“Optimized Photon Tracing Using Spherical Harmonic Light Maps”

Photon mapping is a widely used technique for global illumination [Jensen 2001]. However, current techniques can be slow and/or can take up prohibitively large amounts of memory. We optimize photon tracing using spherical harmonic light maps (SHLMs) to represent photon accumulation on parameterized surfaces. We use a photon tracing algorithm where SHLMs replace the traditional photon map data structure. The SHLM stores irradiance environment maps [Ramamoorthi and Hanrahan 2001] across all surfaces. This gives us many advantages over traditional photon map renderers. Our photon tracing time is faster since there is no kd-tree insertion for photon storage. Our memory usage and density estimation time stay constant regardless of how many photons have been traced through the scene. Finally, SHLMs can be used in conjunction with any normal map to light realtime scenes.

Jensen, H. W. 2001. Realistic Image Synthesis Using Photon Mapping. A K Peters, Ltd.
Ramamoorthi, R., and Hanrahan, P. 2001. An efficient representation for irradiance environment maps. In Proceedings of SIGGRAPH 2001, ACM Press / ACM SIGGRAPH, Computer Graphics Proceedings, Annual Conference Series, ACM, 497–500.