Cryopng is OptiPNG with a dirty hack at the libpng level to actually perform a kind of advanced "Dirty Transparency" cleaning.
As explained by Sergey Chikuyonok (in this Smashing Magazine article) fully transparent pixels could have any RGB component since they are actually invisible, he suggested to turn them to black (0,0,0) to improve compression. A few tools have since implemented this trick (PunyPNG, PngOptimizer, ImageOptim...), but if you look closely at the PNG specifications you'll notice that images are filtered before being compressed. Cryopng turns fully transparent pixels into something that will be nullified once filtered, and this can positively affect compression.
For instance the following mouse sample image used by PunyPNG (sizes in bytes):
105,035 Cryogen-x (cryopng script, also uses PNGOUT and DeflOpt)
For illustration purposes:
Top left: picture with alpha channel activated.
Top center: filter 0 (None) is nullified, fully transparent pixels are all black (classic dirty transparency)
Top right: filter 1 (Sub) is nullified, a fully transparent pixel has the same colour as its left neighbour
Bottom left: filter 2 (Up) is nullified, a fully transparent pixel has the same colour as its upper neighbour
Bottom center: filter 3 (Average) is nullified, a fully transparent pixel has the same colour as the average of its left and upper neighbours
Bottom right: filter 4 (Paeth) is nullified, a fully transparent pixel has the same colour as whether its left, upper or upper left neighbour (Paeth predicator is a powerful mess)
I released cryopng source code.
To build it you need to download OptiPNG 0.7.5 source code first, the optipng2cryopng075.tgz tarball holds only 6 files meant to replace those from OptiPNG (full drop-ins not diffs), building OptiPNG afterwards should produce cryopng.
You'll notice that all the work is done at the libpng level (version 1.6.10 here), I never really had time to polish the code, it's really an ugly hack. But instead of letting it rust on my harddrive I quickly made the changes needed to catch current versions of OptiPNG & libpng and released it (licences are the same as OptiPNG and libpng)
Did some early work to include cryopng inside zopflipng (zopfli current source is needed -now hosted on GitHub since Google Code is closing-, replace the 3 zopflipng source files in src/zopflipng by those in the archive zopflipng-cryoinside-source.zip below, and "make zopflipng").
This version is a bit (unnecessary) verbose and even holds some new undocumented options:
--maxblocks (to help compress some very large PNGs set it to more than 15 -the current cap-)
--always_zopflify has been renamed --force
But the new option you are really looking for is --alpha_cleaner.
--alpha_cleaner=0 is equivalent to cryopng -f0 and so on for the other values 1,2,3 and 4
A good way to try it is to use these options:
zopflipng -m --alpha_cleaner=0124 infile.png outfile.png
(will automatically pick the best alpha cleaning and associated filtering using a "fast" compression method and use zopfli only during the last part, here the alpha cleaner 3 has been dismissed since it is rarely the best one)
For instance on the sample mouse image these options produce a 105321 bytes file (pretty close to best result presented here, not bad for a single tool).