zopfli VS PNG-spec

This project uses HTMLMinifier, optipng, and zopfli to create a custom production Elm build pipeline. You can see how I make use of these tools in this build script. Here are the results in case you're interested. I used the same ideas from this project to build and deploy dwayne/elm-conduit, which you can learn more about in my article Yet Another Tour of an Open-Source Elm SPA.

I'm making a 2D game with lots of PNG assets. I recently found that ZopfliPNG regularly reduces the sizes of my assets by ~3x compared to Asesprite's output. I've tried a few other PNG compression tools, but ZopfliPNG consistently outperforms the others.

I mean something better than Google's best engineers trying to optimize LZ77's compression as much as humanly possible, while remaining compatible with the DEFLATE/zlib bitstream.

zopflipng typically beats pngcrus and optipng (on Linux at least) but by default it drops auxillary PNG chunks [0] which can result in browsers (and other applications) using a different color space, causing the resulting images to look more washed out than the original. To prevent this you need to explicitly pass --keepchunks=cHRM,gAMA,pHYs,iCCP,sRGB,oFFs,sTER to zopflipng.

Unfortunately it (and most other tools) don't have APNG support, keeping only the first frame.

[0] https://github.com/google/zopfli/issues/113

I tried going with a lot of different compression algorithms, in the end a lot of iterations of Zopfli delivers the best result. With that, I got the image down to 591 kB.

I tried zopflipng [1] with the very expensive option suggested from the usage and got 2,493 bytes (original 2,740 bytes, nowhere practical as it took 3 minutes) so you don't have to sacrifice the palette, though I'm not sure if 16 or even 4 color palette will significantly alter the visual.

[1] https://github.com/google/zopfli

Pull requests welcome. :D

There are multiple github issues on the subject where we work on future editions of the PNG spec. (3rd Edition coming soon!)

Especially enticing as the PNG file format does allow for additional compression/filter methods and new ones could be added to a PNG 2.0 standard. A small wishlist discussion about that at the W3C's PNG specification Github.

> But its not like any reader ever supports it fully, hell most readers dont support showing the other images in a png file.

Do you mean APNG? In all fairness, that is not even in the specification although there is discussion to add it. https://github.com/w3c/PNG-spec/issues/26

The one that was specified was MNG but it was a different format and practically no one used it since it was not parsable as a PNG.