Level 2 Help for DIFNEAR

The default (1,1,0,0) means use the whole image.

INTEGER - Starting line (see SIZE)

INTEGER - Starting sample (see SIZE)

INTEGER - Number of lines (see SIZE)

INTEGER - Number of samples (see SIZE)

For example, in the 3 x 3 case, the closest value will be over the
nine pixels in the window and the eight half steps to the eight
neighbors.  These values are the averages of the center pixel and
the neighbor pixel.

comparing     5 in the first image to 

4 4 7
2 6 6
3 8 8

in the second image, not using halfpix would give -1 as the dif
but using halfpix would give 0 because of the values 4 and 6.