This paper presents a method that reduces the number of shadow rays used in ray tracing. This reduction is achieved by extending the concept of the photon map. Here the photons are characterized as either shadow photons or direct illumination photons and in the visualization step this information is used to eliminate shadow rays in areas that are either completely illuminated or completely shadowed. The method handles both sharp and soft shadows and it handles complex objects (ie. procedurally based objects). We have integrated the method into a two-pass global illumination renderer and our results indicate that we are able to reduce the number of shadow rays with more than 90 %
Henrik Wann Jensen and Niels Jørgen Christensen: "Efficiently Rendering Shadows using the Photon Map". In Proceedings of Compugraphics '95, pp. 285-291, Alvor 1995.