alpha channel

Meaning

Noun

• A numerical value specifying a level of translucency to be applied to a colour.

Origin

• Coined by Alvy Ray Smith and Ed Catmull in the 1970s. Smith writes: "We called it that because of the classic linear interpolation formula $\backslash alpha\; A\; +\; (1-\backslash alpha)B$ that uses the Greek letter $\backslash alpha$ (alpha) to control the amount of interpolation between, in this case, two images A and B". That is, when compositing image A atop image B, the value of $\backslash alpha$ in the formula is A's alpha channel.