Soft Shadow Volumes for Ray Tracing

Samuli Laine
Helsinki University of Technology
Hybrid Graphics Ltd.

Timo Aila
Helsinki University of Technology
Hybrid Graphics Ltd.

Ulf Assarsson
Chalmers University of Technology

Jaakko Lehtinen
Helsinki University of Technology
Remedy Entertainment Ltd.

Tomas Akenine-Möller
Lund University

To appear in ACM SIGGRAPH 2005


We present a new, fast algorithm for rendering physically-based soft shadows in ray tracing-based renderers. Our method replaces the hundreds of shadow rays commonly used in stochastic ray tracers with a single shadow ray and a local reconstruction of the visibility function. Compared to tracing the shadow rays, our algorithm produces exactly the same image while executing one to two orders of magnitude faster in the test scenes used. Our first contribution is a two-stage method for quickly determining the silhouette edges that overlap an area light source, as seen from the point to be shaded. Secondly, we show that these partial silhouettes of occluders, along with a single shadow ray, are sufficient for reconstructing the visibility function between the point and the light source.