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Junction Types

It is now possible to say that all line junctions will belong to one of four types, as shown in Fig. 43.


Fig. 43 Types of junction in a line drawing

Consideration of what such junctions can represent in a view of a polyhedral model leads to the conclusion that only certain labellings of the edges at a junction can arise in images of a real scene. These are illustrated in Fig. 44.


Fig. 44 Permissible labellings at a junction

There are far fewer of these than the total number of all possible labellings. Some of the permissible labellings are shown occurring in a scene in Fig. 45.


Fig. 45 Labelled lines in an image

One immediate deduction that can be made is that T-junctions only occur where one object occludes another, or perhaps an object occludes another part of itself.