The model animated but deformed wrong: arms crossed the torso and foot heels
stretched to the model's center-bottom. Two root causes, both fixed:
1. Pose mismatch. Sketchfab models are modelled in an A-pose (arms angled
down) but the Quaternius library rests in a T-pose. Binding the A-pose mesh
to T-pose arm bones misweighted the arms, and the library clips (which
rotate arms ~70° down from the T rest) then shoved the already-drooped arms
across the body.
Fix: autorig.fit_arms_to_mesh() reorients each arm bone chain along the
model's real arm direction; merge_animations retargets every clip from the
library's T-pose rest onto the fitted rest via world-space constraint bake
(Copy Rotation/Location + nla.bake), so world motion is reproduced
regardless of the rest difference.
2. Feet-to-center stretch. The library's deform-flagged `root` bone runs up
the body centre-line; nearest-bone weighting assigned inner-foot/heel verts
to it, stretching them to center when animated.
Fix: exclude root/control/mechanism bones from weighting (_NON_DEFORM_HINTS).
Also upgraded nearest-bone weighting from rigid (1 bone/vert) to a smooth
inverse-distance blend across the nearest 4 bones, so joints deform cleanly.
Bone-heat still fails on Miku's joined hair/clothing mesh, so this is the
primary path.
Verified by rendering Idle/Walk/Run poses (Blender): natural stance, arms at
sides swinging correctly, feet planted, proper running stride. Smoke test
still 30/30.
Co-Authored-By: Claude Fable 5 <[email protected]>
