Apply the contact cfm_factor to normal rows only, not friction tangents - #26
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Apply the contact cfm_factor to normal rows only, not friction tangents#26haixuanTao wants to merge 1 commit into
haixuanTao wants to merge 1 commit into
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The PGS sweep multiplied every contact row by cfm_factor, including the friction tangents. Rapier applies the soft-constraint cfm only to the normal part (ContactConstraintTangentPart::solve takes no cfm_factor); softening the tangents multiplies the effective friction coefficient by cfm_factor. At an overdamped contact config (natural_frequency 30 Hz, damping_ratio 5, dt 1.25 ms) cfm_factor is ~0.02, so mu=1.0 colliders behaved like mu~0.03: statically loaded feet on a humanoid slid steadily and a passive stand toppled. Measured (G1 passive stand): tangential slide insensitive to contact stiffness before the fix, reduced ~15% by this change in isolation, and the effective mu returned to the collider value (verified via a per-row impulse dump: tangent impulses reach mu*N instead of ~0.02*N). 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 PGS sweep applies
cfm_factorto every row:Rapier applies the soft-contact cfm to the normal part only —
ContactConstraintTangentPart::solvetakes nocfm_factor(seecontact_constraint_element.rs; onlyNormalPart::solve/generic_solveand the MLCP path receive it). Softening the tangents effectively multiplies the friction coefficient bycfm_factor.Impact
At an overdamped contact config (
contact_natural_frequency = 30 Hz,damping_ratio = 5, substep dt 1.25 ms),cfm_factor ≈ 0.02— so μ = 1.0 colliders behave like μ ≈ 0.03. Measured on a G1 humanoid passive stand: the loaded feet slid steadily regardless of contact stiffness (the slide was insensitive tonatural_frequencyacross a 16× range — the signature that friction, not the normal stack, was leaking), and the robot toppled. A per-row impulse dump showed tangent impulses saturating at ~0.02·N instead of μ·N; after the fix they reach the true cone.Fix
Solve tangent rows without the cfm factor; normal rows unchanged. One site, a few lines.
🤖 Generated with Claude Code
https://claude.ai/code/session_01B7NC7U2wDx2tSmT9mkeF9h