Microquad Soft Shadow Mapping Revisited

Recently, real-time soft shadow research saw many important contributions from the class of soft shadow mapping algorithms, where approximate occluder geometry is reconstructed from a shadow map and backprojected onto the light source to determine its occlusion. An interesting approximation primitive is the microquad obtained from unprojected shadow map texel centers. However, its full potential has not yet been realized since correct clipping against the light and exact occlusion bitmask updates have previously been ignored motivated by performance considerations. In this paper, we first extend the microquad definition to also allow for triangles as micro-occluders. We then demonstrate how to determine the exact occlusion bitmask of a microquad with correct clipping.