I saw this interessting video about variable roughness
The new roughness microfacet reflections are based on that each faced has its own IOR reflection angle and its reflection amount.This is “physical correct”.
The effect shown in the video is based geometrys or objects which are manufactured almost flat and smooth. Alttough the roughness is quite high you get a mirror like reflection because if you look at the floor at a grazing angle,even at streets with flat sun shine gives this effect.
This is because the rough but flat/smooth surface at grazing angle is reflecting light along flat surface.
Vs a really rough bumpy diffuse surface would behave like each IOR angle of its own and you would have a more diffuse reflection plus selfshadowing depending on the profile.
What is missing here?Its the roughness profile, especially the height of the roughness profile and even better the dimension of the roughness patch.
Similar like the old Tea render has with its roughness profile.
The cycles microfacets are more or less dimensionless based on a glossy to diffuse angled microfacets shading model.
With such view depending roughness curve, like in the video,you can “simulate” such effect of the roughness profile look you would get.
In the video was not shown how to use roughness maps with it.
For this reason i created this post to show a simple setup with a Alumium material and a roughness map.
With the RGB curve you can adjust the roughness falloff like in the video.In this example to a round .25 at grazing angles.
With the maprange you can tweak the overall max roughness for finetune .