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