Alpha compositing

Compositing is used extensively in film when combining computer-rendered image elements with live footage.

Alpha blending is also used in 2D computer graphics to put rasterized foreground elements over a background.

The concept of an alpha channel was introduced by Alvy Ray Smith and Ed Catmull in the late 1970s at the New York Institute of Technology Computer Graphics Lab.

Bruce A. Wallace derived the same straight over operator based on a physical reflectance/transmittance model in 1981.

[2] A 1984 paper by Thomas Porter and Tom Duff introduced premultiplied alpha using a geometrical approach.

[3] The use of the term alpha is explained by Smith as follows: "We called it that because of the classic linear interpolation formula

In addition to over, Porter and Duff[3] defined the compositing operators in, held out by (the phrase refers to 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:

The most significant advantage of premultiplied alpha is that it allows for correct blending, interpolation, and filtering.

Another advantage of premultiplied alpha is performance; in certain situations, it can reduce the number of multiplication operations (e.g. if the image is used many times during later compositing).

[8] One disadvantage of premultiplied alpha is that it can reduce the available relative precision in the RGB values when using integer or fixed-point representation for the color components.

This may cause a noticeable loss of quality if the color information is later brightened or if the alpha channel is removed.

This loss of precision also makes premultiplied images easier to compress using certain compression schemes, as they do not record the color variations hidden inside transparent regions, and can allocate fewer bits to encode low-alpha areas.

Knowing whether a file uses straight or premultiplied alpha is essential to correctly process or composite it, as a different calculation is required.

The most popular image formats that support the alpha channel are PNG and TIFF.

GIF supports alpha channels, but is considered to be inefficient when it comes to file size.

The file format BMP generally does not support this channel; however, in different formats such as 32-bit (888–8) or 16-bit (444–4) it is possible to save the alpha channel, although not all systems or programs are able to read it: it is exploited mainly in some video games[9] or particular applications;[10] specific programs have also been created for the creation of these BMPs.

The RGB values of typical digital images do not directly correspond to the physical light intensities, but are rather compressed by a gamma correction function: This transformation better utilizes the limited number of bits in the encoded image by choosing

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.

More alpha channels can be added for accurate spectral color filtration applications.