Quote:
Originally Posted by tarasdi
After reading a bit about entropy, I actually think that it would be possible to losslessly compress an already lossy compressed filed, as I doubt that a lossy compression would reach the theoretical maximum of information per bit as provided by Shannon's theorem.
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It certainly is possible -- JPEG re-compressors like Stuffit and WinZip 12 remove the huffman coding, re-organize the data and use prediction and better coding to remove redundancy and shrink it significantly (20-40%). You can do something similar with older MPEG standards (up to MPEG-4 part 2, aka Divx/Xvid), but each generation gets a little better so there's a little less to improve. I'm not sure if it would even be possible to losslessly improve H.264 with CABAC. Maybe through the introduction of HEVC's wavefront, which can actually sometimes reduce size by 1% or so, and playing with CABAC contexts. Maybe you could do something really exotic with long-term references.
Compatibility is so much more important that most of these techniques never get any traction, though; people just update to newer standards as they appear.