a comparison module configured to compare the brightness of each pixel in the local region to determine {HEAD ~ the darkest pixels} in {TAIL ~ the region}; and 
in
a comparison module configured to compare the brightness of each pixel in the local region to determine { HEAD ~ the darkest pixels } in { TAIL ~ the region } ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH