Image Trace: same sub-pixel shift from either side of an edge

Layers::shift read a crack's sub-pixel shift from its inside pixel,
looking for a thin run on the inside before one on the outside. Where
both sides are thin (a one- or two-pixel sliver beside another, as in
soft gradients and noisy scans) the two regions of the crack got
different shifts, so their shared piece got different points and the
abutting curves left slivers that no path covers.

The shift is now the mean of the readings from both sides,
(s(c) - s(c flipped)) / 2, which is exactly antisymmetric in floating
point (Crack::flipped gives the crack seen from its other side).

Tests (code-drawn images):
- a_crack_between_thin_slivers_has_the_same_shift_from_either_side:
  every crack's shift is the exact negative of its flipped crack's, on
  a noisy ripple of 1-2 px columns at three thresholds.
- shared_boundaries::thin_slivers_on_both_sides_of_an_edge_leave_nothing_uncovered:
  a 48 px noisy ripple in Black and White at thresholds 140/157/170 and
  Paths 0/50/100, sampled on the shared-boundary check's 4x4 grid per
  pixel: 482-1,228 uncovered points per trace before, 0 after.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018tBcdwNhptqtmX32JCGFzB

docs/image-trace-quality.md

@@ -1,5 +1,6 @@
 # Image Trace quality
 
+> **Last reviewed:** 2026-10-14 · **Last updated:** 2026-10-16 · **Change:** minor (Image Trace: an edge's sub-pixel shift is the same from either side)
 > **Last reviewed:** 2026-10-14 · **Last updated:** 2026-10-16 · **Change:** minor (Image Trace: a simpler contour walk with no precomputed direction table)
 
 What Image Trace (`crates/trace`) makes of images whose right answer is known, measured by a harness and
@@ -7,8 +8,25 @@ compared with the numbers before each change. Nothing here is a judgement by eye
 with any other product except the open tracer named under [Yardstick](#4-yardstick).
 
 
-## A simpler contour walk
+## Same sub-pixel shift from either side of an edge
 
+Each crack between two regions is moved by a sub-pixel shift read from the image. The reading
+looked for a thin run (one to three pixels) on the crack's inside before one on its outside, so where
+both sides were thin (a one- or two-pixel sliver beside another, as in soft gradients and noisy
+scans) the two regions of the crack got different shifts, their shared piece different points, and
+the abutting curves left slivers that no path covers. `Layers::shift` now averages the reading from
+either side, `(s(c) - s(c flipped)) / 2`, which is exactly antisymmetric in floating point, so both
+regions get the same points.
+
+Tests on code-drawn images: `a_crack_between_thin_slivers_has_the_same_shift_from_either_side` (a
+noisy ripple of 1–2 px columns at three thresholds: every crack's shift is the exact negative of its
+flipped crack's, and the image does have cracks the one-sided reading gets wrong) and, in
+`tests/shared_boundaries.rs`, `thin_slivers_on_both_sides_of_an_edge_leave_nothing_uncovered` (a
+48 px noisy ripple in Black and White at thresholds 140, 157, 170 and Paths 0, 50, 100, sampled on
+the 4 × 4 grid per pixel the shared-boundary check uses). With the one-sided reading 482–1,228
+sample points per trace were uncovered; now none are. The crate's other tests pass unchanged.
+
+## A simpler contour walk
 `contour::trace_mask` follows each region's outline along the pixel cracks with the inside on the
 right. It used to build a table of outgoing directions for every pixel corner in a raster pass and
 then follow it. It now works out each turn during the walk from the two pixels ahead of the current
@@ -16,14 +34,12 @@ corner (right if the one ahead on the right is outside, straight if the one ahea
 else left; so at a saddle it turns right and diagonal pixels stay apart, as before). Loop starts are
 found by scanning the pixels for a top edge on the outline, with one flag per pixel for a top edge
 already walked. A mask of the wrong length now gives no loops instead of failing an assertion.
-
 The output is identical: `the_walk_equals_the_stored_direction_walk` compares it with a test-only
 copy of the previous walk on all 65,536 masks of 4 × 4 pixels and on 400 seeded 23 × 17 masks.
 Time (best of 7, release build, 4 cores): the walk alone on 2000 × 2000 masks 19.1 ms vs 26.6 ms
 (a smooth pattern), 81.3 vs 93.2 ms (12% random pixels), 223.1 vs 238.1 ms (50% random pixels);
 whole traces of 1000 × 1000 images 73.1 vs 77.1 ms (Black and White), 249.6 vs 251.7 ms (Color, 6
 colours) and 214.1 vs 218.6 ms (Color, 6 colours, Noise 1), with the same segment counts.
-
 ## Noise threshold stability (local development, 2026-10-14)
 
 Noise must merge whole 4-connected regions, including pixels absorbed by an earlier merge.
@@ -1027,6 +1043,7 @@ One trace at a time on this machine (all cores); memory is the peak allocated ab
 
 | date | change |
 |:--|:--|
+| 2026-10-16 | Image Trace: a crack's sub-pixel shift is the mean of the readings from its two sides, exactly antisymmetric, so abutting regions get the same points on thin slivers (a code-drawn noisy ripple went from 482–1,228 uncovered sample points per trace to 0). |
 | 2026-10-16 | Image Trace: the contour walk works out each turn from the pixels as it goes instead of following a precomputed table of per-corner directions; identical loops on all 65,536 4 × 4 masks and 400 seeded masks, and no slower (whole 1 MP traces 1–5% faster). |
 | 2026-10-14 | Closed fitting retains the positioned boundary when straightening collapses a nonzero-area loop; four generated regressions; preset scores unchanged |
 | 2026-10-14 | Tiny closed fits retain their positioned outlines instead of losing islands/holes; three generated regression tests; quality-harness baseline unchanged |