with premultiplied alpha, pre-multiplication is done in linear space, prior to gamma compression. This results in the following formula: Note that only the color components undergo gamma-correction; the alpha channel is always linear. Other transparency methods Although used for similar purposes, transparent colors and image masks do not permit the smooth blending of the superimposed image pixels with those of the background (only whole image pixels or whole background pixels allowed). A similar effect can be achieved with a 1-bit alpha channel, as found in the 16-bit RGBA high color mode of the Truevision TGA image file format and related TARGA and AT-Vista/NU-Vista display adapters' high color graphic mode. This mode devotes 5 bits for every primary RGB color (15-bit RGB) plus a remaining bit as the "alpha channel". Screendoor transparency can be used to simulate partial occlusion where only 1-bit alpha is available. For some applications, a single alpha channel is not sufficient: a stained-glass window, for instance, requires a separate transparency channel for each RGB channel to model the red, green and blue transparency separately. More alpha channels can be added for accurate spectral color filtration applications. Some order-independent transparency methods replace the over operator with a commutative approximation. History The concept of an alpha channel was introduced by Alvy Ray Smith and in the late 1970s at the New York Institute of Technology Computer Graphics Lab, and fully developed in a 1984 paper by Thomas Porter and Tom Duff. The use of the term alpha is explained by Smith as follows: "We called it that because of the classic linear interpolation formula that uses the Greek letter (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 in the formula is taken directly from A's alpha channel. See also References External links Compositing Digital Images - Thomas Porter and Tom Duff (Original Paper) Image Compositing Fundamentals Understand Compositing and Color extensions in SVG 1.2 in 30 minutes! Alpha Matting and Premultiplication Computer graphics algorithms