Vertex-Tracing - Interaktives Ray-Tracing durch adaptiv progressives Refinement im Objektraum

Abstract This dissertation presents an approach for interactive, physically exact simulation of specular reflections and specular refractions in virtual environments. The introduced approach is called Vertex Tracing and allows a hybrid rendering to add global illumination effects in traditional hardware rendering systems. The core of the Vertex Tracing is an adaptive progressive ray tracing. In contrast to standard ray tracing we use image coherence to compute only pixels (samples) that are essential for the final image reconstruction. The step by step adaption towards the final image is performed by geometry refinement of chosen polyhedra. These are scene objects with visual characteristics as specular reflections or refractions and should be handled particularly for Vertex Tracing. The object vertices are the starting point of computation. First, primary rays are shot from the eye point to the object vertices and after that, like in classical ray tracing approaches, a recursive ray shooting is performed from each vertex. If needed new vertices are inserted and consequently a step by step refinement of the object geometry is done. The reconstruction of the final image is performed by bilinear interpolation via graphics hardware. Beside the possibility of a combined rendering with OpenGL-objects, the use of graphics hardware additionally allows an efficient handling of textures. In this context, we introduce a defered texture lookup to prevent a costly sampling of high frequent textures. In addition, this thesis considers aspects of a distributed and parallel computation to speed up Vertex Tracing. In detail we implemented a distributed Vertex Tracing for a heterogenous network as well as a parallel approach for shared memory machines. Despite the adaptive progressive characteristic of the Vertex Tracing both techniques show that a significant speed-up can be achieved.Die vorliegende Dissertation beschreibt ein Verfahren zur interaktiv physikalisch exakten Simulation spekularer Reflexionen sowie spekularer Brechungen in virtuellen Umgebungen. Unter dem Begriff Vertex-Tracing wird in dieser Arbeit ein Ansatz vorgestellt, der es durch hybrides Rendering erlaubt, traditionelles Hardware-Rendering mit globalen Beleuchtungsphänomenen zu ergänzen. Kern des Verfahrens Vertex-Tracing bildet ein adaptiv progressives Ray-Tracing. Im Gegensatz zum Standard-Ray-Tracing besteht das Ziel darin, vorhandene Bildkohärenzen auszunutzen, indem nur diejenigen Pixel (Samples) berechnet werden, die für die Rekonstruktion des Finalbildes erforderlich sind. Die schrittweise Annäherung an das gewünschte Finalbild erfolgt durch Verfeinerung (Refinement) der Geometrie ausgewählter Polyeder. Diese sind Szenenobjekte, die aufgrund ihrer visuellen Charakteristik in Form spekularer Reflexionen oder Brechungen einem Vertex-Tracing unterzogen werden sollen. Ausgangspunkt der Berechnung dieser Objekte stellen ihre Objekt-Vertices dar. Sie bilden jeweils den Aufpunkt eines geschossenen Primärstrahles vom Betrachter und sind zugleich Startpunkt für eine weitere rekursive Strahlenverfolgung im Sinne des klassischen Ray-Tracing. Je nach Bedarf erfolgt das Einfügen neuer Vertices, dass eine schrittweise Verfeinerung der Objektgeometrie nach sich zieht. Die Rekonstruktion des Finalbildes erfolgt durch bilineare Interpolation mit Hilfe von Graphik-Hardware. Ihre Nutzung gestattet nicht nur ein kombiniertes Rendering mit herkömmlichen OpenGL-Objekten, sondern erlaubt darüber hinaus eine effiziente Behandlung von Texturen im Vertex-Tracing. In diesem Zusammenhang wird ein verzögerter Textur-Lookup vorgestellt. Er verhindert ein vollständiges Sampling von Texturen, das vor allem bei hochfrequenten Texturen einen erheblichen Mehraufwand bedeuten würde. Im Hinblick auf die Beschleunigung des Verfahrens werden ferner Aspekte einer verteilt, parallelen Berechnung untersucht beziehungsweise umgesetzt. Im Vordergrund steht dabei die Verteilung des Vertex-Tracings im Rechner-Cluster sowie eine Parallelisierung des Algorithmus auf Shared-Memory-Maschinen. Beide Ansätze zeigen, dass trotz des adaptiv progressiven Charakters des Verfahrens Vertex-Tracing ein signifikanter Speed-Up erzielbar ist

