This can pretty quickly get an okay approximation of the shape of a concave sprite. I originally wanted to do this with a giant circle but had to do it the cheaty way and use a rotating square instead due to Scratch limitations. It is able to be faster than it otherwise would be because it uses a binary search, and the min and max constraints are adjusted based on the previous iteration, there's only a 20-pixel range of deviation possible between each one-degree turn allowing five iterations of a binary search to do the trick.
If there is a divot in the outline that you want to keep you can split the costume along that divot to split the work between two easier to digest costumes. Strange errors occurred when I added the part that actually extrapolates the shape based on the planes, this should be easy to fix just by culling the points that are outside of the planes. Or by changing the algorithm that calculates them in the first place.
