Rectilinear Planarity of Partial 2-Trees

TL;DR

This paper presents improved algorithms for testing rectilinear planarity of partial 2-trees, reducing complexity from O(n^3 log n) to O(n^2), and achieving linear time in specific cases, advancing understanding of orthogonal graph drawings.

Contribution

It introduces a new O(n^2)-time algorithm for rectilinear planarity testing of partial 2-trees and an O(n)-time solution for special cases, improving previous bounds.

Findings

O(n^2) algorithm for general partial 2-trees

O(n) algorithm for special partial 2-trees

Enhanced understanding of orthogonal spirality

Abstract

A graph is rectilinear planar if it admits a planar orthogonal drawing without bends. While testing rectilinear planarity is NP-hard in general (Garg and Tamassia, 2001), it is a long-standing open problem to establish a tight upper bound on its complexity for partial 2-trees, i.e., graphs whose biconnected components are series-parallel. We describe a new O(n^2)-time algorithm to test rectilinear planarity of partial 2-trees, which improves over the current best bound of O(n^3 \log n) (Di Giacomo et al., 2022). Moreover, for partial 2-trees where no two parallel-components in a biconnected component share a pole, we are able to achieve optimal O(n)-time complexity. Our algorithms are based on an extensive study and a deeper understanding of the notion of orthogonal spirality, introduced several years ago (Di Battista et al, 1998) to describe how much an orthogonal drawing of a subgraph is rolled-up in an orthogonal drawing of the graph.