Digital compositing

The basic operation used is known as 'alpha blending', where an opacity value, 'α' is used to control the proportions of two input pixel values that end up a single output pixel.

Consider three pixels;

• a foreground pixel, f
• a background pixel, b
• a composited pixel, c

• α, the opacity value of the foreground pixel. (α=1 for opaque foreground, α=0 for a completely transparent foreground). A monochrome raster image where the pixel values are to be interpreted as alpha values is known as a matte[?].

Then, considering all three colour channels, and assuming that the colour channels are expressed in a γ=1 colour space (that is to say, the measured values are proportional to light intensity), we have:

c_r = α f_r + (1 - α) b_r

c_g = α f_g + (1 - α) b_g

c_b = α f_b + (1 - α) b_b

Note that if the operations are performed in a colour space where γ is not equal to 1 then the operation will lead to non-linear effects which will be seen as aliasing artifacts (or 'jaggies') along sharp edges in the matte.

Should cite Porter-Duff[?] paper

Compositing often also includes scaling, retouching and colour correction of images.

See also:

• Gamma correction
• Bluescreen
• Digital cinema