I previously criticised JPEGmini, a commercial program that promised to reduce JPEG size by focusing on the way humans perceive colour. My repeated finding in using the program was that it was unable to provide better compression than GIMP’s JPEG export – and GIMP is free!
Now Google has fielded Guetzli, a new JPEG compression engine that Google describes as drawing on psychovisual research. So in that aspect, it doesn’t sound much different from JPEGmini. It is, however, open-sourced, and according to Google, promises 35% smaller files compared to the open source JPEG library libjpeg, which without much doubt must be the engine used by GIMP.
Guetzli, like other recent advances in imaging, is slow. So slow, in fact, that it sounds like its use case is currently limited to very frequently downloaded items like website banners, where spending a lot of time optimizing will considerably reduce traffic.
Alternatively, though, Google could put its weight behind FLIF, supporting it in Chrome/Chromium and thereby putting pressure on other browsers to also support it. FLIF promises to provide current-best lossless compression and is interlaceable – and open source.
Guetzli might give a smaller file than FLIF, but only with more lossy compression, and it sounds like it’s currently a lot slower (I have seen no figures comparing the two).
From photographers’ perspectives, Guetzli may have one further use case. Images with such highly optimised compression are unlikely to be very usable for further editing, so for showing off your work on the web while spoiling the fun for potential thieves, Guetzli or any higher-compression tool might be a good choice (but remember – Google can “guesstimate” the image back with RAISR).