Thus, the value {HEAD ~ Δe} is used as {TAIL ~ a threshold} to check whether an extremum is an edge or an error 
Thus , the value { HEAD ~ Δe } is used as { TAIL ~ a threshold } to check whether an extremum is an edge or an error
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH