# How to avoid collisions in 3D-realizations for moving graphs

## TL;DR

This paper presents a method to ensure collision-free 3D realizations of moving graphs using L-linkages, providing conditions and algorithms for their construction, especially for Dixon-1 graphs.

## Contribution

It introduces a sufficient condition for collision-free L-linkages and an algorithm to construct them, with specific results for Dixon-1 and Dixon-2 moving graphs.

## Key findings

- Dixon-1 moving graphs always have collision-free L-linkages.
- Dixon-2 moving graphs do not have collision-free L-linkages.
- A new algorithm guides the construction of collision-free L-linkages.

## Abstract

If we parameterize the positions of all vertices of a given graph in the plane such that distances between adjacent vertices are fixed, we obtain a moving graph. An L-linkage is a realization of a moving graph in 3D-space, by representing edges using horizontal bars and vertices by vertical sticks. Vertical sticks are parallel revolute joints, while horizontal bars are links connecting them. We give a sufficient condition for a moving graph to have a collision-free L-linkage. Furthermore, we provide an algorithm guiding the construction of such a linkage when the moving graph fulfills the sufficient condition, via computing a height function for the edges (horizontal bars). In particular, we prove that any Dixon-1 moving graph has a collision-free L-linkage and no Dixon-2 moving graphs have collision-free L-linkages, where Dixon-1 and Dixon-2 moving graphs are two classic families of moving graphs.