Current statistical models to simulate pandemics miss the most relevant information about the close atomic interactions between individuals which is the key aspect of virus spread. Thus, they lack a proper visualization of such interactions and their impact on virus spread. In the field of computer graphics, and more specifically in computer animation, there have been many crowd simulation models to populate virtual environments. However, the focus has typically been to simulate reasonable paths between random or semi-random locations in a map, without any possibility of analyzing specific individual behavior. We propose a crowd simulation framework to accurately simulate the interactions in a city environment at the individual level, with the purpose of recording and analyzing the spread of human diseases. By simulating the whereabouts of agents throughout the day by mimicking the actual activities of a population in their daily routines, we can accurately predict the location and duration of interactions between individuals, thus having a model that can reproduce the spread of the virus due to human-to-human contact. Our results show the potential of our framework to closely simulate the virus spread based on real agent-to-agent contacts. We believe that this could become a powerful tool for policymakers to make informed decisions in future pandemics and to better communicate the impact of such decisions to the general public.

Authoring meaningful crowds to populate a virtual city can be a cumbersome, time-consuming and an error-prone task. In this work, we present a new framework for authoring populated environments in an easier and faster way, by relying on the use of procedural techniques. Our framework consists of the procedural generation of semantically-augmented virtual cities to drive the procedural generation and simulation of crowds. The main novelty lies in the generation of agendas for each individual inhabitant (alone or as part of a family) by using a rule-based grammar that combines city semantics with the autonomous persons’ characteristics. A new population or city can be authored by editing rule files with the flexibility of reusing, combining or extending the rules of previous populations. The results show how logical and consistent sequences of whereabouts can be easily generated for a crowd providing a good starting point to bring virtual cities to life.

Following Archimedes Principle, any object immersed in a fluid is subject to an upward buoyancy force equal to the weight of the fluid displaced by the object. This simple description is the origin of a set of effects that are ubiquitous in nature, and are becoming commonplace in games, simulators and interactive animations. Although there are solutions to the fluid‐to‐solid coupling problem in some particular cases, to the best of our knowledge, comprehensive and accurate computational buoyancy models adequate in general contexts are still lacking. We propose a real‐time Graphics Processing Unit (GPU) based algorithm for realistic computation of the fluid‐to‐solid coupling problem, which is adequate for a wide generality of cases (solid or hollow objects, with permeable or leak‐proof surfaces, and with variable masses). The method incorporates the behaviour of the fluid into which the object is immersed, and decouples the computation of the physical parameters involved in the buoyancy force of the empty object from the mass of contained liquid. The dynamics of this mass of liquid are also computed, in a way such that the relation between the centre of mass of the object and the buoyancy force may vary, leading to complex, realistic beha viours such as the ones arising for instance with a sinking boat.

Research on seismic simulations has focused mainly on methodologies specially tailored to civil engineering. However, we have detected a lack in the area of interactive cultural heritage applications, where speed and plausibility are the main requirements to satisfy. We designed a tool that allows setting up and recreating earthquakes in a simple way. We coupled our earthquake simulator with a structural simulator of physics, specifically tailored to masonry buildings, achieving a high degree of accuracy in the simulations. To validate our model, we performed a series of tests over a set of ancient masonry structures such as walls and churches. We show the feasibility of including earthquake simulations and structural vulnerability, a building property that limits the damage of this under seismic movements, into historical studies for helping professionals understand those events of the past where an earthquake took place.

The visual appearance of materials depends on their intrinsic light transfer properties, the illumination and camera conditions, and other environmental factors. This is in particular the case of porous, rough, or absorbent materials, where the presence of liquid on the surface alters significantly their BRDF, which in turn results in considerable changes in their visual appearance. For this reason, rendering materials change their appearance when wet continues to be a relevant topic in computer graphics. This is especially true when real-time photo-realistic rendering is required in scenes involving this kind of materials in interaction with water or other liquids. In this paper, we introduce a physically inspired technique to model and render appearance changes of absorbent materials when their surface is wet. First, we develop a new method to solve the interaction between the liquid and the object surface using its own underlying texture coordinates. Then, we propose an algorithm to model the diffusion phenomenon that occurs in the interface between a solid porous object and a liquid. Finally, we extend a model that explains the change of appearance of materials under wet conditions, and we implement it achieving real-time performance. The complete model is developed using GPU acceleration.

