Edge detectors yield pixels in an image lie on edges.

The next step is to try to collect these pixels together into a set of edges.

Thus, our aim is to replace many points on edges with a few edges themselves.

The practical problem may be much more difficult than the idealised case.

- Small pieces of
edges may be missing,
- Small edge segments may appear to be present
due to noise where there is no real edge,
*etc*.

In general, edge linking methods can be classified into two categories:

**Local Edge Linkers**- -- where edge points are grouped to form edges by considering each point's relationship to any neighbouring edge points.
**Global Edge Linkers**- -- where all edge points in the
image plane are considered at the same time and sets of edge points
are sought according to some similarity constraint, such as points
which share the same edge equation.

David Marshall 1994-1997