EMSUSD-1626 : Support MaterialX document in USD Exporter

[derlerk]

ShaderWriter for the LookdevX MaterialXStack material.
Test that validate an export of MaterialXStack with surfaceshader and displacement

[JGamache-autodesk]

Very good. I would say 99% done. Sadly there are a few corner cases in MaterialX that need to be handled correctly. I have described the ones I encountered in the past.

[JGamache-autodesk]

Sorry about that, but I remembered another corner case that will hit us at some point. Simplest form is:

You need to handle shared_image1 correctly if exporting both surfacematerials. I suspect a first implementation will split it and create one image node per material. Maybe later we can consider using USD referencing to keep it shared.

[derlerk]

Sorry about that, but I remembered another corner case that will hit us at some point. Simplest form is: You need to handle shared_image1 correctly if exporting both surfacematerials. I suspect a first implementation will split it and create one image node per material. Maybe later we can consider using USD referencing to keep it shared.

This use case is already working, but indeed it is duplicating the image node. We could in the future come up with a way of referencing it to avoid duplication.

[JGamache-autodesk]

Thanks for doing the work! Looks good to me now.