Visibility graph --- граф видимости.
The visibility graph of a set of disjoint line segments in a plane has a vertex for every endpoint of a segment in , two of them being adjacent when the corresponding points see each other, i.e., the segment they define does not cross any segment in .
The contracted visibility graph of a set of disjoint line segments has a vertex for every segment in , two of them, and , being adjacent when some endpoint of sees some endpoint of .
Let be a set of disjoint horizontal line segments in the plane without two endpoints having equal abscissa. The bar-visibility graph has a vertex for every segment of , and two segments and are adjacent, if there is a vertical segment joining and and touching no other segment of .