A closed loop of segments in the XY plane, "2d" only (§24).
The chain of §24's 2D list: N vertices make N segments, the last one
running from the final vertex back to the first. Everything
PolylineShape says about compositeness applies here too — a chain is
a boundary, not a filled region, which is what distinguishes it from
PolygonShape (convex, filled) and makes a concave closed outline
expressible at all.
At least three vertices, in order around the loop, with no two consecutive
vertices equal — including the closing pair (last, first). Vertices are in
the collider's local frame.
A closed loop of segments in the XY plane,
"2d"only (§24).The chain of §24's 2D list:
Nvertices makeNsegments, the last one running from the final vertex back to the first. Everything PolylineShape says about compositeness applies here too — a chain is a boundary, not a filled region, which is what distinguishes it from PolygonShape (convex, filled) and makes a concave closed outline expressible at all.