Very interesting the real photograph, too bad it has light bouncing from bellow and it's a little harder to compare the lower area, because it's standing on a white plane.
You are right. That and also the fact that the studio setup should produce way softer shadows than the HDRI used in the tests which should greatly influence the shadow terminator on the sphere. I'm gonna reproduce the setup to see how it behaves in Corona.
Anyway, after a lot of testing and observation, I think the diffuse roughness implemented in Fstorm is kind of weird. It shouldn't affect the diffuse that much, plus it does not affect what it is intended to. It gives a kind of coated material effect on every single shader created.
The PxrDisney shader affects both the diffuse at grazing angles and the shadow terminator, and it's much more subtle.
BUT we lose control over the diffuse appearance as it's fully automated and there is no parameter exposed. It seems that they realized that since they have backtracked on that particular point. Indeed, the PxrDisney shader is already "outdated" and has been relegated to the legacy tab in the lastest Renderman version. It's still there but only for back-compatibility. They are now using the PxrSurface shader which is much more complex than the Disney one. The diffuse part is now pure Oren-Nayar. And I personally think it's the way to go.
In fact, I think CoronaMtl is not that far from the perfect shader, it just misses some features here and there.
To me, the perfect shader would probably be a carbon copy of the Alsurface shader + some features of the PxrSurface shader. This is what I'd go for :
_Better diffuse model (Oren-Nayar)
_An additional specular lobe. We will then be able to produce proper coated materials.
WE NEED THAT. The CoronaLayeredMtl is.. well it's not a layered material, it's a blend material which is not the same. A proper layered material would allow us to stack multiple specular lobes in an energy conservative way, the top layers affecting the ones underneath.
At that point, with these only two features, the CoronaMtl would offer everything the Alsurface shader does. (minus the choice between Beckmann and GGX microfacets distribution).
I'd then add some nice features from the PxrSurface shader. Some of them are available following two modes, artistic/physical. I'd stick with the physical mode only :
_Layer thickness for the clearcoat: Everything is in the name, it simulates thickness for the additional specular lobe, really cool.
_Iridescence
_Fuziness: Full featured with gain, color, cone angle and bump. Not physically accurate but still nice to have
_Bump To Roughness: Insane feature! Basically, it can use normal, displacement or even bump maps to affect microfacets distribution and anisotropy, in order to recreate surface imperfections in a way that is less prone to diverge according to the distance and produce more physically correct light interaction. (cf. paper below)
You can refer to the renderman documentation for more info on their surface shader:
https://rmanwiki.pixar.com/display/REN/PxrSurfaceAnd I strongly recommend you to read their recent paper on their material pipeline, really interesting:
http://graphics.pixar.com/library/PxrMaterialsCourse2017/paper.pdf
