Ray tracing has become a very popular method for image synthesis due to its unparalleled flexibility and its ability the generate images of high quality and realism. Simulation of effects such as reflection and refraction have been the hallmarks of ray tracing since its introduction [Whitted80]. With the advent of stochastic ray tracing [Cook84], the range of effects expanded to include motion blur, soft shadows, depth of field, and both blurry reflections and translucency. A notable omission from the repertoire of ray tracing, however, is diffuse reflection of indirect light. Though several new algorithms have been introduced which simulate multiple diffuse reflections of light in polygonal environments [Cohen85] [Nishita85], ray tracing continues to neglect these higher order effects, and consequently falls short of a truly global model of illumination.
These notes describe a simple extension to ray tracing which takes a first step toward alleviating this deficiency. The problem addressed is that of simulating diffuse reflection of specularly reflected and/or refracted light originating from point light sources. The technique involves one or more passes of backward ray tracing (from the light) and the construction of illumination maps as a pre-processing step.