So how does every cut-out on your phone keep its soft edge? Guess first.

At New York Tech in the late 1970s, Ed Catmull wanted to render an object once and lay it over any background, instead of "re-rendering the same image over different backgrounds". Film's answer was a matte, "another piece of film that is transparent where the spacecraft, for example, exists in the frame and opaque elsewhere". One bit per pixel gives "no partial transparencies" at the edges: stairs. Alvy Ray Smith "started right then and by the next morning had the full package": a fourth number in every pixel, beside red, green and blue, saying how much of the pixel the picture covers. One more frame buffer cost $60,000 in 1975 dollars. The name comes from "the classic linear interpolation formula αA + (1-α)B". In Smith's words: "There is no second strip of film for the matte."

Our own lantern, skies and pixels; no frames from any film.

Did you guess the fourth number?

Sources: Alvy Ray Smith, "Alpha and the History of Digital Compositing", Microsoft Technical Memo 7, 1995.
Sounds: our own, made in code (earshot, CC0).