Google has announced the open source release of Guetzli, a new JPEG encoder able to reduce a JPEG's file size by up to 35%, without any significant loss of quality.
Guetzl is a JPEG encoder for digital images and web graphics that produces smaller JPEG files while still maintaining compatibility with existing browsers, image processing applications and the JPEG standard. From the practical viewpoint this is very similar to Google's 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. The visual quality of JPEG images is directly correlated to its multi-stage compression process: color space transform, discrete cosine transform, and quantization. Guetzli specifically targets the quantization 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. 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.
However, Guetzli is slow in compressing images. "Guetzli is rather slow to encode," the researchers said, suggesting it's most likely useful on image-heavy websites. "Although Guetzli may be too slow for many practical uses, we hope that it can show direction for future image format design," the researchers said.
Mozilla, maker of the Firefox web browser, also began a project in 2014 called Mozjpeg designed to improve on standard compression engines. Google's tests showed that Guetzli outdoes Mozilla's tool by 29 to 45 percent.