USD identifiers cannot start with a digit and, like all unsupported
characters, they will be turned into an `_`. This is a bit unfortunate
since object names like "1" or "2" both become "_" in the output file.
This not only generates a naming collision, which is not currently
handled, but it needlessly loses information even when there is no
collision.
Treat leading digits separately and turn names like "1" and "2" into
"_1" and "_2" respectively. This will allow us to eventually enable the
`light_tree_node_subtended_angle` render test which uses such names for
the lights. Another recent test required investigation and then rework
due to the same problem.
Pull Request: https://projects.blender.org/blender/blender/pulls/134595
Make use of USD's new UTF-8 support to allow our import/export code to
accept and generate appropriate USD files. This has been a long standing
shortcoming since USD's introduction, with incomplete and complicated
DCC-specific workarounds often attempted.
Summary of changes
- Export gets a new "Allow Unicode" option defaulting to "false". The
new Unicode USD files are not backward compatible. DCCs using older
versions of USD (before 24.03) will not be able to load such files so
we want to provide this as an opt-in option for now.
- Every location which used to call either `USDHierarchyIterator::make_valid_name`
or `pxr::TfMakeValidIdentifier` will now go through a new `make_safe_name`
API instead
- Export code is responsible for passing in the `allow_unicode` option
- Import code will always pass in `true` meaning Blender will happily
accept both existing and new Unicode USD files
Strangely, USD does not provide a convenient way of making valid UTF-8
identifiers and they left their old API unchanged. We had to roll our
own per their advice: https://forum.aousd.org/t/how-to-make-a-unicode-identifier-valid/1435
Pull Request: https://projects.blender.org/blender/blender/pulls/122471
