Digital compositing

Digital compositing is the process of digitally assembling multiple to make a final image, typically for print, motion pictures or screen display. It is the digital analogue of optical film compositing. The basic operation used in digital compositing 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. As a simple example, suppose two images of the same size are available and they are to be composited. The input images are referred to as the foreground image and the background image. Each image consists of the same number of pixels. Compositing is performed by mathematically combining information from the corresponding pixels from the two input images and recording the result in a third image, which is called the composited image. Consider three pixels; a foreground pixel, f a background pixel, b a composited pixel, c and α, the opacity value of the foreground pixel. (α = 1 for an opaque foreground, α = 0 for a completely transparent foreground). A monochrome 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: cr = α fr + (1 − α) br cg = α fg + (1 − α) bg cb = α fb + (1 − α) bb 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 can potentially be seen as aliasing artifacts (or "jaggies") along sharp edges in the matte. More generally, nonlinear compositing can have effects such as "halos" around composited objects, because the influence of the alpha channel is non-linear. It is possible for a compositing artist to compensate for the effects of compositing in non-linear space. Performing alpha blending is an expensive operation if performed on an entire image or 3D scene.