Precomputing BRDF LUTs

In the previous recipes, we learned how to initialize compute pipelines in Vulkan and demonstrated the basic functionality of compute shaders. Let's switch gears back to PBR and learn how to precompute the Smith GGX BRDF LUT. To render a PBR image, we have to evaluate the BRDF at each point based on surface properties and viewing direction. This is computationally expensive, and many real-time implementations, including the reference glTF-Sample-Viewer implementation from Khronos, use precalculated tables of some sort to find the BRDF value based on surface roughness and viewing direction. A BRDF LUT can be stored as a 2D texture where the x axis corresponds to the dot product between the surface normal vector and the viewing direction, and the y axis corresponds to the 0...1. surface roughness. Each texel stores two 16-bit floating-point values—namely, a scale and bias to F0, which is the specular reflectance at normal incidence.

Important note...