Towards Predictive Rendering in Virtual Reality

The strive for generating predictive images, i.e., images representing radiometrically correct renditions of reality, has been a longstanding problem in computer graphics. The exactness of such images is extremely important for Virtual Reality applications like Virtual Prototyping, where users need to make decisions impacting large investments based on the simulated images. Unfortunately, generation of predictive imagery is still an unsolved problem due to manifold reasons, especially if real-time restrictions apply. First, existing scenes used for rendering are not modeled accurately enough to create predictive images. Second, even with huge computational efforts existing rendering algorithms are not able to produce radiometrically correct images. Third, current display devices need to convert rendered images into some low-dimensional color space, which prohibits display of radiometrically correct images. Overcoming these limitations is the focus of current state-of-the-art research. This thesis also contributes to this task. First, it briefly introduces the necessary background and identifies the steps required for real-time predictive image generation. Then, existing techniques targeting these steps are presented and their limitations are pointed out. To solve some of the remaining problems, novel techniques are proposed. They cover various steps in the predictive image generation process, ranging from accurate scene modeling over efficient data representation to high-quality, real-time rendering. A special focus of this thesis lays on real-time generation of predictive images using bidirectional texture functions (BTFs), i.e., very accurate representations for spatially varying surface materials. The techniques proposed by this thesis enable efficient handling of BTFs by compressing the huge amount of data contained in this material representation, applying them to geometric surfaces using texture and BTF synthesis techniques, and rendering BTF covered objects in real-time. Further approaches proposed in this thesis target inclusion of real-time global illumination effects or more efficient rendering using novel level-of-detail representations for geometric objects. Finally, this thesis assesses the rendering quality achievable with BTF materials, indicating a significant increase in realism but also confirming the remainder of problems to be solved to achieve truly predictive image generation

Information theoretic refinement criteria for image synthesis

Aquest treball està enmarcat en el context de gràfics per computador partint de la intersecció de tres camps: rendering, teoria de la informació, i complexitat.Inicialment, el concepte de complexitat d'una escena es analitzat considerant tres perspectives des d'un punt de vista de la visibilitat geomètrica: complexitat en un punt interior, complexitat d'una animació, i complexitat d'una regió. L'enfoc principal d'aquesta tesi és l'exploració i desenvolupament de nous criteris de refinament pel problema de la il·luminació global. Mesures de la teoria de la informació basades en la entropia de Shannon i en la entropia generalitzada de Harvda-Charvát-Tsallis, conjuntament amb les f-divergències, són analitzades com a nuclis del refinement. Mostrem com ens aporten una rica varietat d'eficients i altament discriminatòries mesures que són aplicables al rendering en els seus enfocs de pixel-driven (ray-tracing) i object-space (radiositat jeràrquica).Primerament, basat en la entropia de Shannon, es defineixen un conjunt de mesures de qualitat i contrast del pixel. S'apliquen al supersampling en ray-tracing com a criteris de refinement, obtenint un algorisme nou de sampleig adaptatiu basat en entropia, amb un alt rati de qualitat versus cost. En segon lloc, basat en la entropia generalitzada de Harvda-Charvát-Tsallis, i en la informació mutua generalitzada, es defineixen tres nous criteris de refinament per la radiositat jeràrquica. En correspondencia amb tres enfocs clàssics, es presenten els oracles basats en la informació transportada, el suavitzat de la informació, i la informació mutua, amb resultats molt significatius per aquest darrer. Finalment, tres membres de la familia de les f-divergències de Csiszár's (divergències de Kullback-Leibler, chi-square, and Hellinger) son analitzats com a criteris de refinament mostrant bons resultats tant pel ray-tracing com per la radiositat jeràrquica.This work is framed within the context of computer graphics starting out from the intersection of three fields: rendering, information theory, and complexity.Initially, the concept of scene complexity is analysed considering three perspectives from a geometric visibility point of view: complexity at an interior point, complexity of an animation, and complexity of a region. The main focus of this dissertation is the exploration and development of new refinement criteria for the global illumination problem. Information-theoretic measures based on Shannon entropy and Harvda-Charvát-Tsallis generalised entropy, together with f-divergences, are analysed as kernels of refinement. We show how they give us a rich variety of efficient and highly discriminative measures which are applicable to rendering in its pixel-driven (ray-tracing) and object-space (hierarchical radiosity) approaches.Firstly, based on Shannon entropy, a set of pixel quality and pixel contrast measures are defined. They are applied to supersampling in ray-tracing as refinement criteria, obtaining a new entropy-based adaptive sampling algorithm with a high rate quality versus cost. Secondly, based on Harvda-Charvát-Tsallis generalised entropy, and generalised mutual information, three new refinement criteria are defined for hierarchical radiosity. In correspondence with three classic approaches, oracles based on transported information, information smoothness, and mutual information are presented, with very significant results for the latter. And finally, three members of the family of Csiszár's f-divergences (Kullback-Leibler, chi-square, and Hellinger divergences) are analysed as refinement criteria showing good results for both ray-tracing and hierarchical radiosity

Previewing techniques in raster graphics

The long processing times inherent in the use of raster graphics techniques for the shading of complex scenes, complicate the design and debugging of a picture. The efficiency of the picture-generating process can, however, be improved by adding several interactive facilities to the system. In particular, fast previewing techniques are valuable for checking the modeling and for getting a good impression in an early stage of the picture-definition process. A discussion of raster graphics system design is followed by examples of fast pre-viewing techniques for the shading and texture mapping of surface models and for the shading of solid models using a raycasting technique