Google Research has released details of its new and more efficient method for encoding JPEG images, promising to cut a third off JPEG image file-sizes with equal (or improved) appearance - but at the cost of greater time needed for compression, relative to the venerable libjpeg method that has been in use for decades.
Google has several other projects to reduce image sizes on the web, including its Zopfli encoder (which similarly creates smaller PNG files without breaking format compatibility) and WebP (a new image format that supports both lossless and lossy compression for improved file sizes).
Google has stated that "Guetzli" - which means "cookie" in Swiss German - targets the quantization stage of a JPEG image which is partly responsible for visual quality. But since the files are so much smaller and there's no real loss in image quality, Google says it's worth the tradeoff. Other compression algorithms such as "libjpeg" can also produce images similar or larger in size than Guetzli, but the blog post confirms that its human raters consistently preferred the images produced by Guetzli. This means that JPEG images weigh less and load times for websites will definitely get a boost. Images created by Guetzli are on average 35% smaller than those from the current industry-leading encoders.
Images pushed through the Guetzli encoder are still available in a regular JPEG format, making them compatible with just about every application and browser now on the market. If that's not something you care about, you can head over to the Github destination instead and check out all the files, instructions on installing the tool, and the code necessary for using it. Guetzli focuses on compression at the quantization stage as that is where more visual degeneration of the image occurs. Because it is more complex than existing encoders, it takes "significantly" longer to complete compression. The time between shots might be reduced if Guetzli compression is used. The second image is encoded with libjpeg and the third image uses Guetzli. 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.