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Google Makes Web Images 35% Smaller

18 March 2017

Google has developed a new open-source JPEG compression algorithm, dubbed Guetzli (apparently that means "cookie" in Swiss German).

A new JPEG compression algorithm doesn't sound as exciting as a method of enhancing small images like fictional forensics investigators in television shows do all the time, but the ability to reduce file sizes without requiring updates to browsers is probably more useful, particularly in the near future. Smaller file sizes means less data is being transferred around the Internet and websites will load faster.

The announcement by Google software engineers Robert Obryk and Jyrki Alakuijala explains that the new compression algorithm does not affect web compatibility (like the average video format, digital image formats are essentially wrappers for content that could be encoded in any number of ways - the important thing is that the browser/viewer/player knows how to handle the different compression formats it might find after the JPEG marker).

Left: original, center: libjpeg, right: Guetzli.

The encoding process for a JPEG can be broken down into six distinct parts: color space transformation, downsampling, block splitting, discrete cosine transform, quantization, and entropy coding. "We think this makes the slower compression a worthy tradeoff".

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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.

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). Guetzli works by producing multiple candidates of compressed JPEGs and then comparing them to see which is best.

Google has made the Guetzli encoder open source, and it's available on Github for anyone to integrate into their own projects or to use on their own. Presumably Google are considering optimisations and rational solutions to speed up Guetzli's compression speed performance.

However, it's worth noting that Guetzli is slower than other options out there. And the popularity and usage of JPEGs have led Google to experiment with the said format instead of building a new one from scratch.

Google Makes Web Images 35% Smaller