Realizable configurations of lines in pictures of polyhedra

In an idealized picture of a scene that contains only polyhedra each line segment that is recorded can have only one of four possible "meanings". In order to understand the picture it is necessary that we be able to label each line with one of the four corresponding labels:, or A " " or "." label is associated, respectively, with a convex or concave edge that has both of its two associated planes visible. A line labelled with an arrow refers to a convex edge oriented so that only one of these two planes is visible from the camera and the other is hidden behind it. The orientation of the arrow along the line is such that the planes are to the right of the arrow. If no consistent set of line labels is possible the picture is of an "impossible"--object.