libtxc_dxtn
We provide a libtxc_dxtn replacement that uses S2TC to compress and
decompress textures, which can be directly used by Mesa/DRI.
How this library compresses textures, can be configured by the
EnvironmentVariables. By default, it is set to be a bit faster
than the S3TC libtxc_dxtn.
Decompressing S2TC textures will work perfectly. However, it can not decompress S3TC input correctly. What will happen if you try doing so anyway, can be seen on this screenshot:
Basically, any pixels S2TC can not decompress (which are "reserved" in the FileFormats specification), will at 50 percent chance (in an ordered dither) pick one of the two per-block candidate colors. This is clearly a quality reduction compared to a real S3TC decoder, but still is usually acceptable.
Note though that S2TC decompression typically is NOT used when using libtxc_dxtn
by Mesa/DRI. The only cases when this is used, are software fallback (that is
usually very slow anyway, and does not happen on modern chipsets at all), and
of course when using the llvmpipe driver (which is how I made this
screenshot).
You can test the library without installing it by compiling it using
./configure
make
and then not using make install, but instead calling a program with
LD_LIBRARY_PATH set to the .libs subdirectory of the S2TC distribution:
LD_LIBRARY_PATH=/home/rpolzer/s2tc/.libs ./darkplaces-sdl