where m is the mean luminance value, yj indexes the
luminance value, nj is the number of points at luminance
level yj, N is the total number of points in the region and
σi is the pseudo-variance of the region. The summation in
(4) is carried out over all pixels belonging to one region
and subscript i Є [1, 3] refers to the region being
processed. The term pseudo-variance is used since the
summation in (4) uses the absolute difference instead of
square differences. This reduces the hardware
complexity.