Interpreting line drawings of curved objects with tangential edges and surfaces

This paper extends previous work on the interpretation of line drawings of curved objects to include objects with tangential edges and surfaces. We present a catalogue of labelled junctions and show that the existence of a legal global labelling can be determined in linear time. However, minimising the number of invisible vertices necessary to produce a legal global labelling is shown to be NPhard. The potential combinatorial ambiguity inherent in the drawings can be mitigated by using information from the intensity image to identify points at which the object surface is tangential to the viewing direction.