FacetForge

FacetForge is a reference C++ codebase that provides benchmark implementations of microfacet BSDFs with unbiased multiple scattering. Unique capabilities:

• Quickly add new NDFs (including full-sphere NDFs) by implementing one simple function (the NDF itself)
• Visible-distribution-of-normals sampling for Student-T NDFs
• Support for biscale roughness: put any BSDF onto the microsurface, including other microsurface BSDFs

The initial release of this codebase was part of:

Student-T and Beyond: Practical Tools for Multiple-Scattering BSDFs with General NDFs
Eugene d'Eon
ACM (SIGGRAPH) Talks, August 2023
Paper / Supplemental

We will update the codebase over time. Pull requests are encouraged. For guidelines, see CONTRIBUTING.

Usage

Microsurface

Different from other approaches, in FacetForge a Microsurface is an operator that takes as input a BSDF and an NDF and outputs a new BSDF. Therefore, there are no RoughDielectric or RoughConductor BSDFs. To create a rough conductor, for example, you first create a smooth ConductorBRDF, plug it into your chosen NDF, and plug that into the Microsurface class to create a new BRDF:

ConductorBRDF micro_brdf(eta, k); // take a smooth conductor BRDF
GGXNDF ndf(&micro_brdf, rough_x, rough_y); // and assign it to microfacets with a GGX distribution
Microsurface macro_brdf(&ndf); // and feed this to a Microsurface operator to create a rough conductor BRDF

More examples of rough BSDFs are included in the test folder.

NDFs

NDFs can be added to FacetForge in two ways:

• heightfield NDFs can derive from the ShapeInvariantNDF and must implement the P22 slope distribution (which defines the NDF), sampling of the visible distribution of slopes when both roughnesses are equal to unity, and the cross section as a function of direction over the full sphere
• general full-sphere NDFs can derive from the NullNDF and implement the NDF D together with a majorant

Limitations

The primary purpose of the codebase is to implement flexible microfacet BSDFs with general NDFs. Achieving this goal comes with some limitations (some of which are straightforward to remove, some not), including:

• pdf() is not implemented
• NullNDFs will suffer crippling inefficiency for very low roughness (analogous to null scattering through a mostly empty inhomogeneous medium with a very large majorant)
• Analytic eval for single-scattering and specular facets is not currently implemented
• Polarization is not currently supported
• There is no notion of spectrum or color - radiance is monochromatic double

Assumptions

The code has three parts:

• C++ implementation (generally portable) in the include folder
  • assumes drand48()
  • tested on Mac OS Arm M1 with clang++
  • assumes std::mt19937 for gamma random variates (for Student-T NDF sampling)
• Mathematica tests (described below) in the test folder

Running the Tests

The test folder contains Mathematica notebooks that compare eval and sample for various BSDFs. To use the tests:

• In Mathematica SetDirectory[] to the facet-forge root dir where you cloned to
• (optional): edit the arguments of the test to vary roughness parameters, iors, incidence angle, numbers of samples
• run the full notebook, which will:
  • compile the required C++ test using clang++
  • execute the test, dumping the output to a .txt file
  • load the output and compare 2D histograms and 1D curves that compare eval and sample

Thanks

This codebase is an extension of Eric Heitz's original implementation from the 2016 SIGGRAPH paper that introduced multiple scattering to microfacet theory: https://eheitzresearch.wordpress.com/240-2/

License and Citation

@incollection{deon2023,
    author = {Eugene d'Eon},
    title = {Student-T and Beyond: Practical Tools for Multiple-Scattering BSDFs with General NDFs},
    booktitle={ACM SIGGRAPH 2023 Talks},
    pages={1--2},
    year={2023},
    url={https://doi.org/10.1145/3587421.3595417}
}

Copyright © 2023, NVIDIA Corporation. All rights reserved.

This code is made available under the Apache-2.0 license.