Radiance interpolants for interactive scene editing and ray tracing

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (p. 189-197).by Kavita Bala.Ph.D

Spatial Decompositions for Geometric Interpolation and Efficient Rendering

Interpolation is fundamental in many applications that are based on multidimensional scalar or vector fields. In such applications, it is possible to sample points from the field, for example, through the numerical solution of some mathematical model. Because point sampling may be computationally intensive, it is desirable to store samples in a data structure and estimate the values of the field at intermediate points through interpolation. We present methods based on building dynamic spatial data structures in which the samples are computed on-demand, and adaptive strategies are used to avoid oversampling. We first show how to apply this approach to accelerate realistic rendering through ray-tracing. Ray-tracing can be formulated as a sampling and reconstruction problem, where rays in 3-space are modeled as points in a 4-dimensional parameter space. Sample rays are associated with various geometric attributes, which are then used in rendering. We collect and store a relatively sparse set of sampled rays, and use inexpensive interpolation methods to approximate the attribute values for other rays. We present two data structures: (1) the <i>ray interpolant tree (RI-tree)</i>, which is based on a kd-tree-like subdivision of space, and (2) the <i>simplex decomposition tree (SD-tree)</i>, which is based on a hierarchical regular simplicial mesh, and improves the functionality of the RI-tree by guaranteeing continuity. For compact storage as well as efficient neighbor computation in the mesh, we present a pointerless representation of the SD-tree. An essential element of this approach is the development of a location code that enables efficient access and navigation of the data structure. For this purpose we introduce a location code, called an LPTcode, that uniquely encodes the geometry of each simplex of the hierarchy. We present rules to compute the neighbors of a given simplex efficiently through the use of this code. We show how to traverse the associated tree and how to answer point location and interpolation queries. Our algorithms work in arbitrary dimensions. We also demonstrate the use of the SD-tree for rendering atmospheric effects. We present empirical evidence that our methods can produce renderings of good quality significantly faster than simple ray-tracing

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

Radiance Caching for Efficient Global Illumination Computation

In this paper we present a ray tracing based method for accelerated global illumination computation in scenes with low-frequency glossy BRDFs. The method is based on sparse sampling, caching, and interpolating radiance on glossy surfaces. In particular we extend the irradiance caching scheme of \cite{ward88ray} to cache and interpolate directional incoming radiance instead of irradiance. The incoming radiance at a point is represented by a vector of coefficients with respect to a spherical or hemispherical basis. The surfaces suitable for interpolation are selected automatically according to the glossiness of their BRDF. We also propose a novel method for computing translational radiance gradient at a point

Fast and interactive ray-based rendering

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonDespite their age, ray-based rendering methods are still a very active field of research with many challenges when it comes to interactive visualization. In this thesis, we present our work on Guided High-Quality Rendering, Foveated Ray Tracing for Head Mounted Displays and Hash-based Hierarchical Caching and Layered Filtering. Our system for Guided High-Quality Rendering allows for guiding the sampling rate of ray-based rendering methods by a user-specified Region of Interest (RoI). We propose two interaction methods for setting such an RoI when using a large display system and a desktop display, respectively. This makes it possible to compute images with a heterogeneous sample distribution across the image plane. Using such a non-uniform sample distribution, the rendering performance inside the RoI can be significantly improved in order to judge specific image features. However, a modified scheduling method is required to achieve sufficient performance. To solve this issue, we developed a scheduling method based on sparse matrix compression, which has shown significant improvements in our benchmarks. By filtering the sparsely sampled image appropriately, large brightness variations in areas outside the RoI are avoided and the overall image brightness is similar to the ground truth early in the rendering process. When using ray-based methods in a VR environment on head-mounted display de vices, it is crucial to provide sufficient frame rates in order to reduce motion sickness. This is a challenging task when moving through highly complex environments and the full image has to be rendered for each frame. With our foveated rendering sys tem, we provide a perception-based method for adjusting the sample density to the user’s gaze, measured with an eye tracker integrated into the HMD. In order to avoid disturbances through visual artifacts from low sampling rates, we introduce a reprojection-based rendering pipeline that allows for fast rendering and temporal accumulation of the sparsely placed samples. In our user study, we analyse the im pact our system has on visual quality. We then take a closer look at the recorded eye tracking data in order to determine tracking accuracy and connections between different fixation modes and perceived quality, leading to surprising insights. For previewing global illumination of a scene interactively by allowing for free scene exploration, we present a hash-based caching system. Building upon the concept of linkless octrees, which allow for constant-time queries of spatial data, our frame work is suited for rendering such previews of static scenes. Non-diffuse surfaces are supported by our hybrid reconstruction approach that allows for the visualization of view-dependent effects. In addition to our caching and reconstruction technique, we introduce a novel layered filtering framework, acting as a hybrid method between path space and image space filtering, that allows for the high-quality denoising of non-diffuse materials. Also, being designed as a framework instead of a concrete filtering method, it is possible to adapt most available denoising methods to our layered approach instead of relying only on the filtering of primary hitpoints

