zopfli-png was a first response to utilize a brand new GZIP compressor to recompress PNGs back in the 2013.
Nowadays there are much faster, stable, just better tools to do the same job:
PNG optimizer using zopfli deflate packer
Recently zopfli DEFLATE algorithm was announced. As PNG IDAT section are actually DEFLATE compressed, this algorithm can be used for compressing PNG. This script actually does it.
Please note that neither pixel data, not filters applied to image data are not changed. This script may be treated mostly like DeflOpt replacement. That means, PngOUT or other PNG compressing tools must be applied before this one.
This is a node.js script. So it requires node.js first of all.
This package includes zopfli.exe. In other OSes it is much easier to use gcc, so go ahead,
git pull https://code.google.com/p/zopfli/ and make it.
Zopfli binary may be placed in bin/ subdirectory or set in path.
node zopfli-png.js [options] file1.png [file2.png ...]
where options is one or more modifiers.
The most important are iteration count modifiers, these are passed to zopfli binary:
--i5
--i10
--i15
--i25
--i50
--i100
--i250
--i500
--i1000
Higher the number, slower and better the compression.
Other options:
--force, force write even if resulting PNG file is bigger. Zopfli is darn good, but it is not a silver bullet.
--silent, do not show any messages. Except maybe most wild errors.
--splitlast, do block splitting last instead of first. This option may reduce the size of output file. Requires Zopfli 1.0.0 (April 25, 2013)
--help, if you want to read this section once again.
Awesome zopfli algorithm and implementation are authored by Jyrki Alakuijala, Ph.D. and Lode Vandevenne, M.Sc. and licensed with Apache Licence 2.0.
Other wrapping code (i.e., zopfli-png.js, crc32crypto.js and pngstream.js)
is licenced with WTFPL Licence 2.0