This enables material displacement for UsdPreviewSurface import and
export. Scenarios are limited by what's supported by the preview surface
itself. Namely only Object Space displacement can be used (no vector
displacement)[1] and the Midlevel and Scale parameters are maintained by
adjusting the scale-bias on the image texture controlling the Height
(this means that Midlevel and Scale must be constants).
Hydra/MaterialX support is more complicated. First, there is a bug which
prevents scalar displacment from working correctly and that needs USD
2408+ for the fix[2]. Second, is that there's an open question about
which coordinate system to use for MaterialX's vector displacement maps.
Lastly, Hydra GL does not render displacement, making verification using
only Blender impossible[3]. As a result, this PR only makes MaterialX
"ready" for support, but stops short of actually connecting the final
piece of the node graph until more of the above can be sorted out.
Tests are added which cover:
- Variations of Midlevel and Scale values
- A constant Height setup
- Negative scenarios checking that only Object space is supported
and that midlevel and scale need to be constants
[1] https://openusd.org/release/spec_usdpreviewsurface.html
[2] https://github.com/PixarAnimationStudios/OpenUSD/issues/3325
[3] https://forum.aousd.org/t/materialx-displacement-hydra-storm/1098/2
Pull Request: https://projects.blender.org/blender/blender/pulls/128909
b4c2feea38