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Item A Practical Analysis of Clustering Strategies for Hierarchical Radiosity(Blackwell Publishers Ltd and the Eurographics Association, 1999) Hasenfratz, Jean-Marc; Damez, Cyrille; Sillion, Francois; Drettakis, GeorgeThe calculation of radiant energy balance in complex scenes has been made possible by hierarchical radiosity methods based on clustering mechanisms. Although clustering offers an elegant theoretical solution by reducing the asymptotic complexity of the algorithm, its practical use raises many difficulties, and may result in image artifacts or unexpected behavior. This paper proposes a detailed analysis of the expectations placed on clustering and compares the relative merits of existing, as well as newly introduced, clustering algorithms. This comparison starts from the precise definition of various clustering strategies based on a taxonomy of data structures and construction algorithms, and proceeds to an experimental study of the clustering behavior for real-world scenes. Interestingly, we observe that for some scenes light is difficult to simulate even with clustering. Our results lead to a series of observations characterizing the adequacy of clustering methods for meeting such diverse goals as progressive solution improvement, efficient ray casting acceleration, and faithful representation of object density for approximate visibility calculations.Item Tightly-Coupled Multiprocessing for a Global Illumination Algorithm(Eurographics Association, 1990) Drettakis, George; Fiume, Eugene; Fournier, AlainA prevailing trend in computer graphics is the demand for increasingly realistic global illumination models and algorithms. Despite the fact that the computational power of uniprocessors is increasing, it is clear that much greater computational power is required to achieve satisfactory throughput. The obvious next step is to employ parallel processing. The advent of affordable, tightly-coupled multiprocessors makes such an approach widely available for the first time. We propose a tightly-coupled parallel decomposition of FIAT, a global illumination algorithm, based on space subdivision and power balancing, that we have recently developed. This algorithm is somewhat ambitious, and severely strains existing uniprocessor environments. We discuss techniques for reducing memory contention and maximising parallelism. We also present empirical data on the actual performance of our parallel solution. Since the model of parallel computation that we have employed is likely to persist for quite some time, our techniques are applicable to other algorithms based on space subdivision.Item A Practical Analysis of Clustering Strategies for Hierarchical Radiosity (Supplementary material)(Blackwell Publishers Ltd and the Eurographics Association, 1999) Hasenfratz, Jean-Marc; Damez, Cyrille; Sillion, Francois; Drettakis, GeorgeItem Filtering, Clustering and Hierarchy Construction: a New Solution for Ray-Tracing Complex Scenes(Blackwell Science Ltd and the Eurographics Association, 1995) Cazals, Frederic; Drettakis, George; Puech, ClaudeData structures that handle very complex scenes (hundreds of thousands of objects) have in the past either been laboriously built by hand, or have required the determination of unintuitive parameter values by the user. It is often the case that an incorrect choice of these parameters can result in greedy memory requirements or severely degraded performance. As a remedy to this problem we propose a new data structure which is fully automatic since it does not require the user to determine any input parameters. The structure is built by first filtering the input objects by size, subsequently applying a clustering step to objects of the same size and finally building a hierarchy of uniform grids . We then show that this data structure can be efficiently constructed. The implementation of the shows that the new structure is stable since it s memory requirements grow linearly with the size of the scene, and that it presents a satisfactory compromise between memory usage and computational efficiency. A detailed comparison with previous data structures is also presented in the results.Item Efficient Glossy Global Illumination with Interactive Viewing(Blackwell Publishers Ltd and the Eurographics Association, 2000) Stamminger, Marc; Scheel, Annette; Granier, Xavier; Perez-Cazorla, Frederic; Drettakis, George; Sillion, FrancoisThe ability to perform interactive walkthroughs of global illumination solutions including glossy effects is a challenging open problem. In this paper we overcome certain limitations of previous approaches. We first introduce a novel, memory- and compute-efficient representation of incoming illumination, in the context of a hierarchical radiance clustering algorithm. We then represent outgoing radiance with an adaptive hierarchical basis, in a manner suitable for interactive display. Using appropriate refinement and display strategies, we achieve walkthroughs of glossy solutions at interactive rates for non-trivial scenes. In addition, our implementation has been developed to be portable and easily adaptable as an extension to existing, diffuse-only, hierarchical radiosity systems. We present results of the implementation of glossy global illumination in two independent global illumination systems.Item Accurate and Consistent Reconstruction of Illumination Functions Using Structured Sampling(Blackwell Science Ltd and the Eurographics Association, 1993) Drettakis, George; Fiume, EugeneThe study of common classes of diffuse emitters, such as planar convex polygons, reveals several interesting properties of the functions of illumination these emitters cast on receiver surfaces. Some properties, such as the position of the maximum and the curvature are of particular interest for sampling and reconstruction of illumination across receivers. A computationally efficient approach is presented that identifies