There are six possible L-junctions, as shown below. There are three possible W-junctions, as shown below. There are three possible Y-junctions, as shown below. Note that the orientation of a Yabd junction cannot be determined by considering the angle sizes, so some junction catalogues list Ybda and Ydab as separate junction types. . The remaining four junction types appear because an occluding line carries no topological information about what it occludes. .
Several algorithms exist for propagating line labels through a sketch.
That described by Kanatani can be recommended. It is not clear that this can be extended in a straightforward way to sketches of non-trihedral objects. . Background Reading Index References and Papers Sketch Interpretation System Index .