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

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@17681 b3059339-0415-0410-9bf9-f77b7e298cf2

1 files changed, 21 insertions, 0 deletions

diff --git a/DOCS/tech/colorspaces.txt b/DOCS/tech/colorspaces.txt
index f4a1f88c2a..dc00fd388d 100644
--- a/DOCS/tech/colorspaces.txt
+++ b/DOCS/tech/colorspaces.txt
@@ -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.