Histogram Equalization (HE)
HE [8] attempts to improve the contrast of an input
image by stretching the peaks of the histogram and
compressing the troughs. Considering a digital image
which has N pixels distributed in M discrete luminance
levels (assuming luminance processing only). Let nk be
the number of pixels with luminance yk , where k Є [0,
M-1]. Then the probability density function (PDF) of the
image is