What is the difference between RGBA and ARGB?

RGBA

RGBA stands for red green blue alpha and is sometimes described as a color space. However, it is actually a three-channel RGB color model supplemented with a fourth alpha channel.

Alpha indicates how opaque each pixel is and allows an image to be combined over others using alpha compositingwith transparent areas and anti-aliasing of the edges of opaque regions.

rgba(red, green, blue, alpha)

The alpha value is declared as a decimal number from 0 to 1, where 0 is fully transparent and 1 is fully opaque.

Characteristics of RGBA

• RGBA is a four-channel format containing data for Red, Green, Blue, and an Alpha value.

• The CSS function rgba() may have limited support in older browsers.

• The opacity of the color can be easily controlled by specifying the opacity in terms of a parameter.

• Example: The rgba(255, 0, 0, 0.3) defines the red color with opacity set to 0.3.

In many systems with more than 8 bits per channel (such as 16 bits or floating-point), the channels are stored in RGBA order, even if 8-bit channels are stored in some other order.

In some software originating on big-endian machines, colors were stored in 32-bits similar to ARGB32, 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 what you would get if RGBA8888 data was read as words on these machines.

ARGB

The channels are arranged in memory in such a 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 channels, 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).

• The first FF in the 80FFFF00 value 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.

This layout became popular when 24-bit color (and 32-bit RGBA) was introduced on personal computers. At the time it was much faster and easier for programs to manipulate one 32-bit unit than four 8-bit units.