It appears that roughness is broken in the VrayMtl for Vray 3.5 in Maya 2017. When roughness is set to 1 on a VrayMtl, the diffuse shading becomes binary, like a two-tone cartoon render where it is either 100% lit or 100% unlit. This can be seen in the attached image, comparing roughness in Arnold and Vray.
I have tested the same scene in Maya 2015 with Vray 3.4 and there the roughness works fine, so I suspect it is a problem with Vray 3.5 in particular.
I have tested this in Maya 2017 with Vray 3.4 and the roughness works fine, so this appears to be a bug specific to Vray 3.52
Thanks for the update. I’ll make a note.
Any chance this can get fixed in nightlies? It’s a pretty significant problem.
Ah, right. If you turn off the “glossy Fresnel” shader, it should render like before.
Then “glossy Fresnel” is ON, we export an entirely different shader under the hood and it has a different implementation of roughness. Will make a note to look into it, but in the meantime, the glossy Fresnel trick should work.
Best regards,
Vlado
Thanks, that’s a good temporary workaround. Of course we then lose the glossy fresnel, which is sub-optimal since glossy Fresnel is super awesome.
I don’t know the math behind it all, but the roughness should look the same even with glossy Fresnel, as can be seen in the image I posted above of the original ALsurface which uses glossy Fresnel (or at least it should not look flat like it does now with glossy Fresnel on in the VrayMtl). So I’m still crossing my fingers that this could be fixed in nightlies soon. I know you have superhuman powers Vlado! 
Yes, absolutely - like I said, it’s a bug that we will fix.
Best regards,
Vlado
Wondering if this has been fixed for the latest Nighlies?
This should be now fixed in the stable nightlies. You can try a newer build and see if it works correctly now. It’s also going to be included in the next official release.
