A way of combining two images. How the mixing is performed is provided by the alpha channel. The little box that appears over the left-hand shoulder of a news anchor is put there by an alpha mixer.

Wherever the pixels of the little box appear in the frame buffer, an alpha number of “1” is put in the alpha channel. Wherever they don’t appear, an alpha number of “0” is placed. When the alpha mixer sees a “1” coming from the alpha channel, it displays the little box. Whenever it sees a “0,” it displays the news anchor. Of course, it doesn’t matter if a “1” or a “0” is used, but you get the point.