Volume rendering is a technique for visualizing 3D arrays of sampled data. It has applications in areas such as medical imaging and scientific visualization, but its use has been limited by its high computational expense. Early implementations of volume rendering used brute-force techniques that require on the order of 100 seconds to render typical data sets on a workstation. Algorithms with optimizations that exploit coherence in the data have reduced rendering times to the range of ten seconds but are still not fast enough for interactive visualization applications. In this thesis we present a family of volume rendering algorithms that reduces rendering times to one second. First we present a scanline-order volume rendering algorithm that exploits coherence in both the volume data and the image. We show that scanline-order algorithms are fundamentally more efficient than commonly-used ray casting algorithms because the latter must perform analytic geometry calculations (e.g. intersecting rays with axis-aligned boxes). The new scanline-order algorithm simply streams through the volume and the image in storage order. We describe variants of the algorithm for both parallel and perspective projections and

We present a new, real-time method for rendering diffuse and glossy objects in low-frequency lighting environments that captures soft shadows, interreflections, and caustics. As a preprocess, a novel global transport simulator creates functions over the object's surface representing transfer of arbitrary, low-frequency incident lighting into transferred radiance which includes global effects like shadows and interreflections from the object onto itself. At run-time, these transfer functions are applied to actual incident lighting. Dynamic, local lighting is handled by sampling it close to the object every frame; the object can also be rigidly rotated with respect to the lighting and vice versa. Lighting and transfer functions are represented using low-order spherical harmonics. This avoids aliasing and evaluates efficiently on graphics hardware by reducing the shading integral to a dot product of 9 to 25 element vectors for diffuse receivers. Glossy objects are handled using matrices rather than vectors. We further introduce functions for radiance transfer from a dynamic lighting environment through a preprocessed object to neighboring points in space. These allow soft shadows and caustics from rigidly moving objects to be cast onto arbitrary, dynamic receivers. We demonstrate real-time global lighting effects with this approach.

University of Utah, We examine a rendering system that interactively ray traces an image on a conventional multiprocessor. The implementation is “brute force ” in that it explicitly traces rays through every screen pixel, yet pays careful attention to system resources for acceleration. The design of the system is described, along with issues related to material models, lighting and shadows, and frameless rendering. The system is demonstrated for several different types of input scenes.

Figure 1: (Left) Uniform 512x512 shadow map and resulting image. (Right) The same with a perspective shadow map of the same size. Shadow maps are probably the most widely used means for the generation of shadows, despite their well known aliasing problems. In this paper we introduce perspective shadow maps, which are generated in normalized device coordinate space, i.e., after perspective transformation. This results in important reduction of shadow map aliasing with almost no overhead. We correctly treat light source transformations and show how to include all objects which cast shadows in the transformed space. Perspective shadow maps can directly replace standard shadow maps for interactive hardware accelerated rendering as well as in high-quality, offline renderers. CR Categories: I.3.3 [Computer Graphics]: Picture/Image Generation—Bitmap and framebuffer operations; I.3.7 [Computer

Shadows are essential to realistic and visually appealing images, but they are di cult to compute in most display environments. This survey will characterize the various types of shadows, describe most existing shadow algorithms, and for each one discuss their complexities, their advantages and their shortcomings. The types of shadows examined are hard shadows, soft shadows, shadows of transparent objects, and shadows for complex modeling primitives. For each type, we examine shadow algorithms within various rendering techniques. The goal of the survey is to provide readers with enough background and insight on the various methods to allow themtochoose the algorithm best suited to their needs. It is also hoped that our analysis will help identify the areas that need more research, and point to possible solutions.

The 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 dynamic environments.

This thesis presents a volumetric representation for the global illumination within a space based on the radiometric quantity irradiance. We call this representation the irradiance volume. Although irradiance is traditionally computed only for surfaces, its de nition can be naturally extended to all points and directions in space. The irradiance volume supports the reconstruction of believable approximations to the illumination in situations that overwhelm traditional global illumination algorithms. Atheoretical basis for the irradiance volume is discussed and the methods and issues involved with building the volume are described. The irradiance volume method is tested within several situations in which the use of traditional global illumination methods is impractical, and is shown to provide good performance.

We present two efficient image-based approaches for computation and display of high-quality soft shadows from area light sources. Our methods are related to shadow maps and provide the associated benefits. The computation time and memory requirements for adding soft shadows to an image depend on image size and the number of lights, not geometric scene complexity. We also show that because area light sources are localized in space, soft shadow computations are particularly well suited to image-based rendering techniques. Our first approach---layered attenuation maps--- achieves interactive rendering rates, but limits sampling flexibility, while our second method---coherence-based raytracing of depth images---is not interactive, but removes the limitations on sampling and yields high quality images at a fraction of the cost of conventional raytracers. Combining the two algorithms allows for rapid previewing followed by efficient high-quality rendering.

Shadow maps provide a fast and convenient method of identifying shadows in scenes but can introduce aliasing. This paper introduces the Adaptive Shadow Map (ASM) as a solution to this problem. An ASM removes aliasing by resolving pixel size mismatches between the eye view and the light source view. It achieves this goal by storing the light source view (i.e., the shadow map for the light source) as a hierarchical grid structure as opposed to the conventional flat structure. As pixels are transformed from the eye view to the light source view, the ASM is refined to create higher-resolution pieces of the shadow map when needed. This is done by evaluating the contributions of shadow map pixels to the overall image quality. The improvement process is view-driven, progressive, and confined to a user-specifiable memory footprint. We show that ASMs enable dramatic improvements in shadow quality while maintaining interactive rates. CR Categories: I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism---Shading,Shadowing; Keywords: Rendering, Shadow Algorithms 1

A coordinated use of hardwareprovided bltplanes and rendering pipelines can create fast ray traced quality illumination effects for dynamic environments by using Multi-pass Pipeline Rendering (MPR) techniques. We provide recttrsive reflections and refractions through the use of secondary viewpoints and projective image mapping. We extend the traditional use of shadow volumes to provide global direct illumination effects which fit into our recursive viewpoint paradiim. Hardware surface shading is fit to a physicallybaaed BRDF to provide a better local model, and the framework permits incorporation of indirect illumination as well. Furthermore, material transmittance is approximated using an extension to projective textures. Together, these techniques provide a platform for producing realistic images in highly dynamic environments. While most appropriate for scenes which specular components contribute largely to the secondary illumination, the integration of MPR with indirect racliosity solutions also provides a dynamic solution for highly diffuse environments. These techniques are immediately applicable to areas such as walkthroughs, animation, and interactive dynamic environments to produce more realistic images in near real-time.