Hello,
I've recently started working on (for now) forward rendering engine and implemented basing brdf but I think that it looks off.
For start - diffuse based on 2012 Disney, modified by Frostbite - https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf#page=11&zoom=auto,-265,449
It doesn't look right to me. Why the roughest surface has some kind of background fresnel?
To compare, here is metalness-roughness render from my engine:
