GGX Microfacet Distribution based BSDF (BRDF + BTDF) for physically plausible shading in Houdini Mantra renderer.The shader consists of cvex eval procedure for evaluating light samples & cvex sample procedure for material sampling. It is availbale to simulate reflection (BRDF) & refraction (BTDF). The model extended by power parameter which influences on the sharpness of the reflection and supports Generalized-Trowbridge-Reitz (GTR) Anisotropy.
Based on papers:
Microfacet Models for Refraction through Rough Surfaces. Bruce Walter, Stephen R. Marschner, Hongsong Li, Kenneth E. Torrance
Accurate fitting of measured reflectances using a Shifted Gamma micro-facet distribution. M. M. Bagher, C. Soler and N. Holzschuch