{HEAD ~ Each edge area pixel} is detected to be a local extremum pixel or {TAIL ~ a nonextremum pixel} 
is detected to be
{ HEAD ~ Each edge area pixel } is detected to be a local extremum pixel or { TAIL ~ a nonextremum pixel }
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL