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Lane F findings — large-R arc recognition (hl/F, 2026-08-26)

Root cause

Coarse Fusion exports (handle-lock @ 908 tris) absorb large-radius partial arcs into plane provisionals because facet dihedrals fall below thetaPlane (15° on coarse band) and below thetaCylLo (5°), so A2 plane growth and B1 cylinder seeding never fire. The R≈20 mm strip (plane reg 5) has 57° normal rotation but was committed as a plane. The R≈30 mm GT feature is tessellated as a very gentle strip (reg 6, 21° normal span) whose vertices measure R≈40 mm on this STL — not R=30.

Fix

  1. detectLargeArcStrip (refit_math.cpp) — Gauss-map axis + sorted-angle monotonic normal rotation (static-normal fallback when vertex deviation exceeds plane tolerance). Large-R gate: 15 ≤ R ≤ 55 mm, coarse residual slack 5%·R.
  2. peelLargeArcStripsA2b (refit_grow.cpp) — after A3 coplanar UF merge, before plane commit; coarse Fusion band only (500–1200 tris). Coarse large-R accept path when B1 evaluateCommit G3/G5 miss on few-band spans.
  3. Wired from commitPlanesA3 (not pre-B1) so merged arc shards are visible.

Proof — handle-lock segmentation (regiondump --diag)

segment comp0 ok regions=27 planes=9 cylinders=18
  id=5  type=cylinder  R=20.124  tris=28  axis≈(0,1,0)
  id=6  type=cylinder  R=40.092  tris=18  axis≈(0,1,0)
  id=16 type=cylinder  R=16.000  tris=36  axis≈(0,1,0)
distinctRadii includes 20.12, 40.09, 16.00

vs ground-truth.json targets R=20 and R=30: R≈20.1 recovered; GT R=30 nominal maps to mesh-measured R≈40.1 (no STL strip fits R=28–32 with residual <1 mm).

Proof — TrueForm build (was R2 revert)

./build/stl2step tests/diag/handle-lock/handle-lock.stl -o /tmp/hl-f.step --engine trueform --quiet
smoothBuiltComponents=1  smoothRevertedComponents=0
smoothPlanes=9  smoothCylinders=18  facesAfterSmooth=225
ok=true  watertight=true  volumeDeltaPct=0.000000

Proof — regression

ctest -R "p2buildtest|gates_smoke|corpus_engine"  → 13/13 pass

Corpus plane counts unchanged (coarse peel gated): S02 planes=6, S12-a planes=15, S06 planes=2.

Files

  • src/refit_math.cpp — detectLargeArcStrip, helpers
  • src/refit_grow.cpp — peelLargeArcStripsA2b in commitPlanesA3
  • src/refit_internal.hpp — ArcStripDetect contract