Volume rendering is a technique for visualizing sampled scalar or vector fields of three spatial dimensions without fitting geometric primitives to the data. A subset of these techniques generates images by computing 2-D projections of a colored semitransparent volume, where the color and opacity at each point are derived from the data using local operators. Since all voxels participate in the generation of each image, rendering time grows linearly with the size of the dataset. This paper presents a front-to-back image-order volume-rendering algorithm and discusses two techniques for improving its performance. The first technique employs a pyramid of binary volumes to encode spatial coherence present in the data, and the second technique uses an opacity threshold to adaptively terminate ray tracing. Although the actual time saved depends on the data, speedups of an order of magnitude have been observed for datasets of useful size and complexity. Examples from two applications are given: medical imaging and molecular graphics.

A fast and simple voxel traversal algorithm through a 3D space partition is introduced. Going from one voxel to its neighbour requires only two floating point comparisons and one floating point addition. Also, multiple ray intersections with objects that are in more than one voxel are eliminated. Introduction In recent years, ray tracing has become the algorithm of choice for generating high fidelity images. Its simplicity and elegance allows one to easily model reflection, refraction and shadows. 1 Unfortunately, it has a major drawback: computational expense. The prime reason for this is that the heart of ray tracing, intersecting an object with a ray, is expensive and can easily take up to 95% of the rendering time. Unless some sort of intersection culling is performed, each ray must intersect all the objects in the scene, a very expensive proposition. There are two general strategies for intersection culling: hierarchical bounding volumes 1, 2, 3, 4 and space partitioning...

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

this paper, we present a system that exploits object-space, rayspace, image-space and temporal coherence to accelerate ray tracing. Our system uses per-surface interpolants to approximate radiance, while conservatively bounding error. The techniques we introduce in this paper should enhance both interactive and batch ray tracers.

While almost all research on image representation has assumed an underlying discrete space, the most common sources of images have the structure of the continuum. Although employing discrete space representations leads to simple algorithms, among its costs are quantization errors, significant verbosity and lack of structural information. A neglected alternative is the use of continuous space representations. In this paper we discuss one such representation and algorithms for its generation from views of 3D continuous space geometric models. For this we use binary, space partitioning trees for representing both the model and the image. Our approach falls under the general rubric of visible surface algorithms, providing an objectspace algorithm which under certain conditions requires only sub-linear time for a partitioning tree represented model, and in general exploits occlusion so that the computational cost converges toward the complexity of the image as the depth complexity increases. Visible edges can also be generated as a step following visible surface determination. However, an important contextual difference is that the resulting image trees are used in subsequent continuous space operations. These include affine transformations, set operations, and metric calculations, which can be used to provide image compositing, incremental image modification in a sequence of frames, and facilitating matching for computer vision/robotics. Image trees can also be used with the hemicube and light buffer illumination methods as a replacement for regular grids, thereby providing exact rather than approximate visibility.

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] Hank Weghorst, Gary Hooper, and Donald P. Greenberg. Improved computational methods for ray tracing. ACM Transactions on Graphics, 3(1):52--69, January 1984. [Whi80] Turner Whitted. An improved illumination model for shaded display. Communications of the ACM, 23(6):343--349, June 1980. [Wil78] Lance Williams. Casting curved shadows on curved surfaces. Computer Graphics, 12(3):270--274, August 1978. [WPF90] Andrew Woo, Pierre Poulin, and Alain Fournier. A survey of shadow algorithms. IEEE Computer Graphics and Applications, 10(6):13--32, November 1990. [WRC88] Gregory J. Ward, Francis M. Rubinstein, and Robert D. Clear. A ray tracing solution for diffuse interreflection. Computer Graphics, 22(4):85--92, August 1988. [Zha91] Ning Zhang. Two methods for speeding up form-factor calculation. In Second Eurographics Workshop on Rendering, Barcelona, Spain, May 1991. 154 [Pol71] E. Polak.<F1

Current graphics hardware can be used to generate shadows using either the shadow volume or shadow map technique. However, the shadow volume technique requires access to a representation of the scene as a polygonal model, and handling the near plane clip correctly and efficiently is difficult; conversely, accurate shadow maps require high-precision texture map data representations, but these are not widely supported. We present a hybrid of the shadow map and shadow volume approaches which does not have these difficulties, and leverages high-performance polygon rendering. The scene is rendered from the point of view of the light source and a sampled depth map is recovered. Edge detection and a template-based reconstruction technique are used to generate a global shadow volume boundary surface, after which the pixels in shadow can be marked using only a one bit stencil buffer and a single-pass rendering of the shadow volume boundary polygons. The simple form of our template-based reconstruction scheme simplifies capping the shadow volume after the near plane clip.

We present an experimental study of some of the important properties of space subdivision structures used to accelerate ray tracing. Location and orientation of partitioning planes, use of bounding volumes, methods to isolate void spaces in hierarchies and choice of traversal methods are examined to determine their effect on performance. This has resulted in the development of a new search structure based on k-d trees, which outperform acceleration schemes that exploit only a subset of the above characteristics. In addition to superior performance and greater adaptivity to scene characteristics, the flexibility of this structure allows it to terminate itself at the correct point, unlike the ad hoc schemes used by existing methods. This structure has been applied successfully to volume visualization applications, resulting in significant performance advantages. 1 Introduction A great deal of research has focused on discovering efficient ways to perform ray tracing, a sophisticated rend...