V.6 - A RECURSIVE SHADOW VOXEL CACHE FOR RAY TRACING

Abstract

This chapter describes a simple technique for speeding up shadow ray computation while ray tracing scenes that exploit any type of spatial subdivision. While ray tracing shadow rays, it is a good idea to store a pointer to the last object that caused a shadow with each light source in a shadow cache. The next shadow ray toward any light source first is intersected with the object in the shadow cache, and if an intersection occurs, the point being shaded can be declared to be in shadow with respect to that light source. This works because it is not necessary to know which object shadows a light source; it is sufficient to know simply that an object does shadow a light source. The shadow cache technique exploits object coherence in shadowing. However, while ray tracing surface tessellations, the amount of object coherence in the shadow is minimal, because the tessellated triangles produced usually are quite small. It is a simple matter to extend the shadow cache idea to include the caching of the voxel in which the last object to shadow the light was intersected, in a voxel cache with each light source.