[Mesa-dev] [Bug 77693] Mesa software rasterizers - decompress textures on load to improve performance

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=77693

--- Comment #2 from Roland Scheidegger <sroland at vmware.com> ---
Ideally you'd just say you don't support s3tc, and leave it up to the
application to not use such textures. I suspect it wouldn't get you very far
though today (though I know some apps indeed can deal with this), and you can't
actually even do it without deleting (or not installing...) libtxc_dxtn.
If you'd really want to fake s3tc support that way for sw renderers, there's
obviously some problems with your implementation:
1) there is a reason why the decode/encode code is not in mesa itself, this
doesn't change if you decode the texture as a whole up front. The code
admittedly is very inefficient for decompressing complete textures, though
nothing would prevent a more efficient whole-block decompression interface.
2) You can't just intercept texstoreimage2d that way, that will give a lot of
trouble. You'd also have to catch the texstoresubimage2d calls, and (more
complicated) deal with all the other stuff which this affects. E.g. things like
getCompressedTexImageARB would more or less require you to store the original
image too, or just hope that it won't get called (which is very likely). You'd
never get all the error conditions right neither (as compressed textures have a
lot of restrictions) though I guess you might not particularly care about these
neither. It would probably be cleaner if you'd still treat it as a compressed
texture everywhere just with hacked up (and converted) format.
In any case it would need to be configurable too.

As a side note, it would of course be possible to make this code faster for
llvmpipe's simd execution (with a different interface).

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