We describe an extension to photon mapping that recasts the most expensive steps of the algorithm -- the initial and final photon bounces -- as image-space operations amenable to GPU acceleration. This enables global illumination for real-time applications as well as accelerating it for offline rendering.

Image Space Photon Mapping (ISPM) rasterizes a light-space bounce map of emitted photons surviving initial-bounce Russian roulette sampling on a GPU. It then traces photons conventionally on the CPU. Traditional photon mapping estimates final radiance by gathering photons from a k-d tree. ISPM instead scatters indirect illumination by rasterizing an array of photon volumes. Each volume bounds a filter kernel based on the a priori probability density of each photon path. These two steps exploit the fact that initial path segments from point lights and final ones into a pinhole camera each have a common center of projection. An optional step uses joint bilateral upsampling of irradiance to reduce the fill requirements of rasterizing photon volumes. ISPM preserves the accurate and physically-based nature of photon mapping, supports arbitrary BSDFs, and captures both high- and low-frequency illumination effects such as caustics and diffuse color interreflection.

An implementation on a consumer GPU and 8-core CPU renders high-quality global illumination at up to 26 Hz at HD (1920x1080) resolution, for complex scenes containing moving objects and lights.

@inproceedings{mcguire09imagespace,
 author = {Morgan Mc{G}uire and David Luebke},
 title = {Hardware-Accelerated Global Illumination by Image Space Photon Mapping },
 booktitle = {Proceedings of the 2009 ACM SIGGRAPH/EuroGraphics conference 
      on High Performance Graphics},
 year = {2009},
 month = {August},
 day = {1},
 location = {New Orleans, Louisiana},
 publisher = {ACM},
 address = {New York, NY, USA},
 url = {http://graphics.cs.williams.edu/papers/PhotonHPG09/}
 }