tool for 3D automatic sport buildings generation. Our system procedurally generates the model from a set of 2D symbolic representations, providing a structured model that allows to obtain the building with dierent levels of element representations according to the user requirements. The system also provides a visualization platform that allows to navigate and inspect the resulted model through animation routes. Results show that our system is a powerful tool for fast creating and editing stadium models and may be feasible for being integrated as a helpful support for sport events organization.
Volume-preserving deformation using generalized barycentric coordinates
Cerveró, M.Àngels; Vinacua, Àlvar; Brunet, Pere
XX Congreso Español de Informática Gráfica, pp 57-66, 2010.
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The cage-based deformation of a 3D object through generalized barycentric coordinates is a simple, e fficient, effective and hence widely used shape manipulation scheme. Editing vertices of the polyhedral cage induces a smooth space deformation of its interior; the vertices thus become control handles of the final deformation. However, in some application fi elds, as medicine, constrained volume preserving deformations are required. In this paper, we present a solution that takes advantage of the potential of the deformations based on generalized barycentric coordinates while adding the constraint of keeping a volume constant. An implementation of the proposed scheme is presented and discussed. A measure of local stress of the deformed volume is also proposed.
A Framework for Triangular Mesh Generation and Modification Processes
Coll, Narcís; Guerrieri, Marité Ethel; Paradinas, Teresa; Rivara, María Cecilia; Sellarès, J. Antoni
31th Iberian-Latin-American Congress on Computational Methods in Engineering, CILAMCE, 2010.
2D Delaunay mesh generation with area/aspect-ratio constraints
Coll, Narcís; Guerrieri, Marité Ethel; Sellarès, J. Antoni
Third Chilean Workshop on Numerical Analysis of Partial Differential Equations (WONAPDE 2010), pp 77, 2010.
Cost-effective Feature Enhancement for Volume Datasets
Díaz, Jose; Marco, Jordi; Vázquez, Pere-Pau
International Workshop on Vision, Modeling and Visualization (VMV), pp 93--100, 2010.
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Volume models often show high complexity. Local details and overall shape may sometimes be difficult to perceive. Unsharp masking techniques improve the perception of those small features by increasing the local contrast. In this paper we present a simple and fast method for feature enhancement based on 3D mipmaps. In contrast to other approaches, in addition to increasing luminance on the feature details, we also darken the valleys of the volume thus increasing local contrast and making neighboring details more visible. Our approach is fast and simple, with small memory requirements thanks to the use of 3D mipmaps. We also propose a color selection strategy, based on harmonic colors, that further enhances the salient features without abrupt or uncomfortable color changes.
Depth-enhanced Maximum Intensity Projection
Díaz, Jose; Vázquez, Pere-Pau
IEEE/EG Volume Graphics, pp 93--100, 2010.
DOI: http://dx.doi.org/10.2312/VG/VG10/093-100
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The two most common methods for the visualization of volumetric data are Direct Volume Rendering (DVR) and Maximum Intensity Projection (MIP). Direct Volume Rendering is superior to MIP in providing a larger amount of properly shaded details, because it employs a more complex shading model together with the use of user-defined transfer functions. However, the generation of adequate transfer functions is a laborious and time costly task, even for expert users. As a consequence, medical doctors often use MIP because it does not require the definition of complex transfer functions and because it gives good results on contrasted images. Unfortunately, MIP does not allow to perceive depth ordering and therefore spatial context is lost. In this paper we present a new approach to MIP rendering that uses depth and simple color blending to disambiguate the ordering of internal structures, while maintaining most of the details visible through MIP. It is usually faster than DVR and only requires the transfer function used by MIP rendering.
Stochastic Tree-based Optimization for Inverse Reflector Design
Mas, Albert; Patow, Gustavo A.; Martín, Ignacio
Congreso Español en Informática Gráfica, pp 165--174, 2010.
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This paper presents a new global optimization algorithm specifically taylored for inverse reflector design. In these problems, the goal is to obtain a reflector shape that produces a light distribution as close as possible to a user provided one. The optimization is an iterative process where each step evaluates the difference between a reflector illumination and the desired one. We propose a tree-based stochastic method that drives the optimization process, using some heuristic rules, to reach a minimum below a threshold that satisfies the user-provided requirements. We show that our method reaches a solution in less steps than most other classic optimization methods, also avoiding many local minima.
