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How does S2TC compare in size to S3TC?
S2TC compressed textures have the very same size as S3TC ones. However, as some bit combinations are unused, they deflate better. For example, a Xonotic build can get reduced by 12% in total size by using S2TC instead of S3TC.
In theory, S2TC files have exactly 25% less entropy than S3TC files, and thus an ideal compressor can yield 25% smaller compressed archive size. But, of course results may vary by other properties of the compressor and the archive format.
How does S2TC compare in quality to S3TC?
A good S2TC compressor is in quality about equivalent to a bad S3TC compressor. See also: QualityComparison.
Which graphics cards can decompress S2TC?
All current graphics cards can decompress S2TC, as S2TC is a special case of the well-known S3TC texture compression format.
libtxc_dxtn decompress S3TC?Can S2TC's
No, it can not decompress S3TC, as the S3TC decompression algorithm is not implemented. When a "reserved" bit combination (i.e. one not defined in S2TC) is encountered, the decompressor instead selects - at equal probabilities - one of the two possible color (or alpha) values. This yields reduced visual quality, but accessibility/readability is not impaired. See the libtxc_dxtn page for a screenshot.