Efficent approximate visibility testing using occluding spheres

Abstract

Practically every ray-casting based image synthesis algorithm relies heavily on the tracing of shadow rays to determine visibility between a sample point on a light source and a surface point. If the light source is relatively large, or indirect illumination is sampled, the necessary amount of such visibility tests may be several magnitudes more then the number of primary rays. In this paper, we discuss an approach using simple spherical occluders to approximate the shadowing effect of scene objects. We elaborate on how spheres allow for an extremely fast evaluation of visibility, and offer a complex automatic method for generating good sphere sets. The virtual light sources or indirect photon mapping technique, and the handling of large light sources are improved using the new method. Results featuring fast global illumination are presented.