This paper focuses on microfacet reflectance models, and more precisely on the definition of a new and more general distribution function, which includes both Beckmann’s and GGX distributions widely used in the computer graphics community. Therefore, our model makes use of an additional parameter γ, which controls the distribution function slope and tail height. It actually corresponds to a bivariate Student’s t-distribution in slopes space and it is presented with the associated analytical formulation of the geometric attenuation factor derived from Smith representation. We also provide the analytical derivations for importance sampling isotropic and anisotropic materials. As shown in the results, this new representation offers a finer control of a wide range of materials, while extending the capabilities of fitting parameters with captured data.

STD: Student’s t-Distribution of Slopes for Microfacet Based BSDFs
M. Ribardiere, B. Bringier, D. Meneveaux, L. Simonot
Computer Graphics Forum, Volume 36 (2017), Number 2 (Eurographics 2017 Proceedings) ( publisher link )

paper (author's submitted version)
material (Fitting)
Mitsuba plugin Disney shader
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