Blender Git Loki

USD: Removed `as_background_job` parameter from export operator

The switch between foreground and background exports is now solely done
by respectively executing and invoking the export operator.

The `op->customdata` pointer is interpreted as an integer, and a bit is
set to indicate whether a background job should be run or not.

Merge remote-tracking branch 'origin/master' into sybren-usd

USD: added gtest for abstract hierarchy iterator

This test loads a blend file and asserts that the export paths are
reported correctly by the abstract hierarchy iterator.

USD: Fix for `BASE_VISIBLE` being renamed

The rename was done in rB0812949bbc3d.

Merge remote-tracking branch 'origin/master' into sybren-usd

USD: removed the temporary code for timing code execution

USD: stricter viewport locking when exporting

This is to prevent crashes like we had with the Alembic exporter (T65816).

Cleanup: added my name to TODO comments

Cleanup: remove scene parameter from USD_export()

The Context is passed to USD_export() anyway, which already contains the
scene.

Merge origin/master

Merge remote-tracking branch 'origin/master' into sybren-usd

Merge remote-tracking branch 'origin/master' into sybren-usd

USD: Allow user to disable export of UV maps and mesh normals

The UV maps and mesh normals take up a significant amount of space, so
it's better to make them optional. They're still enabled by default,
though.

Comparison: a shot of Spring (03_035_A to be exact) is 1.2 GiB when
exported with UVs and normals, and 262 MiB without. We probably have
room for optimisation of written UVs and normals.

USD: Export vertex velocities for fluid simulations

Currently only fluid simulations have explicit vertex velocities. This
is the most important case for exporting velocities, though, as the
baked mesh changes topology all the time, and thus computing the
velocities at import time in a post-processing step is hard.

USD: Cleanup, removed unnecessary `struct` keyword

No functional changes.

USD: Export mesh normals

We now write face-varying mesh normals to USD. When the mesh has custom
loop normals those are written. Otherwise the poly flag `ME_SMOOTH` is
inspected to determine the normals.

This commits also changes the subdivision scheme from the default value
'Catmull Clark' to 'None', indicating we're exporting a polygonal mesh.
This is necessary for USD to understand our normals; otherwise the mesh
is always rendered smooth. In the future we may want to expose this
choice of subdivision scheme to the user, or auto-detect it when we
actually support exporting pre-subdivision meshes.

A possible optimisation could be to inspect whether all polygons are
smooth or flat, and mark the USD mesh as such. This can be added when
needed.

USD: Remove the "uv_" prefix from exported UV maps

The primvar name is now the same as the UV Map name. This is to allow
the standard name "st" for texture coordinates by naming the UV Map as
such, without having to guess which UV Map is the "standard" one.

USD: simplified nested condition in USDAbstractWriter::write()

By flipping a condition the code became much simpler.

No functional changes.

USD: replaced throwing temp C++ exception with BLI_assert() call

The exception was for making things easier to debug for me. Now the issues
are gracefully ignored when things go bad, which is better for users.

No functional changes.

USD: Removed debug prints

No functional changes.