  We study unit disk visibility graphs, where the visibility relation between a
pair of geometric entities is defined by not only obstacles, but also the
distance between them. That is, two entities are not mutually visible if they
are too far apart, regardless of having an obstacle between them. This
particular graph class models real world scenarios more accurately compared to
the conventional visibility graphs. We first define and classify the unit disk
visibility graphs, and then show that the 3-coloring problem is NP-complete
when unit disk visibility model is used for a set of line segments (which
applies to a set of points) and for a polygon with holes.
