I am having the same problem.  I looked into it a bit and found that the
subtitles are not actually being XORed; I think what's happening is that
when the subtitles are alpha blended into the image, an 8-bit alpha
channel is being passed to a blending function designed for 4-bit alpha
channels, causing the pixel values to overflow and wrap around.

The attached patch fixes the DVB subtitles for me, but it is not the
correct fix because it has the side effect of making DVD (as opposed to
DVB) subtitles very faint (perhaps they actually use the 4-bit alpha the
original code expects?).  I'm including it anyway just in case it
provides some clues that might help someone who understands the code to
come up with a real fix.


** Attachment added: "libxine.patch"
   http://launchpadlibrarian.net/10379107/libxine.patch

-- 
DVB subtitle rendering faulty with xshm
https://bugs.launchpad.net/bugs/129114
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