A hierarchical shadow volume algorithm

Abstract

The shadow volume algorithm is a popular technique for real-time shadow generation using graphics hardware. Its major disadvantage is that it is inherently fillrate-limited, as the performance is inversely proportional to the area of the projected shadow volumes. We present a new algorithm that reduces the shadow volume rasterization work significantly. With our algorithm, the amount of per-pixel processing becomes proportional to the screen-space length of the visible shadow boundary instead of the projected area. The first stage of the algorithm finds 8 x 8 pixel tiles, whose 3D bounding boxes are either completely inside or outside the shadow volume. After that, the second stage performs per-pixel computations only for the potential shadow boundary tiles. We outline a two-pass implementation, and also describe an efficient single-pass hardware architecture, in which the two stages are separated using a delay stream. The only modification required in applications is a new pair of calls for marking the beginning and end of a shadow volume. In our test scenes, the algorithm processes up to 11.5 times fewer pixels compared to current state-of-the-art methods, while reducing the external video memory bandwidth by a factor of up to 17.1.