Other methods mostly get the job done by leaving out minor details, while Guetzli compresses the image without compromising on the quality. It hopes that web site designers will use this new tool to cut down on image file sizes and thus improve load times on their sites, especially on mobile browsers.
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 announced the release of Guetzli, an open source JPEG encoder that creates high quality JPEG images with file sizes 35% smaller than now available methods. According to Google, this makes the slower compression a "worthy tradeoff". "This implies the Butteraugli psychovisual image similarity metric which guides Guetzli is reasonably close to human perception at high-quality levels". "However, while Guetzli creates smaller image file sizes, the tradeoff is that these search algorithms take significantly longer to create compressed images than now available methods", the company notes in a blog. Guetzli focuses on compression at the quantization stage as that is where more visual degeneration of the image occurs. It would be interesting to see if Guetzli scores a wider acceptance. You can view Guetzli's repository on GitHub here. Therefore any compression technique needs to downsample in a manner which leads to the highest perceptual image quality possible, since there is no "way back". Guetzli may indeed produce better perceived quality at a given file size, but note for example how some green areas are washed out in the eye comparison image above. Right, Guetzli compression. Credit: Google.
Slower compression is likely to be an insignificant factor for desktop design environments, but has far bigger usage and resource ramifications for the millions of process threads which run every day over the internet, converting selfies, food shots and other user-generated content into optimised formats for viewing. Right: Guetzli. Google claims that Guetzli has fewer artifacts without a larger file size.
Google says it asked people whether they preferred libjpeg-encoded JPEGs or Guetzli JPEGs and most picked the latter.