Xavier Chermain, Frédéric Claux and Stéphane Mérillou
Rendering materials such as metallic paints, scratched metals and rough plastics requires glint integrators that can capture all micro-specular highlights falling into a pixel footprint, faithfully replicating surface appearance. Specular normal maps can be used to represent a wide range of arbitrary micro-structures. The use of normal maps comes with important drawbacks though: the appearance is dark overall due to back-facing normals and importance sampling is suboptimal, especially when the micro-surface is very rough. We propose a new glint integrator for specular surfaces relying on a multiple-scattering patch-based BRDF addressing these issues. To do so, our method uses a modified version of microfacet-based normal mapping [Schüssler et al. 2017] designed for glint rendering, leveraging symmetric microfacets. To model multiple-scattering, we re-introduce the lost energy caused by a perfectly specular, single-scattering formulation instead of using expensive random walks. This reflectance model is the basis of our patch-based BRDF, enabling robust sampling and artifact-free rendering with a natural appearance. Additional calculation costs amount to about 40% in the worst cases compared to previous methods [Yan et al. 2016, Chermain et al. 2018].