Importance Sampling of the Micrograin Visible NDF

Abstract Importance sampling of visible normal distribution functions (vNDF) is a required ingredient for the efficient rendering of microfacet‐based materials. In this paper, we explain how to sample the vNDF for the micrograin material model LRPB23, which has been recently improved to handle height‐normal correlations through a new Geometric Attenuation Factor (GAF) LRPB24, leading to a stronger impact on appearance compared to the earlier Smith approximation. To this end, we make two contributions: we derive analytic expressions for the marginal and conditional cumulative distribution functions (CDFs) of the vNDF; we provide efficient methods for inverting these CDFs based respectively on a 2D lookup table and on the triangle‐cut method Hei20.