ABSTRACT
Variance Shadow Maps (VSMs) were introduced as an alternative to bilinear percentage closer filtering (PCF) to speed up rendering of smoothed shadows. The idea of variance shadow mapping is to store in the shadow map, instead of a single depth value, a distribution of depth values over some region and to use elementary statistics to evaluate the shadowing term. One of the possibilities for reducing the problem of light-bleeding is, simply to avoid it. The trick is to combine standard depth-comparison shadow mapping with VSM applied only to regions that really need it—shadow boundaries. Exponential Variance Shadow Maps (EVSM) is a combination of two techniques—variance shadow maps and exponential shadow maps. EVSM suffers from light-bleeding only in cases when both VSM and ESM fail, which rarely happens. A very important feature of EVSM is that the lightbleeding can be controlled by two factors: VSM with the tail cut off and ESM with k constant.
