USD Skeletons (armatures) are joint-based rather than bone-based in
construction. This means that there's no native bone concept nor is
there any bone lengths. Currently, Blender's USD import uses an
estimation, roughly corresponding to the average distance between
joints in the entire Skeleton, to set the bone length in situations
where it's not obvious that 2 joints are directly connected. This is
imperfect but shouldn't affect actual functioning of the rig.
For armatures coming from external software this is probably the best we
can do. However, for armatures originating from Blender, we can use a
custom primvar to store the bone lengths directly. This allows Blender
armatures to be exported and re-imported in much better fidelity.
This is superior to prior techniques, like those employed by FBX, which
alter the actual Skeleton, inserting extra joints where unnecessary. Not
only is this detrimental when using these files in external software,
but it's still imperfect when importing back into Blender too.
Pull Request: https://projects.blender.org/blender/blender/pulls/126954
832d243249