Matching colored points with rectangles

TL;DR

This paper studies algorithms and complexity for matching colored points with axis-aligned rectangles, providing approximation algorithms and proving NP-hardness for maximum monochromatic and bichromatic matchings.

Contribution

It introduces polynomial-time approximation algorithms with 1/4 ratio for both problems and proves NP-hardness even in restricted cases, extending previous work.

Findings

Approximation algorithms achieve at least 1/4 of optimal matchings.

Maximum monochromatic and bichromatic matchings are NP-hard to compute.

Deciding a perfect matching with same-colored points is NP-complete.

Abstract

Let $S$ be a point set in the plane such that each of its elements is colored either red or blue. A matching of $S$ with rectangles is any set of pairwise-disjoint axis-aligned rectangles such that each rectangle contains exactly two points of $S$. Such a matching is monochromatic if every rectangle contains points of the same color, and is bichromatic if every rectangle contains points of different colors. In this paper we study the following two problems: 1. Find a maximum monochromatic matching of $S$ with rectangles. 2. Find a maximum bichromatic matching of $S$ with rectangles. For each problem we provide a polynomial-time approximation algorithm that constructs a matching with at least $1/4$ of the number of rectangles of an optimal matching. We show that the first problem is $\mathsf{NP}$-hard even if either the matching rectangles are restricted to axis-aligned segments or $S$ is in general position, that is, no two points of $S$ share the same $x$ or $y$ coordinate. We further show that the second problem is also $\mathsf{NP}$-hard, even if $S$ is in general position. These $\mathsf{NP}$-hardness results follow by showing that deciding the existence of a perfect matching is $\mathsf{NP}$-complete in each case. The approximation results are based on a relation of our problem with the problem of finding a maximum independent set in a family of axis-aligned rectangles. With this paper we extend previous ones on matching one-colored points with rectangles and squares, and matching two-colored points with segments. Furthermore, using our techniques, we prove that it is $\mathsf{NP}$-complete to decide a perfect matching with rectangles in the case where all points have the same color, solving an open problem of Bereg, Mutsanas, and Wolff [CGTA (2009)].