summing the negative of {HEAD ~ the pixel values} of image pixels corresponding to each of {TAIL ~ columns} 2M+1 to M+2 in said next row and adding the pixel values of image pixels corresponding to each of columns M to 1 in said next row to produce a next row value 
summing the negative of { HEAD ~ the pixel values } of image pixels corresponding to each of { TAIL ~ columns } 2M+1 to M+2 in said next row and adding the pixel values of image pixels corresponding to each of columns M to 1 in said next row to produce a next row value
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH 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 OTH OTH OTH OTH OTH OTH