Ridorsa, Remei; Patow, Gustavo A.
Congreso Español en Informática Gráfica 2010 - Valencia, pp 207--216, 2010.
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In this paper we present skylineEngine, an urban procedural modelling tool developed as a testbed for new algorithms and techniques in urban modelling. In spite of being a starting open project, it has many features only available on high-end commercial modelling systems, like pattern-based district styling de?nitions, possibility to import city maps from images or from OpenStreetMap, parameterizable models of cities and buildings, global city control through image maps (districts, landuse, height, etc.), and a user-friendly building modelling module based on shape grammars. This system also presents some novel features that make it a unique system, like a graph-based paradigm that allows the user to create content-rich cities with distinct districts, major and minor roads, blocks, lots and buildings, but also other urban elements like streets, sidewalks, parks, bridges and landmarks. Also, during its development we have developed new ways of generating urban content which increase the realism of the resulting environments.
The skylineEngine System
Ridorsa, Remei; Patow, Gustavo A.
Proceedings Congreso Español de Informática Gráfica, pp 207--216, 2010.
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In this paper we present skylineEngine , an urban procedural modelling tool developed as a testbed for new algorithms and techniques in urban modelling. In spite of being a starting open project, it has many features only available on high-end commercial modelling systems, like pattern-based district styling denitions, possibility to import city maps from images or from OpenStreetMap les, parameterizable models of cities and buildings, global city control through image maps (districts, land- use, height, etc.), and a user-friendly building modelling module based on shape grammars. This system also presents some novel features that make it a unique system, like a graph-based paradigm that allows the user to create content-rich cities with distinct districts, major and minor roads, blocks, lots and buildings, but also other urban elements like streets, sidewalks, parks, bridges and landmarks. Also, during its development we have developed new ways of generating urban content which increase the realism of the resulting environments.
Argelaguet, Ferran; Andújar, Carlos
Proceedings of the 16th ACM Symposium on Virtual Reality Software and Technology, pp 163--170, 2009.
DOI: http://dx.doi.org/10.1145/1643928.1643966
The act of pointing to graphical elements is one of the fundamental tasks in Human-Computer Interaction. In this paper we analyze visual feedback techniques for accurate pointing on stereoscopic displays. Virtual feedback techniques must provide precise information about the pointing tool and its spatial relationship with potential targets. We show both analytically and empirically that current approaches provide poor feedback on stereoscopic displays, resulting in low user performance when accurate pointing is required. We propose a new feedback technique following a camera viewfinder metaphor. The key idea is to locally flatten the scene objects around the pointing direction to facilitate their selection. We present the results of a user study comparing cursor-based and ray-based visual feedback techniques with our approach. Our user studies indicate that our viewfinder metaphor clearly outperforms competing techniques in terms of user performance and binocular fusion.
A Daylight simulation method for inverse opening design in buildings
Besuievsky, Gonzalo; Tourré, Vincent
SIACG2009 - IV Iberoamerican Symposium in Computer Graphics, pp 39--46, 2009.
The daylight management has a crucial impact on opening design in buildings. Lighting visualization tools are currently used for simulation prediction for a given geometric model as an aid for this task. An alternative and more efficient approach is the use of an inverse model where the opening shape can be obtained from some given light intention input. In this paper we present a daylight simulation method for an inverse opening design model in buildings. Our approach considers openings composed as a set of small elements and computes, using hardware graphics, directional incoming light from the sky accurately. At each opening element a spatial directional representation is stored. These representation is converted into anisotropic light sources that are used to evaluate their importance for a given indoors light intention. Finally, geometric shapes of the openings are derived. The main contribution of our method is the treatment of generic models containing outdoors and indoors specifications with occlusion, an imposed restriction in previous approaches. Moreover, the outdoor simulation method is based on hardware-accelerated parallel projection procedures that computes fast and accurately the visibility of each opening element, improving in this way time processing and precision. Results shows the use of our method as a helpful tool in the early stages of architectural design.
GPU-based Computation of Distance Functions on Road Networks with Applications
Díez, Yago; Fort, Marta; Sellarès, J. Antoni
Proc. 24th ACM Symposium on Applied Computing, pp 1320--1324, 2009.