Brain-Computer Interface (BCI) systems establish a channel for direct communication between the brain and the outside world without having to use the peripheral nervous system. While most BCI systems use evoked potentials and motor imagery, in the present work we present a technique that employs visual imagery. Our technique uses neural networks to classify the signals produced in visual imagery. To this end, we have used densely connected neural and convolutional networks, together with a genetic algorithm to find the best parameters for these networks. The results we obtained are a 60% success rate in the classification of four imagined objects (a tree, a dog, an airplane and a house) plus a state of relaxation, thus outperforming the state of the art in visual imagery classification.

Ancient cities and castles are ubiquitous cultural heritage structures all over Europe, and countless digital creations (e.g. movies and games) use them for storytelling. However, they got little or no attention in the computer graphics literature. This paper aims to close the gap between historical and geometrical modelling, by presenting a framework that allows the forward and inverse design of ancient city (e.g. castles and walled cities) evolution along history. The main component is an interactive loop that cycles over a number of years simulating the evolution of a city. The user can define events, such as battles, city growth, wall creations or expansions, or any other historical event. Firstly, cities (or castles) and their walls are created, and, later on, expanded to encompass civil or strategic facilities to protect. In our framework, battle simulations are used to detect weaknesses and strengthen them, evolving to accommodate to developments in offensive weaponry. We conducted both forward and inverse design tests on three different scenarios: the city of Carcassone (France), the city of Gerunda (Spain) and the Ciutadella in ancient Barcelona. All the results have been validated by historians who helped fine‐tune the different parameters involved in the simulations. Code available at: https://github.com/neich/BattleField

One important use of realistic city environments is in the video game industry. When a company works on a game whose action occurs in a real-world environment, a team of designers usually creates a simplified model of the real city. In particular, the resulting city is desired to be smaller in extent to increase playability and fun, avoiding long walks and boring neighborhoods. This is manual work, usually started from scratch, where the first step is to take the original city map as input, and from it create the street network of the final city, removing insignificant streets and bringing important places closer together in the process. This first draft of the city street network is like a kind of skeleton with the most important places connected, from which the artist can (and should) start working until the desired result is obtained. In this paper, we propose a solution to automatically generate such a first simplified street network draft. This is achieved by using the well-established seam-carving technique applied to a skeleton of the city layout, built with the important landmarks and streets of the city. The output that our process provides is a street network that reduces the city area as much as the designer wants, preserving land- marks and key streets, while keeping the relative positions between them. For this, we run a shrinking process that reduces the area in an irregular way, prioritizing the removal of areas of less importance. This way, we achieve a smaller city but retain the essence of the real-world one. To further help the designer, we also present an automatic filling algorithm that adds unimportant streets to the shrunken skeleton.

Abstract Modelling flow phenomena and their related weathering effects is often cumbersome due their dependence on the environment, materials and geometric properties of objects in the scene. Example-based modelling provides many advantages for reproducing real textures, but little effort has been devoted to reproducing and transferring complex phenomena. In order to produce realistic flow effects, it is possible to take advantage of the widespread availability of flow images on the Internet, which can be used to gather key information about the flow. In this paper, we present a technique that allows the transfer of flow phenomena between photographs, adapting the flow to the target image and giving the user flexibility and control through specifically tailored parameters. This is done through two types of control curves: a fitted theoretical curve to control the mass of deposited material, and an extended colour map for properly adapting to the target appearance. In addition, our method filters and warps the input flow in order to account for the geometric details of the target surface. This leads to a fast and intuitive approach to easily transfer phenomena between images, providing a set of simple and intuitive parameters to control the process.

