BRDF Measurement Modelling using Wavelets for Efficient Path Tracing

Physically based rendering needs numerical models from real measurements, or analytical models from material definitions, of the Bi-directional Reflectance Distribution Function (BRDF). However, measured BRDF data sets are too large and provide no functionalities to be practically used in Monte Carlo path tracing algorithms. In this paper, we present a wavelet based generic BRDF model suitable for both physical analysis and path tracing. The model is based on the separation of spectral and geometrical aspect of the BRDF and allows a compact and efficient representation of isotropic, anisotropic and/or spectral BRDFs. After a brief survey of BRDF and wavelet theory, we present our software architecture for generic wavelet transform and how to use it to model BRDFs. Then, modelling results are presented on real and virtual BRDF measurements. Finally, we show how to exploit the multi-resolution property of the wavelet encoding to reduce the variance by importance sampling in a path tracing algorithm.