We present a GPU-based algorithm for computing discretized distance functions on road networks. As applications, we provide algorithms for computing discrete Order-k Network Voronoi diagrams and for approximately solving k-Nearest Neighbor queries and Aggregate k-Nearest Neighbor queries on road networks. Finally, we present experimental results obtained with the implementation of our algorithms.
Solving Reverse k-Nearest Queries on Road Networks with the GPU
Díez, Yago; Fort, Marta; Sellarès, J. Antoni
Actas XIII Encuentros de Geometría Computacional, pp 117--124, 2009.
We present a GPU-based approach for computing discretized distance functions on road net- works. As applications, we provide algorithms for computing discrete Order-k Nearest Neighbor diagrams and for approximately solving (Bichromatic) Reverse k-Nearest Neighbor queries on road networks. Finally, we present experimental results obtained with the implementation of our algorithms that demonstrate the e®ectiveness and e±ciency of our approach.
Reporting flock patterns via GPU
Fort, Marta; Sellarès, J. Antoni; Valladares, Ignacio
Proc. 19th Annual Fall Workshop on Computational Geometry, pp 14--15, 2009.
Forés, Adrià; Pattanaik, Sumanta N.; Bosch, Carles; Pueyo, Xavier
XIX Congreso Español de Informática Gráfica (CEIG), 2009.
DOI: http://dx.doi.org/
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This paper introduces a novel system for interactive modeling and designing of arbitrary BRDFs. The system is able to deal with BRDFs de?ned in a variety of forms, such as analytical models, measured data or data obtained by simulation. The system also allows designing BRDFs from scratch using a combination of different analytical lobes. Using the programmable graphics hardware, it then performs interactive display of the designed BRDF, and its rendering on objects lit by complex illumination. The system also allows the fitting of an input BRDF de?ned in any form to our analytical lobe combination, so that it can be efficiently evaluated with GPU based rendering. The idea behind this work is to make available a general system for designing, fitting and rendering BRDFs, that is intuitive and interactive in nature. We plan to use this as a tool for simulation and modeling of complex physically-based BRDFs, and thus provide access to a larger variety of material models to the rendering community.
Accurate Water Simulation for Visibility Driving
Marcó, Jordi; Patow, Gustavo A.; Reinhard, Erik; Pueyo, Xavier
SIACG Iberoamerican Simposium in Computer Graphics, pp 1--8, 2009.
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In this paper we will present a system for the real-time simulation of rain falling on a windscreen. Our model incorporates external forces like gravity and wind, and also simulates how the rain gets removed from the windshield by the windscreen wipers. The algorithm is based on the Lattice-Boltzmann Method (LBM), which consists of a regular lattice that represents the fluid in discrete locations, and equations to simulate its flow. We perform all the computations of the LBM on graphics processors for accelerating the calculations. We render the results of the LBM simulation using an approximate image-space approach for interactive refraction, which allows the computation of refractions of a distant environment through two interfaces.
Monclús, Eva; Díaz, Jose; Vázquez, Pere-Pau; Navazo, Isabel
The 16th ACM Symposium on Virtual Reality Software and Technology, pp 119--122, 2009.
DOI: http://dx.doi.org/10.1145/1643928.1643955
In Volume Rendering, it is difficult to simultaneously visualize interior and exterior structures. Several approaches have been developed to solve this problem, such as cut-away or exploded views. Nevertheless, in most cases, those algorithms usually require either a preprocess of the data, or an accurate determination of the region of interest, previous to data inspection. In this paper we present the Virtual Magic Lantern (VML), an interaction tool tailored to facilitate volumetric data inspection. It behaves like a lantern whose virtual illumination cone provides the focal region which is visualized using a secondary transfer function or different rendering style. This may be used for simple visual inspection, surgery planning, or injure diagnosis. The VML is a particularly friendly and intuitive interaction tool suitable for an immersive Virtual Reality setup with a large screen, where the user moves a Wanda device, like a lantern pointing to the model. We show that this inspection metaphor can be efficiently and easily adapted to a GPU ray casting volume visualization algorithm. We also present the Virtual Magic Window (VMW) metaphor as an efficient collateral implementation of the VML, that can be seen as a restricted case where the lantern illuminates following the viewing direction, through a virtual window created as the intersection of the virtual lantern (guided by the Wanda device) and the bounding box of the volume.
