Summed-area variance shadow maps

Figure 1: Comparison of regular variance shadow maps (left) with our technique (right). The scene demonstrates a difficult case for shadow filtering algorithms. Our algorithm removes the light bleeding artifacts (seen in the left image where multiple shadows overlap) by partitioning the light’s depth range into multiple layers. Shadow maps are commonly used in real-time rendering, but they cannot be filtered linearly like standard color, resulting in severe aliasing. Variance shadow maps resolve this problem by represent-ing the depth distribution using moments, which can be linearly fil-tered. However, variance shadow maps suffer from “light bleeding” artifacts and require high-precision texture filtering hardware. We introduce layered variance shadow maps, which provide si-multaneous solutions to both of these limitations. By partitioning the shadow map depth range into multiple layers, we eliminate all light bleeding between different layers. Using more layers in-creases the quality of the shadows at the expense of additional stor-age. Because each of these layers covers a reduced depth range, they can be stored in lower precision than would be required with typical variance shadow maps, enabling their use on a much wider range of graphics hardware. We also describe an iterative optimiza-tion algorithm to automatically position layers so as to maximize the utility of each. Our algorithm is easy to implement on current graphics hard-ware and provides an efficient, scalable solution to the problem of shadow map filtering