This paper by Purcell et al. presents a photon mapping algorithm that runs entirely on the GPU. The paper presents details for tracing photons, building the photon map, and computing the radiance estimate at each pixel using a k-nearest neighbor search.(Photon Mapping on Programmable Graphics Hardware. Timothy J. Purcell, Craig Donner, Mike Cammarano, Henrik Wann Jensen, and Pat Hanrahan. Proceedings of Graphics Hardware 2003, July 2003.)