ON CHARACTERIZING TERRAIN VISIBILITY GRAPHS

A terrain is an x-monotone polygonal line in the xy-plane. Two vertices of a terrain are mutually visible if and only if there is no terrain vertex on or above the open line segment connecting them. A graph whose vertices represent terrain vertices and whose edges represent mutually visible pairs of terrain vertices is called a terrain visibility graph. We would like to find properties that are both necessary and sufficient for a graph to be a terrain visibility graph; that is, we would like to characterize terrain visibility graphs. Abello et al. [Discrete and Computational Geometry, 14(3): 331{358, 1995] showed that all terrain visibility graphs are "persistent". They showed that the visibility information of a terrain point set implies some ordering requirements on the slopes of the lines connecting pairs of points in any realization, and as a step towards showing sufficiency, they proved that for any persistent graph M there is a total order on the slopes of the (pseudo) lines in a generalized configuration of points whose visibility graph is M. We give a much simpler proof of this result by establishing an orientation to every triple of vertices, reflecting some slope ordering requirements that are consistent with M being the visibility graph, and prove that these requirements form a partial order. We give a faster algorithm to construct a total order on the slopes. Our approach attempts to clarify the implications of the graph theoretic properties on the ordering of the slopes, and may be interpreted as de fining properties on an underlying oriented matroid that we show is a restricted type of 3-signotope.