OpenPBR: Implementation of MaterialX_coat_darkening

[takahashi_masaki]

It was found that there is a significant difference in appearance for materials with a coat when rendered using a common MaterialX file.
https://github.com/AcademySoftwareFo..._carpaint.mtlx

The likely cause is the implementation of coat_darkening.

Although the official reference states that coat_darkening is not supported, vrayMtl has a parameter with the same name, so is there a possibility that it will be supported in the future?

[hermit.crab]

That particular material doesn't have "coat_darkening", but it should work if you add it manually. The parameter is already implemented, it's just not exposed yet. I think the major difference come from the material's described color space - in the file it's "lin_rec709", but if you change to color space to "sRGB", you'll get similar to Arnold results.