[rorrr2]

> For the PNGs, I had used 4 different optimizers (PNGCrush, OptiPNG, AdvPNG, and PNGOut), but almost all images saw size reductions in lossless mode ranging from under 1% to over 50%

I seriously doubt you can squeeze more than 1-2% after properly running PNGOut on an image.

This service only makes sense if you have huge images that you want to optimize. $19 for 1000 8-megabyte images is a pretty good deal. That's a rare case though. If you have tiny image, just run PNGOut on them for free.

[rb2k_]

1-2% might be possible by using aggressive gzip encoders such as https://code.google.com/p/zopfli/

( as seen in https://github.com/sayurin/optipng-zopfli and mentioned in https://twitter.com/pornelski/status/356843309118922756 )

[kraken-io]

We have been experimenting with Google's Zopfli but found out that the optimization time is too long for such a insignificant optimization gain.

Optimization speed has always a key factor for Kraken.io.

[depsypher]

Another option using zopfli is pngtastic (disclaimer, my own project): https://github.com/depsypher/pngtastic

It really is significantly slower, but if you want to reduce every last byte in a png zopfli is your best bet.

[Yver]

You can get decent savings (~90-95% of the optimized size) with some brute force at the encoder level, more time spent testing different filters and rearranging the palette, but it takes a lot of CPU for little gain. At some point, you might spend minutes to save bytes.

This service is much faster than that, so they probably don't use brute force at all.