A file containing an RGB image plus an alpha channel for transparency information.

RGBA stands for red green blue alpha. While it is sometimes described as a color space, it is actually the three-channel RGB color model supplemented with a 4th alpha channel. Alpha indicates how opaque each pixel is and allows an image to be combined over others using alpha compositing, with transparent areas and anti-aliasing of the edges of opaque regions.

The term does not define what RGB color space is being used. It also does not state whether or not the colors are premultiplied by the alpha value, and if they are it does not state what color space that premultiplication was done in. This means more information than just "RGBA" is needed to determine how to handle an image.

To further confuse the issue, in some contexts the term "RGBA" means a specific layout in memory of the four channels, with other terms such as "BGRA" used for alternatives. In other contexts, the alternatives are used more often but the term "RGBA" means any layout.

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 order is used when referring to the encoding.

Adapted from content published on wikipedia.org
  • Image By Spitzak - Own work, CC BY-SA 4.0 — from wikimedia.org
Last modified on November 16, 2019, 4:20 pm
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