Guetzli is a freely licensed JPEG encoder that Jyrki Alakuijala, Robert Obryk, and Zoltán Szabadka have developed in Google's Zürich research branch.

[5] Guetzli is resource-intensive, requiring orders of magnitude more processing time and random-access memory than other JPEG encoders.

[8] Google says it is a demonstration of the potential of psychovisual optimizations, intended to motivate further research into future JPEG encoders.

[2] Two tests found that Guetzli is very slow (about 4 magnitudes slower than normal JPEG encoder) and not necessarily better than mozjpeg.

It is significantly more complex than traditional metrics like PSNR and SSIM, but claimed to perform better with high-end quality, where degradations are not or barely noticeable.

[1] An in-house performance evaluation with 614 ratings from 23 people on their own test set of 31 images yielded 75% of ratings favouring of JPEGs encoded for Butteraugli scores over libjpeg-turbo encodes,[12] which usually score higher on SSIM and PSNR-HVS-M.[13][14] Translating to "butter eye", the Swiss-German name originally signifies a dimple on top of some sweet pastry that has been filled with butter and sugar before baking.