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