A Unified Hierarchical Algorithm for Global Illumination with Scattering Volumes and Object Clusters

This paper presents a new radiosity algorithm that allows the simultaneous computation of energy exchanges between surface elements, scattering volume distributions, and groups of surfaces, or object clusters. The new technique is based on a hierarchical formulation of the zonal method, and efficiently integrates volumes and surfaces. In particular no initial linking stage is needed, even for inhomogeneous volumes, thanks to the construction of a global spatial hierarchy. An analogy between object clusters and scattering volumes results in a powerful clustering radiosity algorithm, with no initial linking between surfaces and fast computation of average visibility information through a cluster. We show that the accurate distribution of the energy emitted or received at the cluster level can produce even better results than isotropic clustering at a marginal cost. The resulting algorithm is fast and, more importantly, truly progressive as it allows the quick calculation of approximate solutions with a smooth convergence towards very accurate simulations

Lightweight Face Relighting

In this paper we present a method to relight human faces in real time, using consumer-grade graphics cards even with limited 3D capabilities. We show how to render faces using a combination of a simple, hardware-accelerated parametric model simulating skin shading and a detail texture map, and provide robust procedures to estimate all the necessary parameters for a given face. Our model strikes a balance between the difficulty of realistic face rendering (given the very specific reflectance properties of skin) and the goal of real-time rendering with limited hardware capabilities. This is accomplished by automatically generating an optimal set of parameters for a simple rendering model. We offer a discussion of the issues in face rendering to discern the pros and cons of various rendering models and to generalize our approach to most of the current hardware constraints. We provide results demonstrating the usability of our approach and the improvements we introduce both in the performance and in the visual quality of the resulting faces

Street Generation for City Modelling

International audienceIn this paper, we present a complete solution for automatically retrieving the street graph of an urban model. Given a set of 2-5D polygons representing the buildings footprints and their heights, the algorithm constructs a graph that represent the street network (a node for a crossing, an edge a street, each associated with a set of surrounding buildings) along with geometric information such as the width of the streets. We demonstrate how this graph can be used to analyze the city structure and give an example of its use with an automatic geometric modeler for city streets

Fast Calculation of Soft Shadow Textures Using Convolution

International audienceThe calculation of detailed shadows remains one of the most difficult challenges in computer graphics, especially in the case of extended (linear or area) light sources. This paper introduces a new tool for the calculation of shadows cast by extended light sources. Exact shadows are computed in some constrained configurations by using a convolution technique, yielding a fast and accurate solution. Approximate shadows can be computed for general configurations by applying the convolution to a representative “ideal” configuration. We analyze the various sources of approximation in the process and derive a hierarchical, error driven algorithm for fast shadow calculation in arbitrary configurations using a hierarchy of object clusters. The convolution is performed on images rendered in an offscreen buffer and produces a shadow map used as a texture to modulate the unoccluded illumination. Light sources can have any 3D shape as well as arbitrary emission characteristics, while shadow maps can be applied to groups of objects at once. The method can be employed in a hierarchical radiosity system, or directly as a shadowing technique. We demonstrate results for various scenes, showing that soft shadows can be generated at interactive rates for dynamics environments

Robust acquisition of 3D informations from short image sequences

International audienceThis paper addresses the problem of 3D reconstruction from a set of viewpoints on a short baseline. Its main contribution is the development of a robust algorithm which can extract 3D informations from a set of images taken with a very small baseline, even in the presence of significant occlusion. To achieve this goal, we use a multi-pass process that works layer by layer. In each pass we begin with a "space carving" step which is made robust through some morphological operations. Then we introduce an original technique to solve the ambiguity inherent to "space carving" in the case of short baseline. Once we have a voxel representation of the objects, we propose a method based on differential geometry to build a smooth mesh. Results are presented, demonstrating the efficiency and usefulness of the method

Accurate Computation of the Radiosity Gradient with Constant and Linear Emitters

International audienceControlling the error incurred in a radiosity calculation is one of the most challenging issues remaining in global illumination research. In this paper, we propose a new method to compute the value and the gradient of the radiosity function at any point of a receiver, with arbitrary precision. The knowledge of the gradient provides fundamental informations on the radiosity function and its behaviour. It can specially be used to control the consistency of the discretisation assumptions

