Anti-aliased shadow mapping for large-scale and dynamic scenes
Authors: Hanqiu Sun
Venue:  (2007)

Shadows are essential for the realism of computer-generated images, which dramatically enhance our perception of virtual scenes by providing useful visual hints. Even tremendous advances in graphics rendering algorithms and programmable GPUs have made real-time and photo-realistic rendering a reality, the synthesis of realistic shadowing effects is still challenging and computationally intensive. Shadow mapping is one of the most popular algorithms for real-time shadow rendering, which has been extensively adopted in real-time applications by its generality and efficiency. However, shadow mapping suffers from the inherent aliasing problems such as jagged shadow boundaries and incorrect self-shadowing due to the image-based nature. In this thesis, we present several shadow rendering techniques to render anti-aliased shadows via shadow mapping especially for large-scale and dynamic scenes. The Parallel-Split Shadow Maps (PSSMs) scheme is first proposed for realistic and real-time shadow rendering towards next generation of three-dimensional computer games. This scheme splits the view frustum into multiple parts using clip planes parallel to the view plane, and then generates multiple smaller shadow maps for the split parts. A fast and robust split strategy based on the analysis of shadow map aliasing is developed, to produce a moderate aliasing distribution over the whole depth range. Hardware-specific accelerations are also implemented and tested to remove the extra rendering passes caused by using multiple shadow maps. With the observation that the sampling density at the light in the post-perspective space better accommodates the requirements for the reconstruction of shadowed images, perspective reparameterization techniques warp shadow maps using the perspective transform to reduce aliasing errors for the objects near the viewer. One limitation in this line of research is that the approximated representation of aliasing errors is only valid for the ideal case in which the light and view directions are orthogonal. We thus establish the generalized perspective reparameterization functions to adaptively adjust the warping strength as the light and/or viewer moves. With the direction-dependent representations of aliasing errors, we subsequently develop three enhanced perspective reparameterizations to better handle general cases, including Generalized Linear Perspective Reparameterization (GLPR), Generalized Minimum-Norm Perspective Reparameterization (GMNPR) and Focus-preserving Trapezoidal Shadow Maps (FTSMs). Previous perspective reparameterizations approximate the aliasing errors for the points in a 2D space&mdash;the points along the view direction (i.e. horizontal plane in view space), from a geometric intuition. There's no explicit way to qualitatively analyze the aliasing elsewhere. We thus derive the global representation of aliasing functions based on the solid mathematical analysis. Using the global representation, we propose the Direction-adaptive Perspective Shadow Maps (DirPSMs) to offer enhanced shadow rendering in real-time applications. These techniques are intuitive to implement without using complex data structures. They are designed to improve both quality and performance of shadow rendering in large-scale and dynamic scenes.