The skin color-filtered image contains tiny pixel areas or image noise from the background or the
clothes color which are similar to the skin. The image noise can prevent a clear split in a later step,
so we have to remove it. As a result of the previous step to produce the black and white images,
we can use a simple binary erosion operator to reduce the noise. At the same time, considering
that erosion operation will shrink the face region which causes some holes, we can use the
following morphological operator to fill holes and enlarge the face regions.
Let f be an image of size m x n, for a given pixel (x,y) in image f, as you can see in equation 4