What do you call "concave hull"?
If that's what I think, there can be lot of concave hulls from a set of points, while there is only one possible convex hull.

hi,

sorry forgot to attach the link:

I see. It relies on the envelope of points close enough to make a continuous path, without going too deep and without closing gaps. Perhaps an algorithm like the Marching Squares is usable there. I haven't meet a library supporting it, yet.

hi, many thanks for this information and I hope that someone could make a lib or share a open source.