If you have a dielectric (non metallic) sphere with a large roughness value (say, 0.95), and put it in a lighting environment that has a constant onmidirectional lighting value (such as 0.5) Cook-Torrance ggx PBR renderers seem to be missing a darkening at glancing angles.
I thought this was a bug in my PBR implementation at first when comparing vs a mitsuba ggx render, but I've found the same issue in google filament (https://github.com/google/filament) as well as in substance.
For instance, below is the iray version from substance compared to the opengl version from substance. 0 metalness, 1 roughness, omnidirectional ibl of 0.5, albedo of (1,1,1).
Is this a known limitation of cook-torrance ggx based PBR? Can anyone explain why this limitation is here, and the reason for the difference?
I've noticed that it's much closer to the path traced "ground truth" at lower roughness values (like up to around 0.5-0.75), and that the most commonly interesting roughness values are near the lower end of roughness. 0.5 even looks almost "completely rough" for instance. Is the issue that the approximations used don't really try to make these less important higher roughness values look correct?