Contact lever arms about the link world COM, not the collider pose - #27
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Contact lever arms about the link world COM, not the collider pose#27haixuanTao wants to merge 1 commit into
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The MB contact init used the contacting collider world translation as a proxy for the link origin when building angular lever arms (a documented follow-up: links_workspace was unbound over a 10-storage-buffer concern that no longer holds). The body jacobians measure link velocity at the link WORLD COM, so for any link whose collider is offset from its COM the contact rows are wrong by that offset. Measured on a G1 humanoid (foot collider ~4.6cm from the link COM), against an analytic MuJoCo mj_jac reference at an identical pose: the ankle-pitch column of a foot contact row was 8x too small (0.008 vs 0.056) — ground reactions transmitted almost no ankle torque, making quiet standing impossible at any motor stiffness. With this fix the contact rows match the reference to ~2% (ankle column within 3%) and J·M⁻¹·Jᵀ effective masses agree with a same-model MuJoCo reference to 2-8%. Fix: bind links_static + links_workspace and use local_to_world · local_mprops.com for both the A side and the self-contact B side. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01B7NC7U2wDx2tSmT9mkeF9h
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Bug
The multibody contact init builds angular lever arms as
pt_world − link_origin, wherelink_originis the contacting collider's world translation — a documented proxy (links_workspacewas left unbound over a 10-storage-buffer concern that no longer holds; backends in use request 14). The body jacobians measure link velocity at the link world COM, so for any link whose collider is offset from its COM, the contact jacobian rows are wrong by that offset.Impact
Measured on a G1 humanoid (foot collider ~4.6 cm from the link COM), comparing GPU contact rows against an analytic MuJoCo
mj_jacreference at an identical pose:With the ankle column ~8× under, ground reactions transmit almost no ankle torque — a quiet stand is impossible at any motor stiffness (the statue topples identically at kp ×1 through ×64, which was the diagnostic signature).
Fix
Bind
links_static+links_workspaceand uselocal_to_world · local_mprops.comfor the A side and the self-contact B side. Compiles clean; ~20 net lines.🤖 Generated with Claude Code
https://claude.ai/code/session_01B7NC7U2wDx2tSmT9mkeF9h