Procedural modeling techniques have emerged as a fundamental tool for automatic design and reconstruction of buildings and urban landscapes. In recent years, we have witnessed an impressive increase in the expressive capabilities of such techniques, being its main strength the possibility of generating large urban scenes with a small ruleset. In this paper, we propose what we consider the next stage in this process, where generic graph-rewriting techniques are used to transform input rulesets into new ones, thus allowing the automatic reuse, transformation and generation of rulesets. We showcase our system with an application to a high-level procedural language (based on the well-known CGA grammars) for facades. We demonstrate the practicality of this new approach by transforming the style of the input facade previously created to different styles. User studies confirm this result.

Analysing the thermal behaviour of buildings is an important goal for any and all of the tasks involving energy flow simulation in urban environments. However, the number of variables to be considered, along with the difficulty of implementing some of them, make it difficult to address the problem on an urban scale. In this paper we propose a procedural approach that, from a 3D urban model and a set of parameters, simulates the thermal exchanges that take place inside and outside buildings in an urban environment. We also provide a technique to efficiently visualise thermal variations over time of both the interior and exterior of buildings in an urban environment. We believe this technique will be helpful for performing a rapid analysis when building parameters, such as materials, dimensions, shape or number of floors, are being changed.

Generalization of 2D city layouts is a relevant operation common to Computer Graphics and GIS, whose goal is to generate simplified representations of street networks. However, most of the contributions in this area belong to the GIS literature, which we intend to bring closer to the CG community. In this paper we propose a three- fold characterization of the algorithms dedicated to generic generalization and we also analyze the techniques proposed for the generation of personalized route maps in CG. We examine their data structures, simplification criteria and theoretical basis. To enable a comparative comprehension, we propose unified terminology and we refer the graphs used in the GIS literature to their name used in graph theory. From our analysis of the generalization techniques, we propose four research lines for further investigation to design new generalization algorithms, either from original ideas or by combining/extending some of the reviewed techniques.

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.

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 users 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.

This paper presents a global optimization algorithm specifically tailored for inverse reflector design problems. In such 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 the current 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 user-provided threshold that satisfies the requirements. When we are close to the solution, we resort to the Hooke and Jeeves method, to reach the minimum faster. Extending our previous work Mas et al. (2010), we show that our method reaches a solution in fewer steps than most other classic optimization methods, and also avoids many local minima. The method has been tested on a real case study based on European road lighting safety regulations.

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.

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.

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.

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.

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.

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.

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.

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.

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 and 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 and 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.

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.

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.

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.

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.

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.

A proposed rule-based editing metaphor intuitively lets artists create buildings without changing their workflow. It is 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 artists toolbox (for example, Boolean operations or local controlling operators). This approach opens new possibilities, from model verification to model editing through graph rewriting.

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.

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.

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.

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.

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.

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.

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.

We present in this paper a GPU-based strategy that allows a fast reuse of paths in the context of shooting random walk applied to radiosity. Given an environment with diffuse surfaces, we aim at computing a basis of n radiosity solutions, corresponding to n light-source positions. Thanks to the reuse, paths originated at each of the positions are used to also dis-tribute power from every other position. The visibility computations needed to make possible the reuse of paths are drastically accelerated using graphic hardware, resulting in a theoret-ical speed-up factor of n with regard to the computation of the independent solutions. Our contribution has application to the fields of interior design, animation, and videogames.

