Google shrinks JPEGs by 35% with Guetzli algorithm

Google is shrinking JPEG image files by 35% to help boost online page load times.

Google researchers have developed an algorithm dubbed Guetzli which is apparently the Swiss-German word for "cookie" which slashes file size while maintaining visual quality and compatibility with browsers by not altering the file format, unlike its previous effort at image compression, WebP.

"From the practical viewpoint this is very similar to our Zopfli algorithm, which produces smaller PNG and gzip files without needing to introduce a new format, and different than the techniques used in RNN-based image compression, RAISR, and WebP, which all need client and ecosystem changes for compression gains at internet scale," Google Research Europe software engineers Robert Obryk and Jyrki Alakuijala said in a blog post.

WebP slashed file sizes by a quarter, but didn't always interact well with image-editing software, with Photoshop requiring a plugin, or with other browsers, which didn't all support the new file format. Guetzli promises more compression, without users or developers having to adopt a new file format.

The researchers said Guetzli targets one aspect of JPEG compression, quantisation dropping the number of colours in an image to cut its size.

Guetzli specifically targets the quantisation stage in which the more visual quality loss is introduced, the smaller the resulting file. "Guetzli strikes a balance between minimal loss and file size by employing a search algorithm that tries to overcome the difference between the psychovisual modeling of JPEG's format, and Guetzli's psychovisual model, which approximates color perception and visual masking in a more thorough and detailed way than what is achievable by simpler color transforms and the discrete cosine transform," the researchers explained. "However, while Guetzli creates smaller image file sizes, the tradeoff is that these search algorithms take significantly longer to create compressed images than currently available methods."

How does it look? Google said it tested JPEGs compressed using the algorithm on "human raters", and they consistently prefered the Guetzil images over other JPEGs, even if the latter were larger files.

The algorithm has been released as open source, as it's naturally in Google's own interest to cut down data use online. "It is our hope that webmasters and graphic designers will find Guetzli useful and apply it to their photographic content, making users' experience smoother on image-heavy websites in addition to reducing load times and bandwidth costs for mobile users," the researchers added.