Confluent Drawing Algorithms Using Rectangular Dualization ⋆ Gianluca Quercini 1 and Massimo Ancona 2 1 Institute for Advanced Computer Studies, University of Maryland College Park, MD, USA quercini@umiacs.umd.edu 2 Dipartimento di Informatica e Scienze dell’Informazione, Universit`a di Genova Genova, Italy ancona@disi.unige.it Abstract. The need of effective drawings for non-planar dense graphs is motivated by the wealth of applications in which they occur, including social network analysis, security visualization and web clustering engines, just to name a few. One common issue graph drawings are affected by is the visual clutter due to the high number of (possibly intersecting) edges to display. Confluent drawings address this problem by bundling groups of edges sharing the same path, resulting in a representation with less edges and no edge intersections. In this paper we describe how to create a confluent drawing of a graph from its rectangular dual and we show two important advantages of this approach. Keywords: Confluent Drawing, Rectangular Dualization, Orthogonal Drawing, Clustered Graphs. 1 Introduction Drawing graphs is a challenging problem, as witnessed by the wealth of ap- proaches (recently surveyed in [6]) that have been proposed over the last two decades. Far from being mere and meaningless collections of nodes and edges, graphs are effective ways to describe relationships between objects (e.g. people, molecules, computers); consequently, visualizing a graph is an important step to clearly reveal all its information. In order to be readable, a drawing must com- ply with precise aesthetic criteria on the way nodes and edges are visualized; nodes (and edges) should not be too close to one another, symmetries should be highlighted, the total area should be minimized and so on. While optimizing all such constraints is clearly impossible, a good balance is usually the right way to go to create nice drawings. Large and dense graphs are inherently difficult to draw, as the amount of information that can be visualized is limited by the size of the medium (computer screen or paper) used to render the drawing. One ⋆ This work was supported in part by the National Science Foundation under Grants IIS-10-18475, IIS-09-48548, IIS-08-12377, CCF-08-30618, and IIS-07-13501. U. Brandes and S. Cornelsen (Eds.): GD 2010, LNCS 6502, pp. 341–352, 2011. c  Springer-Verlag Berlin Heidelberg 2011