Plumb the configured substep count into GpuMultibodySet - #25
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GpuMultibodySet::num_solver_iterations was hardcoded to 4 (its setter existed but had no caller — the FIXME noted it) while the pipeline runs sp.num_solver_iterations substeps (8 in the biped configs). Since set_visible_dt divides the visible dt by the stale field, every multibody kernel integrated substeps with 2x the true substep dt (4x at num_solver_iterations=16), inconsistent with the constraint softness coefficients computed from the correctly divided params. Measured on a G1 passive-stand scene: foot slide 66mm -> 8mm per 0.5s, and the SOLVER_ITERS=16 NaN-in-4-steps divergence is eliminated (more substeps now stabilize instead of exploding). Fix: pass the configured count at both GpuMultibodySet build sites before set_visible_dt. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01B7NC7U2wDx2tSmT9mkeF9h
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Bug
GpuMultibodySet::num_solver_iterationsis hardcoded to 4 at construction and its setter has no caller (theFIXME: should be read from the simulation settingscomment marks the spot). The pipeline, however, runssp.num_solver_iterationssubsteps. Sinceset_visible_dtdivides the visible dt by the stale field, every multibody kernel integrates substeps with the wrong dt whenever the configured count differs from 4 — e.g. 2× too large at 8 substeps, 4× at 16 — inconsistent with the constraint-softness coefficients, which are computed from the correctly divided params.Symptoms
Measured on a G1 humanoid passive-stand scene (RTX 5090, WebGPU):
num_solver_iterations = 16The divergence-with-substep-count signature was the tell: each added substep integrated with an unchanged, too-large dt.
Fix
Call
set_num_solver_iterations(...)with the configured value beforeset_visible_dtat bothGpuMultibodySetbuild sites. 3 insertions, 2 files.🤖 Generated with Claude Code
https://claude.ai/code/session_01B7NC7U2wDx2tSmT9mkeF9h