Waltz labelling algorithm
Take the line drawing and on each vertex heap up the possible junctions. Now suppose we exit the junction along an edge via a (+) label, for example. This would mean we must enter the next junction via a (+) label. Suppose there was no such possibility: we should obviously discount the initial choise of junction and so on. Thus we travel around loops propagating the constraints, until we can do no more in that loop. Then we pass to another and so on. Only when there is a complete irreducible interpretation do we stop.
Ambiguous drawings
All 4 labellings shown below are consistent and correspond to different objects
and backgrounds.
Problems
