Each of {HEAD ~ these bits} in {TAIL ~ a given line} is compared with a dither coded second stream of bits generated in response to the background illumination along a line immediately adjacent to the line being scanned 
in
Each of { HEAD ~ these bits } in { TAIL ~ a given line } is compared with a dither coded second stream of bits generated in response to the background illumination along a line immediately adjacent to the line being scanned
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH