  Bounding hull, such as convex hull, concave hull, alpha shapes etc. has vast
applications in different areas especially in computational geometry. Alpha
shape and concave hull are generalizations of convex hull. Unlike the convex
hull, they construct non-convex enclosure on a set of points. In this paper, we
introduce another generalization of convex hull, named alpha-concave hull, and
compare this concept with convex hull and alpha shape. We show that the
alpha-concave hull is also a generalization of an NP-complete problem named
min-area TSP. We prove that computing the alpha-concave hull is NP-hard on a
set of points.

Computational Geometry