RGBA color space

RGBA stands for red green blue alpha. While it is sometimes described as a color space, it is actually simply a use of the RGB color model, with extra information. The color is RGB, and may belong to any RGB color space, but an integral alpha value as invented by Catmull and Smith between 1971 and 1972 enables alpha compositing. The inventors named alpha after the Greek letter in the classic linear interpolation formula α A + (1 − α) B.

The alpha channel is normally used as an opacity channel. If a pixel has a value of 0% in its alpha channel, it is fully transparent (and, thus, invisible), whereas a value of 100% in the alpha channel gives a fully opaque pixel (traditional digital images). Values between 0% and 100% make it possible for pixels to show through a background like a glass, an effect not possible with simple binary (transparent or opaque) transparency. It allows easy image compositing.

Representation

In computer graphics, pixels encoding the RGBA color space information must be stored in computer memory (or in files on disk), in well defined formats. There are several ways to encode RGBA colors, which can lead to confusion when image data is exchanged. These encodings are often denoted by the four letters in some order (e.g. RGBA, ARGB, etc). Unfortunately, the interpretation of the these 4-letter mnemonics is not well established, leading to further confusion. There are two typical ways to understand a mnemonic such as "RGBA":