The value of {HEAD ~ each local extremum pixel} is modified as {TAIL ~ a function} of the current local reference point 
The value of { HEAD ~ each local extremum pixel } is modified as { TAIL ~ a function } of the current local reference point
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH