Editing Alpha compositing (section)

==Description==
In a 2D image a color combination is stored for each picture element (pixel), often a combination of red, green and blue ([[RGB color model|RGB]]). When alpha compositing is in use, each pixel has an additional numeric value stored in its '''alpha channel''', with a value ranging from 0 to 1. A value of 0 means that the pixel is fully [[transparency and translucency|transparent]] and the color in the pixel beneath will show through. A value of 1 means that the pixel is fully opaque.

With the existence of an alpha channel, it is possible to express compositing image operations using a ''compositing algebra''. For example, given two images ''A'' and ''B'', the most common compositing operation is to combine the images so that ''A'' appears in the foreground and ''B'' appears in the background. This can be expressed as ''A'' '''over''' ''B''. In addition to '''over''', Porter and Duff<ref name=Duff/> defined the compositing operators '''in''', '''held out by''' (the phrase refers to [[Matte (filmmaking)#Garbage and holdout mattes|holdout matting]] and is usually abbreviated '''out'''), '''atop''', and '''xor''' (and the reverse operators '''rover''', '''rin''', '''rout''', and '''ratop''') from a consideration of choices in blending the colors of two pixels when their coverage is, conceptually, overlaid orthogonally:

[[Image:Alpha compositing.svg]]

As an example, the '''over''' operator can be accomplished by applying the following formula to each pixel:<ref name=Wallace/>
:<math>\begin{align}\alpha_o &= \phantom{~C_a}\alpha_a + \phantom{C_b}\alpha_b(1 - \alpha_a) \\
C_o &= \frac{ C_a \alpha_a + C_b \alpha_b (1 - \alpha_a) }{\alpha_o}
\end{align}</math>

Here <math>C_o</math>, <math>C_a</math> and <math>C_b</math> stand for the color components of the pixels in the result of the "over", image A, and image B respectively, applied to each color channel (red/green/blue) individually, whereas <math>\alpha_o</math>, <math>\alpha_a</math> and <math>\alpha_b</math> are the alpha values of the respective pixels.

The '''over''' operator is, in effect, the normal painting operation (see [[Painter's algorithm]]). The '''in''' and '''out''' operators are the alpha compositing equivalent of [[Clipping path|clipping]]. The two use only the alpha channel of the second image and ignore the color components. In addition, '''plus''' defines additive blending.<ref name=Duff/>