Practical Rendering of Thin Layered Materials with Extended Microfacet Normal Distributions

Abstract

We propose a practical reflectance model for rendering thin transparent layers with different sides varying in roughness and levels of gloss. To capture the effect of subsurface reflection, previous methods rely on importance sampling for each light-surface interaction. This soon becomes a computationally demanding task since a recursive sampling scheme is required to handle multiple internal reflections. In this paper, we first provide a comprehensive analysis of the relationship between the directional distribution of scattered light and the roughness of each layer boundary using joint spherical warping. Based on the analysis, we generalize the traditional microfacet theory for layered materials and introduce the extended normal distribution function (ENDF) to accurately model the behavior of subsurface reflection. With the ENDF, the number of sampling processes can be reduced to only once for each bounce of subsurface reflection. We demonstrate that our BSDF model based on the ENDF is easy to be implemented on top of Monte Carlo sampling based offline renderers and it incurs little computational overhead. Moreover, it can be also efficiently used in real-time applications with the help of GPU acceleration.