Implement 2D minkowski() as a boundary sweep

[revarbat]

minkowski() only ever looked at 3D bodies. 2D sections among its children were dropped on the floor — silently:

linear_extrude(6) minkowski() { square([30,20], center=true); circle(4, $fn=24); }

produced no geometry at all, where the reference produces the rounded square asked for. No warning; the whole statement just vanished from the model. Found while writing an example for minkowski(), which rendered two of its three copies and nothing where that one should have been.

How

Manifold has no 2D Minkowski, so this builds one from the boundary-sweep identity: for B convex and containing the origin,

A (+) B  =  A  ∪  (∂A (+) B)

and ∂A is a chain of segments, each of which sums with a convex B to the hull of B at its two ends. So A is never cut up at all — however concave it is, however many holes it has, only its edges are walked. Only B is decomposed, and only because the per-segment hull needs it convex.

That is one hull per edge of A per convex piece of B, and the piece count is one in the case that actually turns up: a circle swept over something.

Both conditions on B are met rather than assumed, and each is silently wrong on its own, so each has its own test:

skipped step	what it costs
decomposing a concave B (hulling it instead)	computes A ⊕ hull(B) — 5092 against the reference's 4992
shifting B onto the origin and back	keeps an untranslated copy of A: starts at x=0 where the reference starts at x=16
walking every contour	a hole is left unswept

Matching the reference

Six cases, all matching the reference exactly:

	ours	reference
convex + circle	6298.16	6298.16
concave A + circle	5120.47	5120.47
concave A + concave B	4992.00	4992.00
B away from the origin	5120.47	5120.47
three operands	4670.21	4670.21
A with a hole	4141.98	4141.98

A mix of 2D and 3D children still takes the 3D path, as before.

Testing

Four tests: a convex outline rounded to the right size and bounding box, a concave one that must stay concave, a single section handed back untouched, and 3D still working.

The concave test took three attempts to become worth having, and the negative control is why. Substituting Hull(parts) for the union — the obvious wrong implementation — first passed a volume bound, because hulling lands within 1% of the right answer on this shape. It then passed a probe of the notch, because I had put the probe outside both shapes, where it proved nothing. It now sits inside what a hull would cover and outside the real shape, and catches the substitution.

825 tests pass. Negative-controlled: hulling the sweeper, skipping the origin shift, and walking only the first contour each fail exactly their own test and nothing else.

🤖 Generated with Claude Code