Byte order for packed images

Question

So from http://en.wikipedia.org/wiki/RGBA_color_space, I learned that the byte order for ARGB is, from lowest address to highest address, BGRA, on a little endian machine in certain interpretations.

How does this effect the naming convention of packed data eg a uint8_t ar[]={R,G,B,R,G,B,R,G,B}?

Answer 1

Little endian by definition stores the bytes of a number in reverse order. This is not strictly necessary if you are treating them as byte arrays however any vaguely efficient code base will actually treat the 4 bytes as a 32 bit unsigned integer. This will speed up software blitting by a factor of almost 4.

Now the real question is why. This comes from the fact that when treating a pixel as a 32 bit int as described above coders want to be able to run arithmetic and shifts in a predictable way. This relies on the bytes being in reverse order.

In short, this is not actually odd as in little endian machines the last byte (highest address) is actually the most significant byte and the first the least significant. Thus a field like this will naturally be in reverse order so it is the correct way around when treated as a number (as a number it will appear ARGB but as a byte array it will appear BGRA).

Sorry if this is unclear, but I hope it helps. If you do not understand or I have missed something please comment.

Answer 2

If you are storing data in a byte array like you have specified, you are using BGR format which is basically RGB reversed:

bgr-color-space