CGI2013 manuscript No. (will be inserted by the editor) A Practical Model for Computing Subsurface BRDF of Homogeneous Materials with A Thin Layer of Paint

Abstract This paper presents a practical model for computing the subsurface BRDF of materials composed of a homogeneous semi-infinite layer and a thin painted layer on the top. Solution of light propagation inside a scattering/absorbing medium is expensive. We pro-pose a fast and exact subsurface BRDF computation model using the Ambartsumian’s integral equation for the bottom layer and using invariant imbedding method to modify the subsurface BRDF when a thin layer of material is painted on top. By using this model, we avoid the expensive light transport computation inside the material and compute the subsurface BRDF of the material directly. We validate our model against the so-lutions from a standard discrete ordinate (DOM) based light transport solver and demonstrate our model by simulating a variety of materials