Parameter Estimation of BSSRDF for Heterogeneous Materials

Rendering of highly scattering media is computationally expensive in general. While existing BSSRDF models can accurately and efficiently approximate light scattering in homogeneous media, we still have to resort to costly Monte Carlo simulation for heterogeneous media. We propose a simple parameter estimation method which enables homogeneous BSSRDF models to approximate the appearance of heterogeneous media. The main idea is to estimate the input optical parameters of a given homogeneous BSSRDF model such that the output well approximates light transport within heterogeneous media. Our method takes spatially varying optical coefficients into account by taking averages of the coefficients around the incident and exitant points. This approach is motivated by the path integral theory which predicts how wide the beam of light will spread in heterogeneous media. Since our method provides parameters for homogeneous BSSRDF models, it is applicable to many existing BSSRDF models and easy to integrate into existing rendering systems. We show that our modification produces more accurate results than the existing heuristics with the same goal.