Efficient Importance Sampling Techniques for the Photon Map
Keller, Alexander; Wald, Ingo
In Global Illumination computations the photon map is a powerful tool for approximating the irradiance, which is stored independent from scene geometry. By presenting a new algorithm, which uses novel importance sampling techniques, we improve the memory footprint fo the photon map, simplyfy the caustic generation, and allow for a much fsater sampling of direct illumination in complicated models as they arise in a production environment.
In Vision Modelling and Visualization 2000, pp. 271-279