[cairo] ARGB, BGRA, RGBA mess

[Carl Worth]

On Thu, 13 Jan 2005 13:24:55 -0800, Bill Spitzak wrote:
> I think what several people here have been saying, me included, is that 
> the "standard" is that "RGBA" means R is in the byte with the lowest 
> address.

"address" to me sounds like we are talking about an image in memory.

> I just checked sevearal interfaces to file reading/writing code 

File reading/writing to me sounds like a bytestream-oriented interface
which is fundamentally different than an in-memory image. (But some of
the confusion here may be a bit of "culture clash" between these two
kinds of image interfaces).

> (libjpeg, libpng, and libtiff, imagemagic, etc) and every single one of 
> them uses RGBA. I do not see ARGB or BGRA anywhere! I also thing ABGR is 
> rarely used, except to handle endian issues.

Quoting from libpng(3) I see the following input transformations
available in that library:

           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG

So this library clearly advertises support for RGBA, BGRA, and
ARGB. And presumably if both transforms are enabled simultaneously one
would get ABGR. Note: I'm just looking at the manual now, not
examining any images in memory nor in files.

-Carl
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