Concave 2D colliders

A table can be one dynamic body again. collider2d takes a convex_decomposition kind: it cuts the polygon into convex pieces, then grows each piece through the seams it shares.

Cut a table into a top and two legs and you leave seams. A beam pushed into one is three collision pairs that disagree, and it stays put. Erin Catto's Stuck Inside overlaps the pieces by hand. This does it for you: each piece carries its two edges at a seam on into the neighbour and keeps what they enclose.

overlap is the dial, 0.9 by default. At 0 you get the plain cut; at 1 a piece grows flush with whatever stopped it.

What it costs and what it buys

• A piece keeps a seam only while it stays convex. Without that rule, a piece with several seams on a spiky outline grows a few percent of its area past the polygon. A sampling test over 25 shapes guards it.
• overlap scales depth, not count, so pieces pile up. A drawn outline stacks two or three pieces on a point, and a 240-point star stacks 80.
• The cost, one box tested against that star, per placement:

kind	ns
trimesh	118
convex_decomposition, overlap 0	187
convex_decomposition, overlap 0.9	367

• What it buys, a beam started inside the table, after four seconds:

kind	result
trimesh, polyline, overlap 0	still inside
convex_hull	out, with the gap under the table filled in
overlap 0.9	out

• A trimesh is the cheaper static collider when bodies only touch it from outside. This kind is for dynamic bodies, and for static ones a body can end up inside.

Also

• Mass comes from the ungrown pieces, so a table weighs what its area says.
• A solids-mode Godot CollisionPolygon2D imports as this kind.
• geometry2d.convex_decomposition hands the pieces to a script.
• 3D keeps VHACD. Its hulls share no faces, so there is nothing to grow across. Overlapping hulls are authored by hand there, as several colliders on one body.