Y is the luminance value. Since it is slightly relevant to the skin
color, the distribution of Cb and Cr values only is used. Fig. 2(a) and
(b) shows the distribution of skin color pixels of Cb and Cr values. The
x-axis shows the values of Cb and Cr ranging from 0 to 255 while the
y-axis normalizes the number of distributed pixels. Fig. 2 shows that
the distribution of Cb and Cr values is far smaller than that of RGB
values and is thus suitable for the skin color modeling of the skin
color region classifier.