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