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M. Schwärzler, O. Mattausch, M. Wimmer, D Scherzer:
"Fast Accurate Soft Shadows with Adaptive Light Source Sampling";
Talk: International Workshop on Vision, Modeling and Visualization, Magdeburg, Germany; 2012-11-12 - 2012-11-14; in: "17th International Workshop on Vision, Modeling and Visualization (VMV 2012)", (2012), 1 - 8.

Physically accurate soft shadows in 3D applications can be simulated by taking multiple samples from all over
the area light source and accumulating them. Due to the unpredictability of the size of the penumbra regions, the
required sampling density has to be high in order to guarantee smooth shadow transitions in all cases. Hence,
several hundreds of shadow maps have to be evaluated in any scene configuration, making the process computationally
expensive. Thus, we suggest an adaptive light source subdivision approach to select the sampling points
adaptively. The main idea is to start with a few samples on the area light, evaluating there differences using
hardware occlusion queries, and adding more sampling points if necessary. Our method is capable of selecting
and rendering only the samples which contribute to an improved shadow quality, and hence generate shadows of
comparable quality and accuracy. Even though additional calculation time is needed for the comparison step, this
method saves valuable rendering time and achieves interactive to real-time frame rates in many cases where a
brute force sampling method does not.

soft shadows, real-time rendering

http://publik.tuwien.ac.at/files/PubDat_212712.pdf