Parameters Sharing Multi-items Non-parametric Factor Microfacet Model for Isotropic and Anisotropic BRDFs

The reflection model of object surfaces is an essential part in photorealistic rendering. While analytical models are unable to express all significant effects of all materials, we turn to data-driven models, which, however, cost a large amount of memory and more computational resources. The non-parametric factor microfacet model designed by Bagher et al. [1] is intended to solve these problems. In this paper, we present a new non-parametric factor microfacet model which has triple specular lobes but retains the original number of parameters by sharing factors among three color channels. The fitting method called AWLS is also extended to solve for the G factor, which makes the fitting process more robust. Moreover, we use the D factor of our model for importance sampling as in the case of analytical models and find it effective for the specular materials. Finally we generalize our model and our fitting method to fit the 150 anisotropic materials. With only 2010 parameters (8KB), it can reconstruct the original data (2 MB) well, which further proves the expressiveness of our microfacet model.