7 results
Search Results
Now showing 1 - 7 of 7
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 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 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.Item Interactive Rendering using the Render Cache(The Eurographics Association, 1999) Walter, Bruce; Drettakis, George; Parker, Steven; Dani Lischinski and Greg Ward LarsonInteractive rendering requires rapid visual feedback. The render cache is a new method for achieving this when using high-quality pixel-oriented renderers such as ray tracing that are usually considered too slow for interactive use. The render cache provides visual feedback at a rate faster than the renderer can generate complete frames, at the cost of producing approximate images during camera and object motion. The method works both by caching previous results and reprojecting them to estimate the current image and by directing the renderer s sampling to more rapidly improve subsequent images. Our implementation demonstrates an interactive application working with both ray tracing and path tracing renderers in situations where they would normally be considered too expensive. Moreover we accomplish this using a software only implementation without the use of 3D graphics hardware.