Smallest bounding geometry of an object or group of objects using concave angles. Also known as an alpha hull or shape.
A concave hull, also referred to as an alpha hull or shape, is the smallest bounding geometry of an object or group of objects using concave angles. This is in contrast to a convex-hull. Where a convex hull is the equivalent of putting a rubber band around a group of points, a concave hull would pull the rubber band in to eliminate any large areas with no points or more closely follow an outer extent of the points.
Some graphic examples are presented at http://ubicomp.algoritmi.uminho.pt/local/concavehull.html