This paper proposes a technique for the design of luminaire reflector shapes from prescribed optical properties (far-field radiance distribution), geometrical constraints and users knowledge. This is an important problem in the field of Lighting Engineering, more specifically for Luminaire Design. The reflectors shape to be found is just a part of a set of pieces called in Lighting Engineering an optical set. This is composed of a light bulb (the source), the reflector and usually a glass that acts as a diffusor for the light, and protects the system from dust and other environmental phenomena. Thus, we aim at the design and development of a system capable of generating automatically a reflector shape in a way such that the optical set emits a given, user-defined, far-field radiance distribution for a known bulb. In order to do so, light propagation inside and outside the optical set must be simulated and the resulting radiance distribution compared to the desired one. Constraints on the shape imposed by industry needs and experts knowledge must be taken into account, bounding the set of possible shapes. The general approach taken is based on a minimization procedure on the space of possible reflector shapes, starting from a user-provided starting shape. The algorithm moves towards minimizing the distance, in the l2 metric, between the resulting illumination from the reflector and the prescribed, ideal optical radiance distribution specified by the user. The initial shape and a provided confidence value are used during the whole process as a boundary for the space of spanned reflectors used during the simulation.

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 paper we present a new technique which, through the use of an electrical analogy and the calculation of sky view factors and form factors, allows to simulate and study the thermal behaviour of an urban environment, taking into account the solar and sky radiation, the air and sky temperatures, and even the thermal interaction between nearby buildings. We also show that it is possible, from a 3D recreation of a large urban environment, to simulate the heat exchanges that take place between the buildings of a city and its immediate surroundings. In the same way, taking into account the terrestrial zone, the altitude and the type of climate with which the simulations are carried out, it is possible to compare the thermal behaviour of a large urban environment according to the chosen conditions.

Mechanical Design (MCAD) tools are used for creating 3D digital prototypes used in the design, visualization, and simulation of products. In this paper we present LeoMCAD, a Lego-based mechanical system designed to be used as an education tool both for kids and Lego hobbyists; but which features a novel solver that naturally and seamlessly computes the interaction between the pieces that build-up a given model, solving an otherwise complex forward kinematic system of equations in a much simpler way. The results show how our system is able to cope with situations that would produce dead-lock situations in more advanced commercial systems.

Thermal behaviour analysis on buildings is an important goal for all tasks involving energy flow simulation in urban environments. One of the most widely used simplified thermal models is based on an electrical analogy, where nodes are set to simulate and solve a circuit network. In this paper we propose a procedural approach for automatically locate the nodes of the circuit, according to the building structure. We provide a conceptual technique to efficiently visualize thermal variations over time in buildings. We show that we can simulate and visually represent the variations of the interior temperatures of a building over a period of time. We believe that the technique could be helpful for rapid analysis for changing building parameters, such as materials, dimensions or number of floors.

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.

Simulating the evolution of urban landscapes is a challenging objective with a large impact not only for Computer Graphics (for its applications in the filming and gaming industries), but also for urban planning, economical and historical studies, urban physics, and many other. However, this target has remained elusive because of the large complexity implied by urban structures and their evolutions. We present a system that aims at simulating the evolution of the commercial structure in a modern city. In particular, given an initial distribution of shops, it studies the evolution when larger commercial areas, like malls, are introduced. This is computed using the Huff model as a measure of the attraction each commerce has on potential consumers, and an agent-based simulation to determine how these aspects affect their choices. Then, after a given simulation time, the system decides whether the shop has retained an income such that it can continue operating, or has gone bankrupt. Our system is used to study the evolution of the commercial structure of Barcelona city over the last century.

