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Varying compression window size? (png, etc.)

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• Peter Ahlstrom at 2007-11-12 07:47
  Do any of the major zip-type optimizers vary compression window size? Optipng lets you set the compression window size, but doesn't let you iterate through the different options automatically. Around four years ago I compiled my own version of optipng which can do this, and it allowed me to squeeze out some extra bytes in some files. Current versions of kzip and the advancecomp tools have a better end result, but I wonder if they do so varying compression window size or not.
  
  I sent my code to Cosmin Truta at the time, but he didn't integrate it, maybe because I changed the way the option worked. Instead of setting compression window size directly as optipng does (as it says in its help):

  -zw window size
                Select the zlib window size (32k,16k,8k,4k,2k,1k,512,256).
                The window size can be specified either in bytes (e.g. 16384) or kilobytes (e.g. 16k).

  I changed it so the zw option is specified from 1 to 7,

  - The zlib window size is set to 256 * 2 ^ zw bytes, or if not specified, to a minimum that does not affect the compression ratio, usually 7 (32k).

  For the files I was working with at the time—4bit indexed grays— -zc9 -zm5-9 -zs0-1,3 -f0 -zw4-7 always worked best, so I added that as an -o8 option.
  
  Anyway my code was based on the 0.4.3+RLE codebase, but it was a simple change that could be added to the newest version with little trouble I'm sure.
• Awesoken at 2007-11-12 10:40

  Re: Varying compression window size? (png, etc.)

  I can only speak for PNGOUT and KZIP. They both use a 32KByte window size. The window size feature was originally meant to produce ZIP files for embedded systems that might not have much memory available for decompression. Selecting a smaller window size can sometimes have the side effect of helping compression ratio, however it is quite rare. When I played around with it, I didn't feel that it was worthy of its own command line option. Deflate compression is an inexact science. The best strategy, if you were programming your own, is to optimize the average compression ratio over a large set of random files.