On layered area-proportional rectangle contact representations

Abstract

Semantic word clouds visualize the semantic relatedness between the words of a text by placing pairs of related words close to each other. Formally, the problem of drawing semantic word clouds corresponds to drawing a rectangle contact representation of a graph whose vertices correlate to the words to be displayed and whose edges indicate that two words are semantically related. The goal is to maximize the number of realized contacts while avoiding any false adjacencies. We consider a variant of this problem that restricts input graphs to be layered and all rectangles to be of equal height, called Maximum Layered Contact Representation Of Word Networks or Max-LayeredCrown, as well as the variant Max-IntLayeredCrown, which restricts the problem to only rectangles of integer width and the placement of those rectangles to integer coordinates.

We classify the corresponding decision problem k-IntLayeredCrown as NP-complete even for internally triangulated planar graphs and k-LayeredCrown as NP-complete for planar graphs. We introduce three algorithms: a 1/2-approximation for Max-LayeredCrown of internally triangulated planar graphs, and a PTAS and an XP algorithm for Max-IntLayeredCrown with rectangle width polynomial in n.