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Image Trace: retain pieces returning to a junction

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Squashed from #2205 onto main.
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bflatastic
2026-10-16 15:27:54 +07:00
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@@ -6,6 +6,21 @@ What Image Trace (`crates/trace`) makes of images whose right answer is known, m
compared with the numbers before each change. Nothing here is a judgement by eye, and nothing is compared
with any other product except the open tracer named under [Yardstick](#4-yardstick).
## Pieces returning to a junction
An outline piece can return to its starting junction while being fitted as an open chain. At loose
Corners/Paths settings, a nonzero positioned boundary could collapse to one cubic and lose a small
corner-touching mark. Fixed-end pieces with identical endpoints and fewer than two fitted segments
now retain their positioned boundary when its area is finite and nonzero. Fitting the shared piece
this way also gives an abutting fill exactly the reversed boundary, including within a compound path.
Six source-generated regressions check 1- and 2-pixel regions through four rotations, both windings
and large translations; fixed junctions; zero positioned area; free ends; Noise controls; and shared
edges through four rotations and black/white inversion. Noise 2 still removes the 1-pixel mark.
The white-hole Noise check is a control, not a reproduced returning-hole case. Defaults are unchanged;
these tests establish survival and edge consistency, not a new artistic-quality or parity score.
This is separate from the closed-contour recovery in #2012 and straightening guard in #2025.
## Thin closed outlines with Snap Curves To Lines
A four-pixel stair-step mark can fit as two straight cubics along the same diagonal, leaving a closed path with zero area. The outline-distance checks accept this because both sides are close to the chord. Closed fitting now checks analytic curve area after straightening and upright levelling; if a nondegenerate source loop has lost its interior, it retains the positioned boundary as an unsimplified polygon. The area integral is translated to a local origin to reduce cancellation. This numerical degeneracy check is not a new Noise or minimum-feature-area setting.
@@ -36,6 +51,7 @@ anti-aliasing):
| kind | inputs |
|:--|:--|
| 2026-10-15 | Fixed-end pieces returning to their junction retain their positioned nonzero boundary when fitted to fewer than two cubics; source-generated tests cover survival, winding, shared-edge equality, translations and Noise controls. Defaults and parity scores unchanged. |
| curved shapes | a circle; two ellipses; a rounded badge with a round hole; a letter D with its counter |
| polygons | a triangle, a turned square and a hexagon; a five-pointed star; a letter A with its counter |
| thin lines | five lines 0.5, 1, 2, 3 and 5 units wide (of a 256-unit square), a few degrees off the pixel grid |