Alan King, Christopher Kulla, Alejandro Conty, Marcos Fajardo
Presented at Siggraph 2013
A simple importance-sampling method for rendering subsurface light transport described by a BSSRDF on arbitrary geometry without any pre-computation.
Several sharp readers reported a typo in the formula for Rm. This has been corrected in the file below.
An implementation of this technique can be found in PBRT version 3. The 3rd edition of the text will also contain a detailed discussion of the limitations of this approach. In particular, if the scattered point is chosen stochastically to minimize branching of the ray tree, the axis MIS computation requires counting of ray hits along the two other axes which can be expensive. Moreover, BSSRDF themselves embed a number of assumptions (planar surface, semi-infinite medium, etc...) which do not hold in practice. This can lead to undesirable energy gain or loss in many common configurations. For these reasons, brute force simulation of subsurface light transport will always produce superior results.