Root cause of v3d/transl NaN identified: roma.rotmat_to_rotvec
uses torch.empty + 8 index_put_ on a buffer that CoreML mlprogram
translates as scatter_nd over a garbage-initialised tensor. Cells
that the scatter chain misses keep NaN; the subsequent quat /
norm propagates NaN to every vertex.
Patch: branchless atan2 formulation (stack/clamp/norm/atan2 only),
no torch.empty, no index_put_. Precision drift vs roma original:
2.26e-6 L_inf on random batches. Mlpackage now validates all
outputs finite (1.27e-4 L_inf vs PyTorch eager on v3d).
Bench standalone: 65 ms median FP16 (15.3 fps, target met).
Live with 3 parallel workers: 8 fps Multi-HMR keyframe rate
(2.3x speedup vs PyTorch MPS baseline 3.5 fps); rigging still
ships at 15-20 fps perceived.
Output names shifted post-patch (var_2541 -> var_2412 etc) so
multihmr_coreml.py constants updated.
L'électron rare
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