Improved grazing angle bidirectional reflectance distribution function model using Rayleigh–Rice polarization factor and adaptive microfacet distribution function

Abstract

The bidirectional reflectance distribution function (BRDF) describes optical scattering off realistic surfaces. The microfacet BRDF, while computationally simple, lacks accuracy especially for grazing angles. An approximation, which replaces mathematically problematic elements of the microfacet model with the polarization factor from wave optics, has proved useful in accurately modeling the grazing region. We now expand upon this analysis by additionally varying the microfacet distribution function—a fundamental part of microfacet BRDF models. We find that after choosing the best microfacet distribution, 12 of the 18 materials studied show a significant improvement in the BRDF fit at grazing angles using the proposed approximation. Additionally, there was one case for which the approximation produced a model statistically tied for best within the experimental uncertainty of the data. The remaining five materials have significant sources of error outside the grazing region and will be further studied in future work.