Computing Cartograms with Optimal Complexity

TL;DR

This paper presents an optimal method for constructing rectilinear duals called cartograms with polygons of at most 8 sides, improving previous complexity bounds and providing linear-time algorithms for specific graph classes.

Contribution

It introduces an optimal 8-sided polygon construction for cartograms, along with linear-time algorithms for Hamiltonian maximal planar graphs, and proves necessity of 8 sides in certain cases.

Findings

Optimal 8-sided polygons for cartograms are sometimes necessary.

Linear-time algorithms for Hamiltonian maximal planar graphs.

Lower bound example showing 8 sides are necessary.

Abstract

In a rectilinear dual of a planar graph vertices are represented by simple rectilinear polygons and edges are represented by side-contact between the corresponding polygons. A rectilinear dual is called a cartogram if the area of each region is equal to a pre-specified weight of the corresponding vertex. The complexity of a cartogram is determined by the maximum number of corners (or sides) required for any polygon. In a series of papers the polygonal complexity of such representations for maximal planar graphs has been reduced from the initial 40 to 34, then to 12 and very recently to the currently best known 10. Here we describe a construction with 8-sided polygons, which is optimal in terms of polygonal complexity as 8-sided polygons are sometimes necessary. Specifically, we show how to compute the combinatorial structure and how to refine the representation into an area-universal rectangular layout in linear time. The exact cartogram can be computed from the area-universal rectangular layout with numerical iteration, or can be approximated with a hill-climbing heuristic. We also describe an alternative construction for Hamiltonian maximal planar graphs, which allows us to directly compute the cartograms in linear time. Moreover, we prove that even for Hamiltonian graphs 8-sided rectilinear polygons are necessary, by constructing a non-trivial lower bound example. The complexity of the cartograms can be reduced to 6 if the Hamiltonian path has the extra property that it is one-legged, as in outer-planar graphs. Thus, we have optimal representations (in terms of both polygonal complexity and running time) for Hamiltonian maximal planar and maximal outer-planar graphs.