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Crash Course in BRDF Implementation Code Sample

This is a code sample accompanying the article at https://boksajak.github.io/blog/BRDF. The file can be compiled as a HLSL shader or C++ code.

Example

A working path tracer which uses this BRDF implementation can be found in the ReferencePT project on GitHub. See file PathTracer.hlsl which includes brdf.h and calls to functions evalCombinedBRDF and evalIndirectCombinedBRDF for details.

How to use

  1. Include brdf.h in your project
  2. If compiling as C++, make sure that supporting GLM library is available and its includes at the beginning of the brdf.h file are valid. This library makes it possible to use HLSL-style structures and functions and compile it as C++ with minimal effort.
  3. Call functions evalCombinedBRDF and evalIndirectCombinedBRDF to evaluate BRDFs as described below and in the example. When compiling for C++, all functions and structures are placed in the brdf namespace.

Direct light evaluation

To evaluate BRDF for light coming from a light source, use the function evalCombinedBRDF, as follows:

const float3 V = -ray.Direction;            //< V is the direction towards the viewer
const float3 L = normalize(lightVector);    //< L is the direction towards the light source
const float3 N = shadingNormal;             //< N is the shading normal

MaterialProperties material = ...           //< Initialize material properties to that of evaluated surface 
material.reflectance = 0.5f;                //< If reflectance of the surface is used (see below) but not specified, 
                                            //< it should be initialized to 0.5 

float3 light = evalCombinedBRDF(N, L, V, material);

Indirect light evaluation

To generate a reflected ray according to the BRDF of the surface (and essentially evaluate indirect light contribution to the surface), use the function evalIndirectCombinedBRDF as follows:

const float2 random = ...        //< 2 random numbers in the interval <0,1). Note that interval is open on the right 
                                 //< side - number 1 can never occur here as it would generate NaNs when sampling the BRDF
const int brdfType = ...         //< Choose between SPECULAR_TYPE and DIFFUSE_TYPE. An example of how to choose in a path tracer 
                                 //< is in [ReferencePT project](https://github.com/boksajak/referencePT)

float3 rayDirection;             //< Sampled ray direction will be here
float3 brdfWeight;               //< The weight of the BRDF sample will be here

if (!evalIndirectCombinedBRDF(random, shadingNormal, geometryNormal, V, material, brdfType, rayDirection, brdfWeight)) 
{
    break; // Failed to generate valid ray direction
}

Options

The brdf.h file is configurable by macros at its beginning.

Minimal reflectance specification

By default, the specular reflectance of dielectrics (materials with metalness set to zero) is specified by definition MIN_DIELECTRICS_F0 to 4%.

If you want to let users to specify dielectrics reflectance per material, you can enable macro USE_REFLECTANCE_PARAMETER and set the reflectance parameter of the MaterialProperties structure. Note that actual reflectance is calculated as 0.16 * reflectance * reflectance. This makes changes to the parameter to appear more linear. If reflectance parameter is used, but some material doesn't specify it, it should be initialized to 0.5 - this will result in the reflectance of 4% as per formula above.

VNDF Sampling

By default, the implementation uses VNDF sampling from "Sampling the GGX Distribution of Visible Normals" by Heitz. If you want to use newer sampling from "Sampling Visible GGX Normals with Spherical Caps" by Dupuy & Benyoub, enable macro USE_VNDF_WITH_SPHERICAL_CAPS. This new method can bring performance benefits.

There is also an option to use older Walter's sampling from "Microfacet Models for Refraction through Rough Surfaces" by enablinf macro USE_WALTER_GGX_SAMPLING.

FUNCTION macro

All functions are prefixed with the macro called FUNCTION, which is set to static keyword when compiling as C++, and is empty in HLSL.

Release notes

brdf.h 1.2 - August 2023

Features: