Currently Spine doesn't have this feature. It's almost a select transparency and then invert, but it needs to stay 2px from non-transparent pixels (to avoid filtering issues) and Spine meshes can't have holes in them. Once we know the pixels that make up the edges, we need to convert that to a polygon. This probably requires a user setting for how detailed you'd like the polygon.

The next best thing is the New button in the Edit Mesh dialog, which lets you quickly create the mesh hull. It is slightly more time consuming, but you are likely to end up with fewer mesh vertices since you can make more intelligent and sophisticated decisions.

Have you guys been working on this since then? You could base your algorithm on the one that Cocos 2D-X has going on. Took me just 1 day to port it to our engine (Point, Rect, Clipper etc).

We've been doing other things and this has been deemed low priority, since it's not too hard to outline a mesh. Sorry! We'll get to it eventually. We do have the Generate button, which is smart about adding vertices. This enables you to 1) create the mesh hull using New, 2) create internal vertices and edges around parts of the mesh where you want to control deformation, then 3) use Generate multiple times to increase the density of the vertices.