These changes were authored by Michael B Johnson (drwave).
The default Blender USD export currently produces files that trigger
errors in the usdchecker that ships with USD 22.11.
The changes are:
- Set the defaultPrim if no defaultPrim is set. This sets it to the
first prim in the hierarchy which matches the behaviour of Pixar's
referencing (where referencing a USD layer without a defaultPrim will
pick the first prim) as well as matches the logic in Pixar's Maya USD
exporter code.
- Applies the MaterialBindingAPI to prims with material binding
attributes. This is a relatively new requirement for USD as it will
help for efficiency with upcoming changes to Hydra.
- Removes the preview scope in the USD shader hierarchy, because it
is no longer valid for shaders to have any non-container ancestors in
their hierarchy up until the enclosing Material prim.
Reviewed by: Michael Kowalski
Differential Revision: https://developer.blender.org/D17041
b67b84bd5d