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
Abutting traces must draw each edge between two neighbouring regions once:
the filled paths on either side follow one curve, one of them reversed, and
together cover the image without gaps or overlaps. These tests check that
from the trace output alone, with no renderer: points along each path's
outline must lie within 0.001 px of another path's outline, the paths' areas
must sum to the image's, and every point of a 4 x 4 per-pixel grid must lie
in exactly one path.
The images are drawn in code, hard-edged and anti-aliased, and traced in
Color (Abutting) and Black and White, each at the default settings, Paths and
Corners 0 and 100, Snap Curves To Lines, and Shapes. They cover a closed island
in a background, a ring with and without a core of a third colour, a wavy
two-colour seam, a three-region junction, outlines that leave a junction and
return to it (the 111 / 101 / 110 pattern at 1, 2, 3 and 6 px, and framed),
squares touching at a corner, a one-pixel hole and a one-pixel island (kept at
Noise 1, dropped together with what they leave at Noise 25), separate areas of
one colour, and slanted hatches with one-pixel paper channels. A further test
shows the check finds a 0.01 px shift and stacked (Overlapping) layers.
Two tests are ignored as documented current gaps:
- At Paths 0 and Corners 0, a piece returning to its own junction collapses:
its region is dropped and its area left uncovered (draft #2205 fixes it).
- With Shapes, a region whose outline runs through a junction becomes an exact
shape its neighbour does not share (up to 0.0039 px in Black and White).
No tracer code changes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018tBcdwNhptqtmX32JCGFzB
