Picks up the lower-completion work a crashed session left mid-debug, with one
failing test and no diagnosis.
The diagnosis is that the pass is a no-op. Not "does not reduce basin count" —
it changes *no pixel's basin at all*, zero of 9216, comparing one plateau
iteration against sixty-four. That assertion is the substance of this commit:
the original test asserted a consequence (fewer basins) which a working pass
need not produce, so it could have been satisfied by weakening it. A no-op
check cannot pass vacuously, and it is what turned an opinion into a fact.
Three candidate causes were tried and none was it. Exact float equality is
genuinely wrong and is fixed regardless — a gradient computed from 8-bit
samples is never exactly equal across a region the eye calls flat, so `==`
never fires and `<` fires everywhere; `LEVEL_EPS` now sits behind all three
comparisons. The test image is not it either: a flat disc, a terraced disc and
a constant-slope ramp all behave the same.
The finding worth keeping is about the domain rather than the code. On a
gradient-magnitude watershed every flat region of the picture is at gradient
zero, the global minimum, and a plateau with no descending exit is a minimum —
one basin already, nothing to resolve. The plateaux lower-completion is defined
for are regions of constant non-zero gradient, which are rarer in a photograph
than F1's phrasing implies. That may be the whole answer, or it may be hiding
a fourth cause; I could not close it.
So `plateau_iterations` defaults to 0. The implementation stays, correct as
far as it goes and costing nothing until someone finishes it; the test stays,
ignored with its reason; docs/segmentation.md §12 records what was ruled out so
the next attempt starts further along than this one did.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
dtourolleandClaude Opus 5
0b20436445