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