Creating soft shadow edges with random sampling
The basic shadow mapping algorithm combined with PCF can produce shadows with soft edges. However, if we desire blurred edges that are substantially wide (to approximate true soft shadows), then a large number of samples is required. Additionally, there is a good deal of wasted effort when shading fragments that are in the center of large shadows, or completely outside of the shadow. For those fragments, all of the nearby shadow map texels will evaluate to the same value. Therefore, the work of accessing and averaging those texels is essentially a wasted effort.
The technique presented in this recipe is based on a chapter published in GPU Gems 2, edited by Matt Pharr and Randima Fernando, Addison-Wesley Professional, 2005. (Chapter 17 by Yury Uralsky.) It provides an approach that can address both of the preceding issues to create shadows with soft edges of various widths, while avoiding unnecessary texture accesses in areas inside and outside...
