Add glTF PBR

[proog128]

This PR adds a node definition and implementation of the glTF PBR material with the following extensions:

• KHR_materials_transmission
• KHR_materials_specular
• KHR_materials_ior
• KHR_materials_sheen
• KHR_materials_clearcoat
• KHR_materials_volume

For extensive discussion and test models see KhronosGroup#1.

Known issues

No thin-walled support yet. In glTF, the material can either be thin-walled (thickness = 0) or volumetric (thickness > 0). A volumetric material has refraction. The implementation in this PR only supports the volumetric case, i.e., the node graph ends with a surface node. The alternative would be thin_surface, but this seems to be not supported by MaterialXView. Besides, I wasn't able to figure out how to switch between surface and thin_surface based on an input.

Occlusion is ignored. There is no concept of occlusion in MaterialX.

Thickness is ignored. There is no concept of a thickness map in MaterialX. Nevertheless, the parameter could be used to switch between thin-walled and volumetric.

Potential future additions

Add node definition and implementation for texture fetches. Most material parameters in glTF have a "factor" and a "texture". For example, there is baseColorFactor that modulates baseColorTexture, i.e., baseColor = baseColorFactor * texture(baseColorTexture, uv). Normal maps, on the other hand, have a scale: normal = normalize(texture(normalTexture) * 2 - 1) * vec3(scale, scale, 1). By using these nodes for texture fetches implementations can ensure compatibility to glTF.

Add support for KHR_materials_emissive_strength. Waiting for the extension to be ratified. This makes it possible to use values for emission that are greater than 1.

Add support for KHR_materials_iridescence. Waiting for the extension to be ratified. Layers a thin_film node on top of the specular BSDF.

[jstone-lucasfilm]

This looks fantastic, thanks @proog128. For the new folder containing example materials in the gltf_pbr shading model, would it be acceptable to use the name GltfPbr for consistency with our other folders of example materials, or would this reduce clarity? The name of the gltf_pbr shading node is fine as it stands, and this proposed change would only affect the name of the new examples folder.

[proog128]

The folder is renamed to GltfPbr. Thanks for your feedback!

[fire]

Given one is trying to test MaterialX gltf principled pbr property mapping is there a unit test-able check for validity when compared against a reference photo.

The idea would be to provide a feature test gltf MaterialX and an associated obj similar to https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/SpecularTest

[emackey]

@fire It looks like @kwokcb is doing something with tests, for example: kwokcb@882c7de