Morphing Planar Graph Drawings with Unidirectional Moves

Alamdari et al. [1] showed that given two straight-line planar drawings of a graph, there is a morph between them that preserves planarity and consists of a polynomial number of steps where each step is a linear morph that moves each vertex at constant speed along a straight line. An important step in their proof consists of converting a pseudo-morph (in which contractions are allowed) to a true morph. Here we introduce the notion of unidirectional morphing step, where the vertices move along lines that all have the same direction. Our main result is to show that any planarity preserving pseudo-morph consisting of unidirectional steps and contraction of low degree vertices can be turned into a true morph without increasing the number of steps. Using this, we strengthen Alamdari et al.’s result to use only unidirectional morphs, and in the process we simplify the proof.