Quality scalability of soft shadow mapping
Michael H. Schwarz, Marc Stamminger · 2008
Figure 1: (a) Soft shadow mapping (micro-occluder budget bmo = 12) without blending can lead to transition artifacts when varying shadow map levels need to be used across pixels. (b–d) Our scheme enables not only smooth transitions for screen-uniform budgets (b: bmo = 12, c: bmo = 5) but also readily supports local budget variations (d: bmo decreases from 12 in the center to 5 at the left and right borders as shown in the inset). Recently, several soft shadow mapping algorithms have been in-troduced which extract micro-occluders from a shadow map and backproject them on the light source to approximately determine light visibility. To maintain real-time frame rates, these algorithms often have to resort to coarser levels of a multi-resolution shadow map representation which can lead to visible quality degradations. In particular, discontinuity artifacts can appear when having to use different shadow map levels across pixels. In this paper, we discuss several aspects of soft shadow quality. First, we motivate and propose a scheme that allows for varying soft shadow quality in screen-space in a visually smooth way and also for its adaptation based on local features like assigned impor-tance. Second, we suggest a generalization of micropatches which yields a better occluder geometry approximation at coarser shadow map levels, thus helping to reduce occluder overestimation. Third, we introduce a new hybrid acceleration structure for pruning the search space of potential micro-occluders that enables employing finer shadow map levels and hence increasing quality. Finally, we address multisampled rendering and suggest a simple scheme for interpolating light visibility that only adds a negligible cost com-pared to single-sample rendering.