Update glTF PBR implementation to support current Khronos definition

[kwokcb]

Target

• To improve glTF <-> USD interop.

Requirements

The glTF PBR implementation is out of date and requires updating to support:

• Anisotropy
• Dispersion
• Diffuse Transmission (future extension)
• Thin Walledness: Update the glTF PBR implementation to support thin-walledness #1936

Requester

There is a RFP for work to be done for Khronos.

[emackey]

Thanks for getting this ball rolling @kwokcb! I'll fill in a bit more detail here on the work Khronos is looking for specifically:

The gltf_pbr.mtlx node of MaterialX should be updated to include new PBR parameters that have been made available since this node was first introduced, specifically:

• The anisotropyStrength and anisotropyRotation parameters from KHR_materials_anisotropy. This MaterialX node doesn't need to consider glTF's anisotropyTexture parameter here, although transformation scripts will need to take it into account (for example if a given MaterialX graph supplies a texture to one of the first two parameters, it will need to be encoded into a texture when it is exported to glTF).
• The dispersion parameter from KHR_materials_dispersion. This is equivalent to OpenPBR's transmission_dispersion_scale only when OpenPBR's transmission_dispersion_abbe_number is set exactly to its default value of 20. If the Abbe number is not 20, one must scale the dispersion accordingly (see formulas; it's just an inverse relationship).
• The diffuseTransmission and diffuseTransmissionColor parameters from KHR_materials_diffuse_transmission. Don't get distracted by the glTF spec's use of the terms "Factor" vs "Texture", those are just choices between uniform vs per-texel in glTF land and have no bearing on the MaterialX node graph, which is always procedural. The diffuseTransmission parameter allows forward-scattering of light through a surface via the diffuse lobe, and diffuseTransmissionColor modulates the color of such light.
• The thin-wall issue mentioned in the original post above.

Also, looking in the MaterialX translation folder, I currently see translation graphs from OpenPBR to SS, and another from SS to glTF. Ideally, Khronos PBR would like to see a graph go directly from OpenPBR to glTF, and another from glTF back to OpenPBR.

[emackey]

/cc @DRx3D who is putting together an RFP