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Add a practical description of endian-independent RGB/BGR coding

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git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@17681 b3059339-0415-0410-9bf9-f77b7e298cf2
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pacman 2006-02-24 22:18:45 +00:00
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DOCS/tech/colorspaces.txt
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@ -135,3 +135,24 @@ that you actually get:
Depending upon the CPU being little- or big-endian, different 'in memory' and
'in register' formats will be equal (LE -> BGRA == BGR32 / BE -> ARGB == BGR32)
Practical coding guide:
The 4, 8, 15, and 16 bit formats are defined so that the portable way to
access them is to load the pixel into an integer and use bitmasks.
The 24 bit formats are defined so that the portable way to access them is
to address the 3 components as separate bytes, as in ((uint8_t *)pixel)[0],
((uint8_t *)pixel)[1], ((uint8_t *)pixel)[2].
When a 32-bit format is identified by the four characters A, R, G, and B in
some order, the portable way to access it is by addressing the 4 components
as separate bytes.
When a 32-bit format is identified by the 3 characters R, G, and B in some
order followed by the number 32, the portable way to access it is to load
the pixel into an integer and use bitmasks.
When the above portable access methods are not used, you will need to write
2 versions of your code, and use #ifdef WORDS_BIGENDIAN to choose the correct
one.