A Framework for Rendering, Simulation and Animation of Crowds
Pelechano, Nuria; Spanlang, Bernhard; Beacco, Alejandro
Congreso Español de Informatica Grafica. CEIG'09, 2009.
A hybrid rugosity mesostructure (HRM) for faster rendering of fine haptic detail
Theoktisto, Víctor; Fairén, Marta; Navazo, Isabel
XXXV Latin American Informatics Conference, 2009.
We propose a faster method for surface haptic rendering using image-based Hybrid Rugosity Mesostructures (HRMs), paired maps with per-face height eld displacements and normalmaps, which are layered on top of a much decimated mesh. The haptic probe's force response algorithm is modulated using the blended HRM coat to render surface features at much lower costs. The proposed method solves typical problems at edge crossings, concave foldings and texture transitions. To prove the wellness of the approach, a usability testbed framework was built to measure and compare experimental results of haptic rendering approaches. Trial results of user testing evaluations show the goodness of the proposed HRM technique, rendering accurate 3D surface detail at high sampling rates, deriving useful modeling and perception thresholds for this technique.
Trueba, Ramón; Andújar, Carlos; Argelaguet, Ferran
Proceedings of the 15th Joint virtual reality Eurographics conference on Virtual Environments, pp 93--100, 2009.
DOI: http://dx.doi.org/10.2312/EGVE/JVRC09/093-100
The World in Miniature Metaphor (WIM) allows users to select, manipulate and navigate efficiently in virtual environments. In addition to the first-person perspective offered by typical VR applications, the WIM offers a second dynamic viewpoint through a hand-held miniature copy of the environment. In this paper we explore different strategies to allow the user to interact with the miniature replica at multiple levels of scale. Unlike competing approaches, we support complex indoor environments by explicitly handling occlusion. We discuss algorithms for selecting the part of the scene to be included in the replica, and for providing a clear view of the region of interest. Key elements of our approach include an algorithm to recompute the active region from a subdivision of the scene into cells, and a view-dependent algorithm to cull-away occluding geometry through a small set of slicing planes roughly oriented along the main occluding surfaces. We present the results of a user-study showing that our technique clearly outperforms competing approaches on spatial tasks performed in densely-occluded scenes.
Portalada: A Virtual Reconstruction of the Entrance of the Ripoll Monastery
Besora, Isaac; Brunet, Pere; Callieri, Marco; Chica, Antoni; Corsini, Massimilliano; Dellepiane, Matteo; Morales, Daniel; Moyés, Jordi; Ranzuglia, Guido; Scopigno, Roberto
Proceedings of 3DPVT08: Fourth International Symposium on 3D Data Processing, Visualization and Transmission, pp 89--96, 2008.
The dichotomy between detail representation and data management is still a big issue in the context of the acquisition and visualization of 3D objects, especially in the field of Cultural Heritage. New technologies give the possibility to acquire very detailed geometry, but very often it’s very hard to process the amount of data produced. In this paper we present a project which aimed at virtually reconstructing the impressive (7x11 m.) portal of the Ripoll Monastery, Spain. The monument was acquired using triangulation laser scanning technology, producing a dataset of more than 2000 range maps for a total of more than 1 billion triangles. All the steps of the entire project are described, from the acquisition planning to the final setup for the dissemination to the public. In particular, we show how timeof- flight laser scanning data can be used to obtain a speed up in the alignment process, and how, after model creation and imperfections repairing, an interactive and immersive setup gives the public the possibility to navigate and visualize the high detail representation of the portal. This paper shows that, after careful planning and with the aim of new algorithms, it’s now possible to preserve and visualize the highly detailed information provided by triangulation laser scanning also for very large surfaces.
Real-Time Exploration of the Virtual Reconstruction of the Entrance of the Ripoll Monastery
Besora, Isaac; Brunet, Pere; Chica, Antoni; Morales, Daniel; Moyés, Jordi
Proceedings of CEIG 2008, pp 219--224, 2008.
