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].

@InProceedings{LPB25,
  author       = "Lucas, Simon and Pacanowski, Romain and Barla, Pascal",
  title        = "Importance Sampling of the Micrograin Visible NDF",
  booktitle    = "Computer Graphics Forum (Proceedings of the Eurographics Symposium on Rendering)",
  year         = "2025",
  url          = "http://www-sop.inria.fr/reves/Basilic/2025/LPB25"
}