Convex blocking and partial orders on the plane

Let C = {c(1),..., c(n)} be a collection of disjoint closed bounded convex sets in the plane. Suppose that one of them, say c(1), represents a valuable object we want to uncover, and we are allowed to pick a direction alpha is an element of [0, 2 pi) along which we can translate (remove) the elements of C, one at a time, while avoiding collisions. We study the problem of finding a direction alpha(0) such that the number of elements that have to be removed along alpha(0) before we can remove c(1) is minimized. We prove that if we have the sorted set D of directions defined by the tangents between pairs of elements of C, we can find alpha(0) in O(n(2)) time. We also discuss the problem of sorting D, in o(n(2)logn) time. 2015 Elsevier B.V. All rights reserved.