VrayMtl - roughness issue with metallic and specular color?

[guido_odendahl]

3ds Max 20222 / V-Ray 5 Update 3

Dear community and V-Ray developers,

I guess I'm facing a roughness issue when using the VrayMtl with metallic and pure black specular color. This is not about a physically or non-physically behavior, this is about a strange behavior when changing a color value of specular.

edit:
This issue is only visible in the CPU renderer, for GPU it seems to be as expected.





Behavior when decreasing the roughness - nothing is changing in the material probe sphere. (same if I use roughness in the BRDF section).


Now i increase the specular color a little bit:


With a specular color more than pure black the behavior of glossiness or roughness behaves as expected in my point of view.

Is this an issue or is this a regular behavior of VrayMtl? In general I would not use the specular color when using metallic property in a physically manner, but from an artistic point it can be useful to have a kind of specular layer on top of the metallic surface. What do you think?

Thanks Guido

[beat_reichenbach]

On a side note:
The fact that these two materials give different results is a bit confusing in the first place:
Material A
Metalness=1
DiffuseColor=1
SpecColor=0

Material B
Metalness=1
DiffuseColor=1
SpecColor=1

[^Lele^]

That looks like a bug.

Two things on using metallicity:
a) a white (1.0 float/255int) color in the reflection is a technical requirement for the metallic workflow.
b) You can colorise your speculars using the coat layer.

[caypiranha]

tried it here on Vray 2.2 and it seems, it does not appear. So it might belong to update 3?
Also wondering, why "roughness" and "Translucency" are greyed out seen on Guido's screenshots, which are accessible in 2.2.. (or is update 3 still a beta?)

[^Lele^]

I hadn't noticed the "update 3" bit, but i have no idea what that is.
The latest stable nightly is still 2.2.

[caypiranha]

... so, that´s the reason why "roughness" and "Translucency" are greyed out seen on Guido's screenshots then...
even I switched to compact mat editor, I still have access to those

[^Lele^]

As i said, i have no idea.

[guido_odendahl]

Many thanks for your feedback