The benefit of using Guetzil, aside from the reduction in image size and barely-noticeable changes in image quality, is that the files it turns out remain compatible with existing browsers, image processing programs and the JPEG standard itself. Guetzli may indeed produce better perceived quality at a given file size, but note for example how some green areas are washed out in the eye comparison image above. Left: The uncompressed original. The tech giant now hopes that their new open-source compression algorithm will enable users to reduce website load times and bandwidth costs. Each can be optimized, and Guetzli focuses specifically on the quantization stage as this is typically where visual quality is lost. Smaller files are achieved by blurring together pixels that are similar, but only just enough to not distort the overall image's structure. "Guetzli strikes a balance between minimal loss and file size by employing a search algorithm that tries to overcome the difference between the psycho-visual modelling of JPEG's format, and Guetzli's psycho visual model, which approximates colour perception and visual masking in a more thorough and detailed way than what is achievable by simpler colour transforms and the discrete cosine transform".
Google insists that despite Guetzli being able to produce smaller image file sizes, it doesn't mean it will sacrifice quality.
Google is offering the Guetzli encoder as an open source software project for anyone to check out and possibly improve.
Secondly, Guetzli would seem set to have a significant resource footprint compared to the long-established image compression libraries now in use (mostly over Apache or other Linux-based frameworks). Mozilla, maker of the Firefox web browser, began a project in 2014 called Mozjpeg created to improve on standard compression engines.
For more details on the nitty-gritty of how Guetzli actually accomplishes the improved encoding (it apparently involves "psychovisual models") check out the Google Research Blog and the published paper on Guetzli. Google's tests showed that Guetzli outdoes Mozilla's tool by 29 to 45 percent.
Google says it asked people whether they preferred libjpeg-encoded JPEGs or Guetzli JPEGs and most picked the latter.