Technical Debt: Shader exporters vary wildly in quality, feature support, and stability (USDU-248)

[hybridherbst]

While going through the code I noticed a couple of places now that need much more testing:

• PBR attribute exporting is handled very differently between HDRP and Built-In
• URP support is missing (given that Unity has now officially mentioned "yes, this will be the default" this needs to happen at some point)
• exporters check for properties, not shader keywords, in most places (in some the keyword checks are in place);
  for example, material.HasProperty("_SpecGlossMap") will return true if this material has ever been set to "Specular", a texture attached, and then set back to "Roughness" workflow. Right way would be to check if actual shader keywords are set for known shaders.
• there's magic hardcoded values scattered in the code (e.g. specular color is set to 0.4, 0.4, 0.4 if material has no spec color)
• the workflow around linear textures, sRGB textures, and color spaces (linear vs. gamma) is unclear (and we're seeing color differences between Unity, USD, and AR QuickLook because of this I assume)
• the shaders for ChannelCombiners etc. are not in Resources, which means they will most likely be missing from builds unless a user explicitly adds them (this will cause certain, but not all, imports/exports to fail in a build)

[jcowles]

Some of the differences are required (e.g. because HDRP implementation is actually different from built-in), but in general, I totally agree with this -- I actually feel like there is a ton of accrued tech debt across the import and export pipelines and all of it should be revisited with a new holistic design.

I actually think there is less tech debt in shader import/export and much more in the geometry pipelines. UsdSkel support is particularly messy.

[riemaeker]

Any updates on this? Especially URP support would be very welcome :)