Increase compression ratio of deflate stream
other people did that.
This is on my to-do list.
I've run quite a lot of images through advpng (the advancecomp utility that offers zopfli and 7-zip compression of some png formats) and I didn't get a huge amount from it. Many files wouldn't go smaller and the ones that did were just a few percent. Possibly the 5%-10% advertised improvement is not relative to the best that optipng can already achieve?
Ian, among the images you tested, how many of them are compressed with delta-filtering different than 0 (None)? You can use "pngcheck -vv" to find that out.
Wondering if zopfli is probably not that much better than Z_FILTERED, in the situations where Z_FILTERED is applicable.
Some are filtered. Most are not.
Two years and one release later I wonder if this is still on the list..?
OptiPNG with the new -strip option and Zopfli built in would leave little for me to be desired. Only one tool for the usual PNG optimization workflow? - hell yeah, I want that!
Maybe some crucial bit of information:
(The Zopfli mode of) advpng seems to be buggy. For some reason it performs much worse than the official ZopfliPNG. I get a solid 17 % difference in filesize here.
So maybe it's worth reevaluating Zopfli another way than through advpng.
advpng does not support any delta filtering (i.e. only filter type 0).