Rational BRDF.

Over the last two decades, much effort has been devoted to accurately measure Bidirectional Reflectance Distribution Functions (BRDFs) of real-world materials and to efficiently use the resulting data during the rendering process. Because of their size, it is difficult to directly use measured BRDFs for real-time applications, and fitting the most sophisticated analytical BRDF models is still a complex task. In this paper, we introduce Rational BRDF, a general-purpose and efficient representation for arbitrary BRDF, based on Rational Functions (RFs). Using an adapted parametrization we demonstrate how Rational BRDFs offer (1) a compact and efficient representation by a low-degree RF, (2) an accurate fitting of measured materials with a guaranteed control of the residual error, and (3) an efficient importance sampling by applying the same fitting process to the inverse of the Cumulative Distribution Function (CDF) generated from the BRDF for use in Monte-Carlo rendering.