Fun with premultiplied alpha
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The computer graphics universe consists of pixels. Pixels, in turn, consist of components: red, green, blue, and the coverage or opacity value alpha. For various reasons it is convenient to store and process a given rgb/spl alpha/ quadruple with the rgb values already multiplied by /spl alpha/. This was first pointed out in the original Porter-Duff compositing paper (1984) and I presented some further justifications in an earlier column ("Image Compositing-Theory", ibid., p.83-7, Sept. 1994). This premultiplication has some other interesting implications, and that's what I discuss this time.
[1] James F. Blinn,et al. Blue screen matting , 1996, SIGGRAPH.
[2] Tom Duff,et al. Compositing digital images , 1984, SIGGRAPH.