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