In computer graphics, when a given image is intended to be
placed over a background, the transparent areas can be specified through a
binary mask. This way, for each intended image there are actually two bitmaps:
the actual image, in which the unused areas are given a pixel value with all
bits set to 0's, and an additional mask, in which the correspondent image areas
are given a pixel value of all bits set to 0s and the surrounding areas a value
of all bits set to 1s. In the sample at right, black pixels have the all-zero
bits and white pixels have the all-one bits.
