Fix COM frame composition in mjCBody::AccumulateInertia#3250
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gholmes829 wants to merge 1 commit intogoogle-deepmind:mainfrom
Open
Fix COM frame composition in mjCBody::AccumulateInertia#3250gholmes829 wants to merge 1 commit intogoogle-deepmind:mainfrom
gholmes829 wants to merge 1 commit intogoogle-deepmind:mainfrom
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mjCBody::AccumulateInertiacallsmjuu_frameaccumwith the body pose and inertial offset arguments swapped, composingbody_ipose ∘ body_poseinstead of the documentedbody_pose ∘ body_ipose. The merged center-of-mass is wrong whenever the body pose and inertial offset don't commute as transforms.Most commonly triggered by URDF fixed-joint children with degenerate-eigenvalue inertia (typical of capsule/cylinder approximations), where
mjuu_fullInertiareturns a non-trivial principal-axes rotation.Distinct from #2982, which patched the inertia-tensor rotation in the same function. #2982's reproduction (child with
pos = 0) collapses both arg orders to the same answer, so it could not have caught this.Added test fails before the fix with merged
body_ipos = (0, 0, 0.5)and passes after with(0.5, 0, 0).