Root cause found via autonomous headless spin (BT_AUTODRIVE+BT_FORCE_TURN) +
frame-level [angsign] probe: the STYLE.H:118 macro
#define Abs(value) ((value>0) ? value : -value)
has NO parens around value, so Abs(a-b) mis-expands to (a-b>0 ? a-b : -a-b) ==
-(a+b) on the false branch, NOT |a-b|. The angular resync velocity gate passed
an EXPRESSION: Abs(localVelocity.angular.y - updateVelocity.angular.y). For a
steady spin the two are equal, so a-b==0 takes the false branch and yields
-(2*rate): on a REVERSE spin that is +2*rate > velDb, firing a type-4 resync
EVERY frame. The resync flood reset the peer's dead-reckon horizon every frame,
pinning its angular slerp at ~half rate and (as the drift periodically ran to
180deg) freezing the rotation for seconds while the leg turn-clip kept stomping.
Confirmed live-autonomous: velDrift 0 (was +2.618), byVel 0 (was 55/55), peer
rendered-rotation median dyaw/expected 1.00 (was 0.49), no multi-second dt gaps.
Fix (mech4.cpp resync gate, reconstruction side -- the engine macro is original,
the original master-perf avoided it by diffing into a temp):
- velDrift: diff into a temp, explicit (d<0?-d:d) so the macro never sees an
expression.
- angDrift: already reworked to a single-variable wrap-safe yaw delta (also
dodges the macro) -- and it corrects a prior units/wrap bug (was comparing a
raw quaternion-Y component against a radian deadband).
- velocity gate now diffs vs updateVelocity (the value the peer extrapolates
with), same scalar representation as localVelocity.
- BT_SPIN / BT_ANGSIGN / updYaw diagnostic probes (env-gated).
KNOWN RESIDUAL (smaller, follow-up): the ANGLE gate still bursts when the crude
large-angle quaternion projection in the reckoner runs updateOrigin stale between
writes and drifts ~180deg; causes occasional single-frame hitches (peer median is
still 1.0), not the freeze. Separate from this fix.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
