Everyone knows about using alpha to create composite images. But what does alpha really mean? Is it how much of a pixel is covered by a fragment? Is it the opacity of the color in that pixel? Is it some combination of these things, or maybe even something else?

The graphics literature is very fluid on this question, interpreting alpha pretty much in whatever way is most convenient on a moment-to-moment basis. In this technical note, we look closely at alpha and the compositing process using the over operator, and come to a clear understanding of what alpha represents.