Google Guetzli compression algorithm squeezes more out of JPEGs

Waiting for images to load is no fun. Neither is paying for bandwidth to serve or receive those images. We've reported in the recent past on Google's efforts to get more detail out of small images and reduce file sizes of images and other web files. The company's Guetzli JPEG encoding algorithm improvement effort looks to have a greater immediate impact because of its ability to potentially reduce image file sizes by up to 35% at the same image quality level when compared to traditional encoders, while maintaining decoding compatibility with existing browsers.

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

Note the reduced artifacting in the image on the right compared to the center image.

Guetzli is the product of Google Research Europe, and has been released under the Apache open source license. The development team says the overall approach is similar to the one used in Google's earlier Zopfli algotithm used to compress gzip files and PNG images, and shares little with the RAISR and WebP methods that require "ecosystem changes for compression gains at internet scale," a fancy way of saying they're new formats and need big changes to server and browser software.

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

Again, note the difference in artifacting between the center and rightmost images.

JPEG compression has several steps, including color space transformation, discrete cosine transformation, and quantization. According to the Guetzli developers, quantization is the key step where visual quality loss is traded for compression efficiency. Google says Guetzli uses a search algorithm to reduce the difference betweem JPEG's psychovisual modeling and the algorithm's own psychovisual model, which it says "approximates color perception and visual masking in a more detailed way." The result is more detail with smaller file sizes, at a cost of substantially higher resource utilization at compression time.

Google says Guetzli can produce smaller file sizes with the same level of quality. The company performed experiments where images of equal file size were shown to study participants who consistently prefered the imaged compressed using Guetzli. It's worth noting that even though the encoding process is slower, that should have a relatively small real-world on servers since image downloads vastly outnumber uploads, where the processing takes place.

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. The developers close their notes with wishes that others will pick up the baton and apply the techniques to other image fromats and to video.

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