RGBA color space

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RGBA color space stands for Red Green Blue Alpha. The color is RGB as usual, but an integral alpha value as invented by Catmull and Smith between 1971 and 1972 enables alpha blending and alpha compositing. The inventors named alpha after the Greek letter in the classic linear interpolation formula αA + (1-α)B.

The alpha channel is the 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 (translucency) or retain their anti-aliasing no matter the background, effects not possible with simple binary (zero/full) transparency. It allows easy image compositing and, in HTML pages, inclusion of images with no background (specially if page background is multi-colour).
Alpha channel can be put as a percentage or as a 0-to-255 scale, like RGB parameters. It also can be written as a decimal number (like percentage. 1.0 is fully opaque and 0.0 is fully transparent.

Sometimes this is referred as ARGB (like RGBA, but first datum is alpha), which is used, for example, in Macromedia products terminology. For example, 0x80FFFF00 is 50%-transparent yellow, because all parameters are expressed on a 0-to-255 scale. 0x80 is 128, very approximately half 255 (alpha 50%); 0xFF is 255, the greatest value a parameter can have (pure red); the second 0xFF is like the previous, but for green; and 0x00 is 0 in decimal (no blue). Red, green, and half transparency mixture are 50% transparent yellow.

PNG is the best known image format that uses the RGBA color space.