Jamie

Great to see this PR in progress! This is functionality that is essential for Arnold renderer, to reduce the peak memory usage of scenes with large volumes. Just to note,...

Just to note, I think it's arguably a slight defect of the Zeltner model that it doesn't explicitly model the color of the fuzz, i.e. one has to add it...

It must be true that the existing approach with a single `base_color` is less expressive than having separate colors, since obviously both the metal and diffuse base albedo must be...

I noticed this slide in the Unreal Substrate presentation. Is this "metalness limitation" the same issue? 

To test the effect of constraining the metal and diffuse base colors to be equal, I did renders with OpenPBR of "rust on top of shiny metal" in two ways:...

@andre-ilm For an extension to OpenPBR to allow for the use cases you describe, a minimal suggestion is to add parameters: ``` metal_override // opt-in, false by default metal_weight //...

Same experiment with a metalness map that has a more gradual transition. In this case, the result with separate metal and base looks like the rust "thinning" in the transition...

By the way, I think `metal_weight` would need to be an overall multiplier of the metallic Fresnel, not just the F0. As otherwise there is no way to completely turn...

>Is there evidence that this would also be useful to eliminate artifacts? If not, it would be nice to avoid adding these parameters because they complicate the implementation. Specifically, when...

>What would be the purpose of adding a metal_weight parameter? Could base_metalness be used for the same purpose? If metal_weight is a more consistent name, could we simply rename base_metalness...