Separating Reflection Functions for Linear Radiosity

Classic radiosity assumes diffuse reflectors in order to consider only pair-wise exchanges of light between elements. It has been previously shown that one can use the same system of equations with separable bi-directional reflection distribution functions (BRDFs), that is BRDFs that can be put in the form of a product of two functions, one of the incident direction and one of the reflected direction. We show here that this can be easily extended to BRDFs that can be approximated by sums of such terms. The classic technique of Singular Value Decomposition (SVD) can be used to compute those terms given an analytical or experimental BRDF. We use the example of the traditional Phong model for specularlike reflection to extract a separable model, and show the results in term of closeness to ordinary Phong shading. We also show an example with experimental BRDF data. Further work will indicate whether the quality of linear radiosity images will be improved by this modification.