Boundary correct real-time soft shadows

Abstract

This paper describes a method to determine correct shadow boundaries from an area light source using umbra and penumbra volumes. The light source is approximated by a circular disk as this gives a fast way to extrude the volumes. The method also gives a crude estimate of the visibility of the area light source as seen from a point in the shadow region. Instead of rendering the volumes to the stencil buffer, we use an extended shadow map - a so-called D-buffer, which among other things stores distances from the center of the light source to the umbra and penumbra volumes. The method is suited for implementation on most programmable hardware. Though some crude approximations are used in the visibility function, the method can be used to produce soft shadows with correct boundaries in real time.