RGBA color space

(Redirected from RGBAX)
"ARGB" redirects here. For the colorspace developed by Adobe in the late 1990s, see Adobe RGB color space.
Example of an RGBA image with translucent and transparent portions, displayed on a white background

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 alpha channel 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 these 4-letter mnemonics is not well established, leading to further confusion. There are two typical ways to understand a mnemonic such as "RGBA":

• In the byte-order scheme, "RGBA" is understood to mean a byte R, followed by a byte G, followed by a byte B, and followed by a byte A. This scheme is commonly used for describing file formats or network protocols, which are both byte-oriented.
• In the word-order scheme, "RGBA" is understood to represent a complete 32-bit word, where R is more significant than G, which is more significant than B, which is more significant than A. This scheme can be used to describe the memory layout on a particular system. Its meaning varies depending on the endianness of the system.

In a big-endian system, the two schemes are equivalent. This is not the case for a little-endian system, where the two mnemonics are reverses of each other. Therefore, to be unambiguous, it is important to state which ordering is used when referring to the encoding.

Format As byte-order As word-order
Little-endian Big-endian Little-endian Big-endian
RGBA (byte-order) RGBA8888 ABGR32 RGBA32
ARGB (word-order) BGRA8888 ARGB8888 ARGB32
RGBA (word-order) ABGR8888 RGBA8888 RGBA32

RGBA (byte-order)

In OpenGL and Portable Network Graphics (PNG), the RGBA (byte-order) is used, where the colors are stored in memory such that R is at the lowest address, G after it, B after that, and A last. On a little endian architecture this is equivalent to ABGR (word-order).[1]

Even when there are more than 8 bits per channel (such as 16 bits or floating-point), the channels are still stored in RGBA order. In PNG, the channels are stored as 16-bit integers in network order (big-endian).

ARGB (word-order)

In the ARGB (word-order) encoding the intensity of each channel sample is defined by 8 bits, and are arranged in memory in such manner that a single 32-bit unsigned integer has the alpha sample in the highest 8 bits, followed by the red sample, green sample and finally the blue sample in the lowest 8 bits:

ARGB values are typically expressed using 8 hexadecimal digits, with each pair of the hexadecimal digits representing the values of the Alpha, Red, Green and Blue channel, respectively. For example, `80FFFF00` represents 50.2% opaque (non-premultiplied) yellow. The `80` hex value, which is 128 in decimal, represents a 50.2% alpha value because 128 is approximately 50.2% of the maximum value of 255 (FF hex); to continue to decipher the `80FFFF00` value, the first `FF` represents the maximum value red can have; the second `FF` is like the previous but for green; the final `00` represents the minimum value blue can have (effectively – no blue). Consequently, red + green yields yellow. In cases where the alpha is not used this can be shortened to 6 digits `RRGGBB`, this is why it was chosen to put the alpha in the top bits. Depending on the context a `0x` or a number sign (#)[2] is put before the hex digits.

On little-endian systems, this is equivalent to BGRA (byte-order). On big-endian systems, this is equivalent to ARGB (byte-order).

In some software originating on big-endian machines such as Silicon Graphics, RGBA (word-order) means color is specified similar to ARGB (word-order) but with the alpha in the bottom 8 bits rather than the top. For example, `808000FF` would be Red and Green:50.2%, Blue:0% and Alpha:100%, a brown. This is used in, e.g. Portable Arbitrary Map (PAM).

The bytes are stored in memory on a little-endian machine in the order ABGR (byte-order).

References

1. ^ PNG specification
2. ^ Microsoft MSDN XAML Color Structure reference (XAML/WPF/Silverlight), including `#aarrggbb` and `sc# scA,scR,scG,scB`