A Real-Time Soft Shadow Volume Algorithm

Doctoral thesis, 2003

Rendering of shadows is a very important ingredient in three-dimensional graphics since they increase the level of realism and provide cues to spatial relationships. Area or volumetric light sources give rise to so called soft shadows, i.e., there is a smooth transition from no shadow to full shadow. For hard shadows, which are generated by point light sources, the transition is abrupt. Since all real light sources occupy an area or volume, soft shadows are more realistic than hard shadows. Fast rendering of soft shadows, preferably in real time, has been a subject for research for decades, but so far this has mostly been an unsolved problem. Therefore, this thesis, which is based on five papers, focuses on how to achieve real-time rendering of soft shadows. The first four papers constitute the foundation and evolution of a new algorithm, called the soft shadow volume algorithm, and the fifth paper provides an essential proof for correctness and generality of this and some previous shadow algorithms. The algorithm augments and extends the well-known shadow volume algorithm for hard shadows. Two passes are used, where the first pass consist of the classic shadow volume algorithm to generate the hard shadows (umbra). The second pass compensates to provide the softness (penumbra). This is done by generating penumbra wedges and rasterizing them using a custom pixel shader that for each rasterized pixel projects the hard shadow quadrilaterals onto the light source and computes the covered area. A result of the thesis is an algorithm capable of real-time soft shadows that utilizes programmable graphics hardware. The algorithm produce high-quality shadows for area light sources and volumetric light sources. It also handles textured light sources, which currently is a very rare capability among real-time soft shadow algorithms. Even video textures are allowed as light sources.