Modélisation de cheveux à partir d'images par étude des variations d'apparence en fonction des conditions d'éclairage

National audienceLes applications graphiques nécessitant l'intégration de maquettes virtuelles d'individus se sont multipliées au cours de ces dernières années. Les jeux vidéos ou la téléconférence en sont les principaux exemples. Les cheveux jouant un rôle significatif dans l'identification d'une personne, il est primordial de les représenter avec le plus de fidélité possible. Les techniques de numérisation tridimensionnelle usuelles ´échouent face à l'extrême complexité géométrique des cheveux, qui sont l'enchevêtrement de dizaines de milliers d'éléments quasi invisibles à l'oeil nu. Cet article présente une méthode qui permet d'extraire la géométrie des mêches de cheveux `à partir de photographies par analyse de réflectance. Le sujet est observé selon un point de vue fixe et est éclairé par une source lumineuse mobile. Chaque mêche de cheveux réfléchissant la lumière selon une direction dépendant de son orientation, l'idée consiste à retrouver celle-ci `à l'aide d'un modèle de réflectance de cheveu et `à partir des cartes de réflectances observées. Après un bref ´état de l'art, nous décrirons dans la section 3 l'approche choisie dans son ensemble. Puis, dans la partie 4, nous donnerons une mise en oeuvre possible pour celle-ci. Enfin, dans les sections 5 et 6 nous présenterons les résultats ainsi que la conclusion et les perspectives

A General Two-Pass Method Integrating Specular and Diffuse Reflection

International audienceWe analyse some recent approaches to the global illumination problem by introducing the corresponding reflection operators, and we demonstrate the advantages of a two-pass method. A generalization of the system introduced by Wallace et al. at Siggraph '87 to integrate diffuse as well as specular effects is presented. It is based on the calculation of extended form-factors, which allows arbitrary geometries to be used in the scene description, as well as refraction effects. We also present a new sampling method for the calculation of form-factors, which is an Mternative to the hemi-cube technique introduced by Cohen and Greenberg for radiosity calculations. This method is particularly well suited to the extended form-factors calculation. The problem of interactive display of the picture being created is also addressed by using hardware-assisted projections and image composition to recreate a complete specular view of the scene

A physiological Plant Growth Simulation Engine Based on Accurate Radiant Energy Transfer

We present a new model for plant growth simulation, taking into account the eco-physiological processes driving plant development with unprecedented fidelity. The growth model, based on a physiological analysis, essentially simulates the internal function of the plant, and has been validated against measured biological data with excellent results. We show how to account for the influence of light through photosynthesis, and thereby incorporate the effects of a given plant's immediate environment on its architecture, shape and size. Since biological matter is controlled by water transpiration and received radiant enery, the model requires efficient and accurate simulation of radiant energy exchanges. We describe a complete lighting simulation system tailored for the difficult case of plants, by adapting state-of-the-art techniques such as hierarchical instanciation for radiosity and general BRDF modeling. Our results show that (a) our lighting simulation system efficiently provides the required information at the desired level of accuracy, and (b) the plant growth model is extremely well calibrated against real plants and (c) the combined system can simulate many interesting growth situations with direct feedback from the environment on the plant's characteristics. Applications range from landscape simulation to agronomical and agricultural studies, and to the design of virtual plants responding to their environment

Simplification et abstraction de dessins au trait

National audienceDans cet article, nous proposons une méthode pour simplifier un ensemble de lignes vectorielles tout en conservant la structure du dessin de départ et en incorporant des choix de style de la part de l'utilisateur. Cette approche a pour avantage d'être assez modulaire pour s'adapter à de nombreuses applications : de l'édition interactive de tracés à la génération de niveaux de détail pour le rendu non-photoréaliste, en passant par la gestion de la densité de dessins. Nous présentons ainsi un cadre commun à l'ensemble de ces méthodes et illustrons son potentiel par le biais de deux applications : un outil de tracé progressif et une méthode de mise à l'échelle d'un dessin