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Edge Tracking Procedure
for each edge pixel P {
   classify its pixel type using masks
   case
       1. isolated point :             ignore it
       2.  start point :                  make a new segment
       3.  interior point :             add to current segment
       4.  end point :                    add to current segment and finish it
       5.  junction or corner :      add to incoming segment
                                                 finish incoming segment
                                                 make new outgoing segment(s)
The ORT package uses a fancier corner finding approach.