Doctor of Philosophy

dissertationThis dissertation explores three key facets of software algorithms for custom hardware ray tracing: primitive intersection, shading, and acceleration structure construction. For the first, primitive intersection, we show how nearly all of the existing direct three-dimensional (3D) ray-triangle intersection tests are mathematically equivalent. Based on this, a genetic algorithm can automatically tune a ray-triangle intersection test for maximum speed on a particular architecture. We also analyze the components of the intersection test to determine how much floating point precision is required and design a numerically robust intersection algorithm. Next, for shading, we deconstruct Perlin noise into its basic parts and show how these can be modified to produce a gradient noise algorithm that improves the visual appearance. This improved algorithm serves as the basis for a hardware noise unit. Lastly, we show how an existing bounding volume hierarchy can be postprocessed using tree rotations to further reduce the expected cost to traverse a ray through it. This postprocessing also serves as the basis for an efficient update algorithm for animated geometry. Together, these contributions should improve the efficiency of both software- and hardware-based ray tracers

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

Reconstruction of the gradient refractive index of the crystalline lens with optimization methods

Esta tesis estudia la óptica del sistema visual y en particular la estructura del cristalino, desarrollando métodos para estimar el gradiente de índice de refracción (GRIN) del mismo. A pesar de que hay una larga tradición en modelar las propiedades refractivas del ojo, todavía se discuten la óptica del cristalino y los valores específicos de índice de refracción. El conocimiento preciso del GRIN permitirá comprender mejor las propiedades ópticas del cristalino y la contribución del mismo a la calidad óptica del ojo. En esta tesis se propone el uso de algoritmos genéticos e imágenes de tomografía de coherencia óptica (OCT) para el estudio del GRIN en cristalinos ex vivo. Se reconstruye tridimensionalmente el gradiente de índice de un cristalino porcino, se estudia la variación con la edad del GRIN en humanos y la distorsión, y posible corrección, de la cara posterior del cristalino en las imágenes de OCT

Efficient Geometry and Illumination Representations for Interactive Protein Visualization

This dissertation explores techniques for interactive simulation and visualization for large protein datasets. My thesis is that using efficient representations for geometric and illumination data can help in developing algorithms that achieve better interactivity for visual and computational proteomics. I show this by developing new algorithms for computation and visualization for proteins. I also show that the same insights that resulted in better algorithms for visual proteomics can also be turned around and used for more efficient graphics rendering. Molecular electrostatics is important for studying the structures and interactions of proteins, and is vital in many computational biology applications, such as protein folding and rational drug design. We have developed a system to efficiently solve the non-linear Poisson-Boltzmann equation governing molecular electrostatics. Our system simultaneously improves the accuracy and the efficiency of the solution by adaptively refining the computational grid near the solute-solvent interface. In addition, we have explored the possibility of mapping the PBE solution onto GPUs. We use pre-computed accumulation of transparency with spherical-harmonics-based compression to accelerate volume rendering of molecular electrostatics. We have also designed a time- and memory-efficient algorithm for interactive visualization of large dynamic molecules. With view-dependent precision control and memory-bandwidth reduction, we have achieved real-time visualization of dynamic molecular datasets with tens of thousands of atoms. Our algorithm is linearly scalable in the size of the molecular datasets. In addition, we present a compact mathematical model to efficiently represent the six-dimensional integrals of bidirectional surface scattering reflectance distribution functions (BSSRDFs) to render scattering effects in translucent materials interactively. Our analysis first reduces the complexity and dimensionality of the problem by decomposing the reflectance field into non-scattered and subsurface-scattered reflectance fields. While the non-scattered reflectance field can be described by 4D bidirectional reflectance distribution functions (BRDFs), we show that the scattered reflectance field can also be represented by a 4D field through pre-processing the neighborhood scattering radiance transfer integrals. We use a novel reference-points scheme to compactly represent the pre-computed integrals using a hierarchical and progressive spherical harmonics representation. Our algorithm scales linearly with the number of mesh vertices