Frustum Casting for Progressive, Interactive Rendering Frustum Casting for Progressive, Interactive Rendering

EEcient visible surface determination algorithms have long been a fundamental goal of computer graphics. We discuss the well-known ray casting problem: given a geometric scene description, a synthetic camera, and a viewport which discretizes the camera lm plane into pixels, ray casting identiies the visible surface at each pixel, i.e., that scene primitive which is rst encountered by an eye ray directed through the pixel center. Interactive rendering systems have not ordinarily been based on ray casting, due to its computational cost. Instead, the dominant method for achieving interactive rendering is hardware-assisted rasterization and depth buuering of polygons, often produced by static or dynamic tesselation of higher-level objects. Modern polygon rasterization architectures are extremely powerful, having undergone an extensive development path. Several trends indicate, however, that alternatives to polygon rasterization and depth buuer-ing deserve examination in the design of future interactive rendering systems. The rst trend is the number and breadth of proposed algorithmic and hardware methods to lessen transformation, rasterizer and depth buuer load while viewing models of high complexity. A second, related trend is that geometric models are, increasingly often, larger than ordinary physical memories, lending greater importance to memory coherence considerations. Finally, general purpose processors have grown very powerful, enabling exible, dynamic retargeting of computational resources to diiering subtasks while maintaining responsiveness. A rendering system based on such processors could have signiicant advantages over dedicated hardware. In light of the above, we explored an alternative, general rendering architecture based on ray casting. In seeking to build an interactive software ray caster, we studied existing visible surface algorithms. Combining three such algorithms, we synthesized frustum casting, a novel algorithm for per-pixel visible surface identiication in general scenes. The algorithm samples discretely, but operates in object space, and is exact and eecient. We demonstrate a prototype software renderer based on frustum casting, which achieves interactivity through \just-in-time" sampling, and progressive image improvement through deferral of intersection and shading operations. Frustum casting well addresses the technological trends listed above. We believe that it and other ray-based rendering methods may be practically incorporable by designers of future high-performance rendering architectures. We gratefully acknowledge the support of Intel Corporation, and of MURI Award SA 1524-258 2386. The opinions and conclusions expressed in this document are not necessarily those of any sponsoring company or agency.