Junction Types

It is now possible to say that all line junctions will belong to one of four types, as shown in Fig. 40.

 

Fig. 40 Types of junction in a line drawing

• At an L-junction only two edges meet,
  
• Three edges meet at the other junction types.
  
• The distinction between an Arrow-junction and a Y-junction is whether the three edges subtend a total angle of less or more than .
  
• The dividing case between an Arrow-junction and a Y-junction is specifically identified as a T-junction.

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. 41.

 

Fig. 41 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. 42.

 

Fig. 42 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.