A Fully-correlated Anisotropic Micrograin BSDF Model

Siggraph 2024 (ACM TOG)

Simon Lucas, Mickaël Ribardière, Romain Pacanowski, Pascal Barla

Author version

We introduce an improved version of the micrograin BSDF model [Lucas et al. 2023] for the rendering of anisotropic porous layers. Our approach leverages the properties of micrograins to take into account the correlation between their height and normal, as well as the correlation between the light and view directions. This allows us to derive an exact analytical expression for the Geometrical Attenuation Factor (GAF), summarizing shadowing and masking inside the porous layer. This fully-correlated GAF is then used to define appropriate mixing weights to blend the BSDFs of the porous and base layers. Furthermore, by generalizing the micrograins shape to anisotropy, combined with their fully-correlated GAF, our improved BSDF model produces effects specific to porous layers such as retro-reflection visible on dust layers at grazing angles or height and color correlation that can be found on rusty materials. Finally, we demonstrate very close matches between our BSDF model and light transport simulations realized with explicit instances of micrograins, thus validating our model.

A Micrograin BSDF Model for the Rendering of Porous Layers

Siggraph Asia 2023 (Conference Track)

Simon Lucas, Mickaël Ribardière, Romain Pacanowski, Pascal Barla

Author version, demo page

We introduce a new BSDF model for the rendering of porous layers, as found on surfaces covered by dust, rust, dirt, or sprayed paint. Our approach is based on a distribution of elliptical opaque micrograins, extending the Trowbridge-Reitz (GGX) distribution [Trowbridge and Reitz 1975; Walter et al . 2007] to handle pores (i.e., spaces between micrograins). We use distance field statistics to derive the corresponding Normal Distribution Function (NDF) and Geometric Attenuation Factor (GAF), as well as a view- and light-dependent filling factor to blend between the porous and base layers. All the derived terms show excellent agreement when compared against numerical simulations.
Our approach has several advantages compared to previous work [d’Eon et al. 2023; Merillou et al . 2000; Wang et al. 2022]. First, it decouples structural and reflectance parameters, leading to an analytical single-scattering formula regardless of the choice of micrograin reflectance. Second, we show that the classical texture maps (albedo, roughness, etc) used for spatially-varying material parameters are easily retargeted to work with our model. Finally, the BRDF parameters of our model behave linearly, granting direct multi-scale rendering using classical mip mapping.

(Fr) Analyse Numérique de Modèles de Matériaux Poreux

JFIG 2022

3rd Best Paper Award

Simon Lucas, Romain Pacanowski, Pascal Barla

Author version

L’apparence des objets est impactée par leur structure microscopique. Ici nous nous intéressons à l’apparence de matériaux poreux. La présence de pores a été modélisée de deux manières dans les travaux existants: en modifiant l’état de surface d’un matériau, ou en modifiant ses propriétés volumiques. Il n’existe cependant pas de modèle de matériaux poreux suffisamment générique pour tenir compte à la fois des effets surfaciques et volumiques. Nous proposons dans cet article une étude de ces effets à l’aide de simulations du transport de la lumière. Cette approche permet de révéler les limitations des modèles existants, ainsi que des pistes prometteuses pour le développement futur d’un modèle de matériau suffisamment général pour tenir compte de variations de porosités et d’état de surface.

Real-Time Geometric Glint Anti-Aliasing with Normal Map Filtering

i3D 2021

Xavier Chermain, Simon Lucas, Basile Sauvage, Jean-Michel Dischler and Carsten Dachsbacher

Author version, project page, video, code

Real-time geometric specular anti-aliasing is required when using a low number of pixel samples and high frequency specular lobes. Several methods have been proposed for mono-lobe bidirectional reflection distribution functions (BRDFs), but none for multi-lobe BRDFs, e.g., a glinty BRDF. We present the first method for real-time geometric glint anti-aliasing (GGAA). It eliminates most of the inconsistent appearing and disappearing of glints on surfaces with significant curvatures during animations. The technique uses the glinty BRDF of Chermain et al. [2020] and leverages hardware GPU-filtering of textures to filter slope distributions on the fly. We also improve this glinty BRDF by adding a correlation factor of slope. This BRDF parameter allows convergence to normal distribution functions that are not aligned on the surface’s axes. Above all, this parameter makes glint rendering compatible with normal map filtering using LEAN mapping. Using GGAA increases the rendering time from 0.6 % to 4.2 % and it requires 1/3 more memory due to MIP mapping of tabulated slope distributions. The results are compared with references using a thousand samples per pixel.