A Hierarchical Volumetric Shadow Algorithm for Single Scattering

摘要

Volumetric effects such as beams of light through participating mediarnare an important component in the appearance of the naturalrnworld. Many such effects can be faithfully modeled by a singlernscattering medium. In the presence of shadows, rendering these effectsrncan be prohibitively expensive: current algorithms are basedrnon ray marching, i.e., integrating the illumination scattered towardsrnthe camera along each view ray, modulated by visibility to the lightrnsource at each sample. Visibility must be determined for each samplernusing shadow rays or shadow-map lookups. We observe thatrnin a suitably chosen coordinate system, the visibility function hasrna regular structure that we can exploit for significant accelerationrncompared to brute force sampling. We propose an efficient algorithmrnbased on partial sum trees for computing the scattering integralsrnin a single-scattering homogeneous medium. On a CPU, wernachieve speedups of 17–120x over ray marching.