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S3TC (DXT* formats, often available on desktops),
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ATITC (ATI compression, available on some mobile devices),
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PVRTC (PowerVR compression, available on some mobile devices),
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ETC1 (Ericsson compression, often available on mobile devices, but without alpha support).
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More compressed formats can be trivially added, just submit a bugreport with a sample KTX file.