The term of art for that is “convex hull” – and there are several tools that do it: the Mesh library, and openCV (I believe), and you can also just implement the algorithm directly.
A Convex Hull is the encompassing shape around a group of points. As if you were to wrap a piece of string around all of the points. This is handy when doing collision tests on complex shapes, or finding the most extreme points within a dataset.
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For a recent related discussion see: