On Visibility Graphs of Convex Fans and Terrains

For two points in the closure of a simple polygon P, we say that they see each other in P if the line segment uniting them does not intersect the exterior of P. The visibility graph of P is the graph whose vertex set is the vertex set of P and two vertices are joined by an edge if they see each other in P. The characterization of visibility graphs has been an open problem for decades, and a significant effort has been made to characterize visibility graphs of restricted polygon classes. Among them is the convex fan: a simple polygon with a convex vertex (i.e. whose internal angle in less than 180 degrees) that sees every other point in the closure of the polygon (often called kernel vertex). We show that visibility graphs of convex fans are equivalent to visibility graphs of terrains with the addition of a universal vertex, that is, a vertex that is adjacent to every other vertex.