ray differentials -> mipmap Level
"10.1 Texture Sampling and Antialiasing" of "PBR Book V4"
"10.1 Sampling and Antialiasing" of "PBR Book V3"
Toksvig
normal variance
the NDF of the average normal and the average roughness (separately) is NOT the average NDF
NormalCurvatureToRoughness
of "Unreal Engine"
TextureNormalVariance
of "Unity3D"
"7.8.1 Mipmapping BRDF and Normal Maps" of "Real-Time Rendering Third Edition"
"9.13.1 Filtering Normals and Normal Distributions" of "Real-Time Rendering Fourth Edition"