Thursday, December 29, 2005


PAQ7 compression released

PAQ7 is a command line file compressor and archiver, that achieves far better compression than the standard ZIP based archivers (and in fact better than almost anything else available).

The better compression is achieved by trading memory usage and speed for compression, and the program uses a neural network to combine a number of predictor models that predict what the next bit in the file will be. Of particular interest are the predictor models that are specialized to work with tiff and jpeg files - in my tests the program achieved around about 20% compression of jpegs in the archive. Technically what it does it to partially decode the jpeg stream, back to the image coefficients, and simply apply better lossless compression to that level of data.

PAQ7, as its name suggests, is one in a long line of experimental leading edge compression programs - and this version is roughly 3 times as fast as its predecessor. The program's source code is available under the GPL, and builds for many platforms, including Windows and Liinux.

Also recently released is a beta of the closed source archiver and compressor WinRK version 3. which similarly achieves very high compression ratios whilst taking a long time to compress files. WinRK is a GUI based program, only available for Windows. It also has a special jpeg recompression algorithm.

Currently neither of these jpeg recompression modes achieves quite as high a compression ratio as Stuffit achieves, which I talked about earlier in the year.

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