Simulating the evolution of urban landscapes is a challenging objective with a large impact not only for Computer Graphics (for its applications in the filming and gaming industries), but also for urban planning, economical and historical studies, urban physics, and many other. However, this target has remained elusive because of the large complexity implied by urban structures and their evolutions. We present a system that aims at simulating the evolution of the commercial structure in a modern city. In particular, given an initial distribution of shops, it studies the evolution when larger commercial areas, like malls, are introduced. This is computed using the Huff model as a measure of the attraction each commerce has on potential consumers, and an agent-based simulation to determine how these aspects affect their choices. Then, after a given simulation time, the system decides whether the shop has retained an income such that it can continue operating, or has gone bankrupt. Our system is used to study the evolution of the commercial structure of Barcelona city over the last century. 1. Introduction Procedural urban modeling has presented us with astonishing results over the last decade, starting with the seminal work by Parish and Muller [PM01] and Muller et al. [MWH 06], and continuing with the recent advances in acquisition [MWA 12], non-regular modeling [LCOZ 11], user interfaces [Pat12], among others. However, in spite of all those improvements, several problems remain open [PBP14], one of the most important ones is simulating the evolution of urban landscapes over time. With only a few exceptions [WMWG09,BWK14], this topic has barely been touched, in spite of its crucial importance for history and archeology, urban planning, socio-economical studies, and many other social-related disciplines. Among these unexplored aspects, the problem of simulating the evolution of the commerce structure in a city is a prominent one, as it is attractive for being computationally tractable and crucial for socio-economic studies. But this study has applications that are broader than a pure social analysis, as the resulting distributions can be used to also model its appearance over time, which is interesting for computer graphics because of its applications to film and videogames, two of the leading industries in the field

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 cells geometry plus the environment map. To test our system, we perform several evaluations inside a cells 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.

One common problem of raster images when used as textures is its resolution dependence, which could produce artifacts such as blurring. On the contrary, vector graphics are resolution independent, and their direct use for real-time texture mapping would be desirable to avoid sampling artifacts. Usually, they composite images from layers of paths and strokes defined with different kinds of lines. Here I present a simple yet powerful technique for representing vector graphics as textures that organizes the graphic into a coarse grid of cells, structuring each cell into simple cell-sized BSP trees, evaluated at runtime within a pixel shader. Advantages include coherent low-bandwidth memory access and, although my implementation is limited to polygonal shapes, the ability to map general vector graphics onto arbitrary surfaces. A fast construction algorithm is presented, and the space and time efficiency of the representation are demonstrated on many practical examples.

In many applications, such as in massive urban models visualization or in the study of the impact of urban simulation at different scales, models with different levels of detail are required. In this paper we propose a flexible system for configuring level of details models using Procedural Modeling aiming to generate only the geometry required for each specific need. We test our system for a solar simulation analysis at urban scale. We evaluate the solar irradiation and the Sky View Factor in order to study the impact at different scales. 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.

In this paper we present a novel solution for the computation of diffuse global illumination in urban environments that takes advantage of the underlying structure of the procedural building models used for generating the city, using them to compute a realistic global illumination solution based on the well known hierarchical radiosity algorithm. As we generate the geometry procedurally, we take advantage of the generation hierarchy to be the base of the hierarchical radiosity algorithm, without using the classic quad-based subdivision scheme. This structure is used for low-frequency global illumination, being later combined with a shadow-map-like system for the highfrequency component, thus resulting in a complete global illumination solution for procedural urban environments.

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].

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

This paper proposes a robust algorithm to compute caustics caused by multiple reflections and refractions on the GPU. The proposed algorithm solves two problems of previous methods, caustic light leaks and caustic undersampling. In order to eliminate caustic light leaks, terminal photon hits are stored and caustic patterns are reconstructed on the faces of a layered distance map attached to the caustic generator. To avoid undersampling, we propose caustic triangles to be drawn instead of splatting photon hits. Unlike splatting, caustic triangles adapt to the local density of the uneven photon distribution, always form continuous patterns, do not cause excessive blurring, and do not require manual user intervention to set the size of these splats.

This paper presents a technique to render in real time complex trees using billboard clouds as an impostor simplification for the original polygonal tree, combined with a new texture-based representation for the foliage. The technique provides several new contributions with respect to previous approaches. The new algorithm allows progressive level of detail both at the geometric and at the shader levels. It also preserves the parallax effects of the original polygonal model keeping leaf positions, orientations, and preserving the overlapping of the leaves as seen from any view point. In addition, the texture-based representation provides high-definition close views without introducing high memory requeriments. We adapted a realistic lighting model with soft shadows and a global illumination precomputation, allowing to render highly complex scenes with thousands of trees in real time.