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