Progressive Backward Photon Mapping

This work is devoted to the study of photon mapping methods applied to realistic rendering. In distinction from the conventional rendering methods, the calculation of the indirect and caustic luminance is based on backward photon maps (which are also called observation maps). The main advantages of this method are described. These advantages are the natural distribution of the photons in the regions forming the image luminance and the reduction of the number of photons formed on the trace of one ray. The main algorithmic difficulty of the backward photon mapping method is discussed, which is the necessity to synchronize data access during the calculations and accumulate the luminance due to indirect and caustic illuminance. To resolve this difficulty, it is proposed to use intermediate photon maps of the indirect and caustic illumination distributed over the computation threads that render the corresponding parts of the image. Based on this analysis, the method of progressive backward photon mapping is introduced, and an algorithm of realistic rendering based on this method is described. The proposed algorithm does not require additional synchronization during the accumulation of luminance at the image points, which makes it possible to efficiently implement it not only on the central processor but also on graphics processors (GPUs). Results of the qualitative and quantitative comparison of the rendering results produced by the methods of progressive forward and backward photon mapping are presented.