A small number of USD files in the wild contain invalid face index data
for some of their meshes. This leads to asserts in debug builds and
crashes for users in retail builds(sometimes). There is already an
import option to Validate Meshes but it turns out that we, and most
other importers, perform validation too late. We crash before getting to
that validate option (see notes).
This PR implements a cheap detection mechanism and will auto-fix if we
detect broken data. The detection may not find all types of bad data but
it will detect what is known to fail today for duplicate vertex indices.
We immediately validate/fix before loading in the rest of the data. The
downside is that this will mean no additional data will be loaded.
Normals, edge creases, velocities, UVs, and all other attributes will be
lost because the incoming data arrays will no longer align.
It should be noted also that Alembic has also chosen this approach. It's
check is significantly weaker though and can be improved separately if
needed.
If auto-fix is triggered, it will typically appear as one trace on the
terminal.
```
WARN (io.usd): <path...>\io\usd\intern\usd_reader_mesh.cc:684
read_mesh_sample: Invalid face data detected for mesh
'/degenerate/m_degenerate'. Automatic correction will be used.
```
A more general downside of these fixes is that this applies to each
frame of animated mesh data. The mesh will be fixed, and re-fixed, on
every frame update when the frame in question contains bad data.
For well-behaved USD scenes, the penalty for this check is between 2-4%.
For broken USD scenes, it depends on how many meshes need the fixup. In
the case of the Intel 4004 Moore Lane scene, the penalty is a 2.7x
slowdown in import time (4.5 s to 12.5 s).
Pull Request: https://projects.blender.org/blender/blender/pulls/138633
04bc3f155e