An Optimal Algorithm for Reconstructing Point Set Order Types from Radial Orderings

TL;DR

This paper presents an optimal algorithm to reconstruct all possible point set order types from a given radial system, enabling efficient orientation queries and convex hull computations, with applications to abstract order types.

Contribution

The authors introduce a new optimal algorithm for constructing convex hulls of all point sets with a given radial system, improving upon previous polynomial-time methods.

Findings

Constructs convex hulls of all point sets with a given radial system in O(n) queries

Answers orientation queries in constant time after preprocessing

Generalizes results to abstract order types

Abstract

Let $P$ be a set of $n$ labeled points in the plane. The radial system of $P$ describes, for each $p\in P$, the order in which a ray that rotates around $p$ encounters the points in $P \setminus \{p\}$. This notion is related to the order type of $P$, which describes the orientation (clockwise or counterclockwise) of every ordered triple in $P$. Given only the order type, the radial system is uniquely determined and can easily be obtained. The converse, however, is not true. Indeed, let $R$ be the radial system of $P$, and let $T(R)$ be the set of all order types with radial system $R$ (we define $T(R) = \emptyset$ for the case that $R$ is not a valid radial system). Aichholzer et al. (Reconstructing Point Set Order Types from Radial Orderings, in ISAAC 2014) show that $T(R)$ may contain up to $n-1$ order types. They also provide polynomial-time algorithms to compute $T(R)$ when only $R$ is given. We describe a new algorithm for finding $T(R)$. The algorithm constructs the convex hulls of all possible point sets with the radial system $R$. After that, orientation queries on point triples can be answered in constant time. A representation of this set of convex hulls can be found in $O(n)$ queries to the radial system, using $O(n)$ additional processing time. This is optimal. Our results also generalize to abstract order types.