Obscuring edges, convex edges, and concave edges.
Physical constraints limit the number of ways
that scene features can form vertices.
Most combinations of line labels simply cannot occur at any
junction.
Furthermore, labeling for adjacent junctions must match
because the nature of a line cannot change along its length.
This makes the selection of labeling
as the assembly of a jigsaw puzzle
because previous selections strongly limit each new choice.
In our sample scene the L junction
starts with the complete set of 123 possible labels
and this number is immediately reduced to 76
because the relative region brightnesses precludes
certain shadow labelings.
Next the mutual constraints between junctions
have an effect the hourly feelings
are reduced from the initial 79 to the 52 compatible
with the neighboring L.
Similarly, only 32 of the options for the L
are now compatible with the remaining arrow labeling.
Sometimes the analysis of a particular junction
can cause and effect to propagate
for a considerable distance paths propagate
through the drawing until activity
dies at junctions where no new changes are forced.
By the time the last junction is considered
the overall arrangement typically
forces a single interpretation for the entire line drawing.
After determining basic facts about the lines
other specialist programs attempt
to find the identity, location, and dimensions.