Witness (Delaunay) Graphs

Proximity graphs are used in several areas in which a neighborliness relationship for input data sets is a useful tool in their analysis, and have also received substantial attention from the graph drawing community, as they are a natural way of implicitly representing graphs. However, as a tool for graph representation, proximity graphs have some limitations that may be overcome with suitable generalizations. We introduce a generalization, witness graphs, that encompasses both the goal of more power and flexibility for graph drawing issues and a wider spectrum for neighborhood analysis. We study in detail two concrete examples, both related to Delaunay graphs, and consider as well some problems on stabbing geometric objects and point set discrimination, that can be naturally described in terms of witness graphs.Comment: 27 pages. JCCGG 200

Rectangular Layouts and Contact Graphs

Contact graphs of isothetic rectangles unify many concepts from applications including VLSI and architectural design, computational geometry, and GIS. Minimizing the area of their corresponding {\em rectangular layouts} is a key problem. We study the area-optimization problem and show that it is NP-hard to find a minimum-area rectangular layout of a given contact graph. We present O(n)-time algorithms that construct $O(n^2)$-area rectangular layouts for general contact graphs and $O(n\log n)$-area rectangular layouts for trees. (For trees, this is an $O(\log n)$-approximation algorithm.) We also present an infinite family of graphs (rsp., trees) that require $\Omega(n^2)$ (rsp., $\Omega(n\log n)$) area. We derive these results by presenting a new characterization of graphs that admit rectangular layouts using the related concept of {\em rectangular duals}. A corollary to our results relates the class of graphs that admit rectangular layouts to {\em rectangle of influence drawings}.Comment: 28 pages, 13 figures, 55 references, 1 appendi

Planar Open Rectangle-of-Influence Drawings

A straight line drawing of a graph is an open weak rectangle-of-influence (RI) drawing, if there is no vertex in the relative interior of the axis parallel rectangle induced by the end points of each edge. Despite recent interest of the graph drawing community in rectangle-of-influence drawings, no algorithm is known to test whether a graph has a planar open weak RI-drawing, not even for inner triangulated graphs. In this thesis, we have two major contributions. First we study open weak RI-drawings of plane graphs that must have a non-aligned frame, i.e., the graph obtained from removing the interior of every filled triangle is drawn such that no two vertices have the same coordinate. We introduce a new way to assign labels to angles, i.e., instances of vertices on faces. Using this labeling, we provide necessary and sufficient conditions characterizing those plane graphs that have open weak RI-drawings with non-aligned frame. We also give a polynomial algorithm to construct such a drawing if one exists. Our second major result is a negative result: deciding if a planar graph (i.e., one where we can choose the planar embedding) has an open weak RI-drawing is NP-complete. NP-completeness holds even for open weak RI-drawings with non-aligned frames

A General Approach to Dominance in the Plane

Given two points p and q and a set of points 0 in the plane, p is said to dominate q with respect to O if p dominates q and there is no o O such that p dominates o and o dominates q. In other words, O is a set of obstacles that might block the "rectangular view" from p to q. Given sets P and O we are interested in determining all pairs (p, q) P x P such that p dominates q with respect to O. This generalizes hotions of direct dominance and rectangular visibility that have been studied before. An algorithm is presented that solves the problem in optimal time O(nlogn k), where n is the size of P U O and k is the number of answers. A second problem asks to store the sets P and O such that queries of the form "given a query point q, compute all points p in P, such that q dominates p with respect to O" can be answered efficiently. A static structure is devised with a query time of O(logn k) using O(nlo 2 n) storage. Using a different approach, we devise a fully dynamic structure in which queries cost O(log 2 n k) time

A general approach to dominance in the plane

Given two points p and q and a (finite) set of points O in the plane, p is said to dominate q with respect to O if p dominates q and there is no o e O such that p dominates o and o dominates q. In other words, O is a set of obstacles that might block the "rectangular view" from p to q. Given sets P and O we are interested in determining all pairs (p, q) e P × P such that p dominates q with respect to O. This generalizes notions of direct dominance and rectangular visibility that have been studied before. An algorithm is presented that solves the problem in optimal time O(n log n + k), where n is the size of P O and k is the number of answers. We also study query versions of the problem in which we ask for all points that are dominated with respect to O by a given query point. Both static and dynamic data structures are presented. Finally, the notion of dominance with respect to obstacles is extended to obstacle sets that may contain arbitrary objects