contour::trace_mask built a table of outgoing directions for every
pixel corner in a raster pass and then followed it. It now works out
each turn during the walk from the two pixels ahead of the current
corner: right if the one ahead on the right is outside, straight if the
one ahead on the left is, else left (so at a saddle it still turns
right and diagonal pixels stay apart). 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. The walk is bounded by the grid
size, and a mask of the wrong length now gives no loops instead of
failing an assert_eq!.
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
4x4 masks and 400 seeded 23x17 masks.
Time (best of 7, release, 4 cores): the walk alone on 2000x2000 masks
19.1 vs 26.6 ms (smooth pattern), 81.3 vs 93.2 ms (12% random pixels),
223.1 vs 238.1 ms (50% random); whole 1000x1000 traces 73.1 vs 77.1 ms
(Black and White), 249.6 vs 251.7 ms (Color, 6 colours), 214.1 vs
218.6 ms (Color, 6 colours, Noise 1), same segment counts.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018tBcdwNhptqtmX32JCGFzB
Every colour layer took a mask, component labels, a fill stack and an edge map the size of the whole image,
and as many layers were traced at once as the machine has cores.
- A layer is traced within the box its pixels lie in.
- The stack that fills components holds `u32` indices (a mask has at most 64 million pixels).
- Layers are traced as many at a time as fit in half a gigabyte (about ten bytes a pixel of the largest
layer's box each), on however many cores.
Peak memory above the image itself, photo-like image (docs/image-trace-quality.md), before -> after:
- 16 megapixels: High Fidelity Photo 3,856 -> 880 MB (2.85 -> 3.65 s), Low Fidelity Photo 1,972 -> 754 MB
(1.49 -> 1.80 s), 16 Colors 1,625 -> 298 MB, Shades of Gray 852 -> 407 MB, Default 256 -> 199 MB
- 4 megapixels: High Fidelity Photo 981 -> 643 MB, 16 Colors 408 -> 122 MB, times unchanged
The traces are the same, path for path.