these properties, and uses them to select samples of illurnination. In addition these properties are used to determine upper bounds on the error due to linear and quadratic interpolants. These bounds are then used to adaptively subdivide the non-uniform sampling grid, resulting in accurate reconstruction. Results show that the method reduces the error compared to uniform approaches, and produces more consistent animated sequences.Item A Light Hierarchy for Fast Rendering of Scenes with Many Lights(Blackwell Publishers Ltd and the Eurographics Association, 1998) Paquette, Eric; Poulin, Pierre; Drettakis, GeorgeWe introduce a new data structure in the form of a light hierarchy for efficiently ray-tracing scenes with many light sources. An octree is constructed with the point light sources in a scene. Each node represents all the light sources it contains by means of a virtual light source. We determine bounds on the error committed with this approximation to shade a point, both for the cases of diffuse and specular reflections. These bounds are then used to guide a hierarchical shading algorithm. If the current level of the light hierarchy provides shading of sufficient quality, the approximation is used, thus avoiding the cost of shading for all the light sources contained below this level. Otherwise the descent into the light hierarchy continues.Our approach has been implemented for scenes without occlusion. The results show important acceleration compared to standard ray-tracing (up to 90 times faster) and an important improvement compared to Wardâ s adaptive shadow testing.Item Interactive Virtual Relighting and Remodeling of Real Scenes(The Eurographics Association, 1999) Loscos, Céline; Frasson, Marie-Claude; Drettakis, George; Walter, Bruce; Granier, Xavier; Poulin, Pierre; Dani Lischinski and Greg Ward LarsonLighting design is often tedious due to the required physical manipulation of real light sources and objects. As an alternative, we present an interactive system to virtually modify the lighting and geometry of scenes with both real and synthetic objects, including mixed real/virtual lighting and shadows. In our method, real scene geometry is first approximately reconstructed from photographs. Additional images are taken from a single viewpoint with a real light in different positions to estimate reflectance. A filtering process is used to compensate for inaccuracies, and per image reflectances are averaged to generate an approximate reflectance image for the given viewpoint, removing shadows in the process. This estimate is used to initialise a global illumination hierarchical radiosity system, representing real-world secondary illumination; the system is optimized for interactive updates. Direct illumination from lights is calculated separately using ray-casting and a table for efficient reuse of data where appropriate. Our system allows interactive modification of light emission and object positions, all with mixed real/virtual illumination effects. Real objects can also be virtually removed using texture-filling algorithms for reflectance estimation.Item Interactive High-Quality Soft Shadows in Scenes with Moving Objects(Blackwell Publishers Ltd and the Eurographics Association, 1997) Loscos, Celine; Drettakis, GeorgeInteractive rendering of soft shadows (or penumbra) in scenes with moving objects is a challenging problem. High quality walkthrough rendering of static scenes with penumbra can be achieved using pre-calculated discontinuity meshes, which provide a triangulation well adapted to penumbral boundaries, and backprojections which provide exact illumination computation at vertices very efficiently. However, recomputation of the complete mesh and back-projection structures at each frame is prohibitively expensive in environments with changing geometry. This recomputation would in any case be wasteful: only a limited part of these structures actually needs to be recalculated. We present a novel algorithm which uses spatial coherence of movement as well as the rich visibility information existing in the discontinuity mesh to avoid unnecessary recomputation after object motion. In particular we isolate all modifications required for the update of the discontinuity mesh by using an augmented spatial subdivision structure and we restrict intersections of discontinuity surfaces with the scene. In addition, we develop an algorithm which identifies visibility changes by exploiting information contained in the planar discontinuity mesh of each scene polygon, obviating the need for many expensive searches in 3D space. A full implementation of the algorithm is presented, which allows interactive updates of high-quality soft shadows for scenes of moderate complexity. The algorithm can also be directly applied to global illumination.Item Efficient Impostor Manipulation for Real-Time Visualization of Urban Scenery(Blackwell Publishers Ltd and the Eurographics Association, 1997) Sillion, Francois; Drettakis, George; Bodelet, BenoitUrban environments present unique challenges to interactive visualization systems, because of the huge complexity of the geometrical data and the widely varying visibility conditions. This paper introduces a new framework for real-time visualisation of such urban scenes. The central concept is that of a dynamic segmentation of the dataset, into a local three-dimensional model and a set of impostors used to represent distant scenery. A segmentation model is presented, based on inherent urban structure. A new impostor structure is introduced, derived from the level-of-detail approach. Impostors combine three-dimensional geometry to correctly model large depth discontinuities and parallax, and textures to rapidly display visual detail. We present the algorithms necessary for the creation of accurate and efficient three-dimensional impostors. The implementation of our algorithms allows interactive navigation in complex urban databases, as required by many applications.