Robust Soft Shadow Mapping with Backprojection and Depth Peeling

Soft shadow mapping is an attractive solution to the problem of realtime soft shadows because it works with any kind of rasterizable geometry (in particular alpha-transparent textures and hair), it does not require any precomputation, and it is simple to implement on the GPU. However, state-of-the-art approaches have several limitations that prevent them from being practical for all scenes. First, parameter tuning is required to avoid surface acne. Second, gaps between shadow map pixels are either ignored, which results in light bleeding, or handled using gap filling, which results in overshadowing. We present a more robust soft shadow mapping technique, based on a recent backprojection algorithm, that uses depth peeling to address the problems of surface acne and light bleeding. Our algorithm uses a multi-layer shadow map to reduce light bleeding, and midpoint shadow maps to handle self-shadowing more robustly. It provides high-quality soft shadowing for complex scenes, while still maintaining interactive rendering rates. Source code is available online.