RGBA color space

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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 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 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 (translucency), an effect not possible with simple binary (transparent or opaque) transparency. It allows easy image compositing. Alpha channel values can be expressed as a percentage, integer, or real number between 0 and 1 like RGB parameters.

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, 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 an image format that uses RGBA.