Alpha Compositing
The notion of an alpha channel carrying the opacities of a digital image is so ordinary now in computer graphics that it may surprise you to discover that it had to be invented. The idea, however, is much deeper than even its inventors understood: Not only does the alpha channel solve the original problem of image compositing but it frees us from the tyranny of the rectangular image and paves the way for the digital convergence of the two main branches of computer picturing, the sampling theory and geometry branches.
Alvy Ray Smith and Ed Catmull invented the alpha channel in 1977. The excerpt above is from Smith's account.
Tom Porter and Tom Duff explored the algebra of the alpha channel in their 1984 SIGGRAPH paper “Compositing Digital Images,” in which they define a full suite of 16 compositing operators analogous to the 16 boolean bitblt operators. They also showed how to perform alpha computations efficiently by storing the color channels in “premultiplied alpha” form.
For the invention and development of the alpha channel, Smith, Catmull, Porter, and Duff won a technical Academy Award in 1996.
Jim Blinn gives a nice overview of compositing in his pair of 1994 columns (Part 1: Theory, Part 2: Practice; subscription required). Alvy Ray Smith also wrote two technical memos that clarify the subject: “Image Compositing Fundamentals” and “Alpha and the History of Digital Compositing.”
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