This paper presents the project of the virtual reconstruction and inspection of the "Portalada", the entrance of the Ripoll Monastery. In a first step, the monument of 7 x 11 meters was acquired using triangulation laser scanning technology, producing a dataset of more than 2000 range maps for a total of more than one billion triangles. After alignment and registration, a nearly complete digital model with 173M triangles and a sampling density of the order of one millimeter was produced and repaired. The paper describes the model acquisition and construction, the use of specific scalable algorithms for model repair and simplification, and then focuses on the design of a hierarchical data structure for data managing and view-dependent navigation of this huge dataset on a PC. Finally, the paper describes the setup for a usable, user-friendly and immersive system that induces a presence perception in the visitors.
A Hardware Accelerated Adaptive Mesh Subdivision for Interactive Daylight Illumination
Bosch, Carles; Besuievsky, Gonzalo; Martín, Ignacio
Computer Graphics International, pp 256--263, 2008.
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Daylight simulation for generic environments involves visibility computations from an infinite area light source (the sky). In order to obtain accurate results, this task tends to be very time consuming and also requires a good model discretization that is not automatically done in general. This paper presents a daylight simulation method for interactive direct lighting visualization that performs adaptive mesh subdivision. In a first step, the visibility of the scene is computed from the sky by means of hardware parallel projections and occlusion queries. Visibility values at receiver surfaces serve to guide the subdivision of the scene using an iterative process, resulting in a final adaptive mesh where each element stores visibility from a set of hemispherical directions. Lighting visualization is then performed at interactive frame rates for any day-time condition, taking advantage of the precomputed visibility. The main contribution of our method is that it provides a fast automatic adaptive mesh operation over the model according to visibility values. In this way, no pre-meshed models are required, making the method well suited for generic environments such as indoors or outdoors of buildings and urban models. We show that our mesh subdivision method is also useful as an optimal input model for other lighting visualization techniques like Precomputed Radiance Transfer or a global illumination radiosity solver.
Visibility-based feature extraction from discrete models
Chica, Antoni
Symposium on Solid and Physical Modeling, pp 347--352, 2008.
DOI: http://dx.doi.org/10.1145/1364901.1364951
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In this paper, we present a new visibility-based feature extraction algorithm from discrete models as dense point clouds resulting from laser scans. Based on the observation that one can characterize local properties of the surface by what can be seen by an imaginary creature on the surface, we propose algorithms that extract features using an intermediate representation of the model as a discrete volume for computational ef?ciency. We describe an ef?cient algorithm for computing the visibility map among voxels, based on the properties of a discrete erosion. The visibility information obtained in this ?rst step is then used to extract the model components (faces, edges and vertices) —which may be curved— and to compute the topological connectivity graph in a very ef?cient and robust way. The results are discussed through several examples.
Vicinity Occlusion Maps: Enhanced Depth Perception of Volumetric Models
Díaz, Jose; Yela, Héctor; Vázquez, Pere-Pau
Computer Graphics International (CGI), pp 56--63, 2008.
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Volume models often show high depth complexity. This poses difficulties to the observer in judging the spatial relationships accurately. Illustrators usually use certain techniques such as halos or edge darkening in order to enhance depth perception of certain structures. Halos may be dark or light, and even colored. Halo construction on a volumetric basis impacts rendering performance due to the complexity of the construction process. In this paper we present Vicinity Occlusion Maps: a simple and fast method to compute the light occlusion due to neighboring voxels. Vicinity Occlusion Maps may be used to generate flexible halos around objects or selected structures in order to enhance depth perception or accentuate the presence of some structures in volumetric models at a low cost. The user may freely select the structure that requires the halos to be generated, its color and size, and our proposed application generates those in real time. They may also be used to perform vicinity shading in realtime, or even to combine both effects.
MTCut: GPU-based Marching Tetra Cuts
Monclús, Eva; Navazo, Isabel; Vázquez, Pere-Pau
Theory and Practice of Computer Graphics Conference, pp 37--44, 2007.
Isosurface construction and rendering based on tetrahedral grids has shown to be feasible on programmable graphics hardware. In this paper we present MTCut: a volume cutting algorithm that is able to cut isosurfaces obtained by a Marching Tetrahedra algorithm on volume data. It does not require a tetrahedal representation and runs in real time for complex meshes of up to 1.8M triangles. Our algorithm takes as input the isosurface to be cut, slices it, and produces the cut geometry in response to the user interaction with a haptic device. The result is a watertight manifold that can be interactively recovered back to CPU in response to a user request.