JPEG is a lossy compression method for images, which allows a tradeoff to occur between file size and the final quality of the images.
Although there are several different ways to adjust the size and quality of a JPEG, Google's Guetzli (German for "cookie") is all about the "quantization stage of compression". Guetzli promises more compression, without users or developers having to adopt a new file format. According to the Google blog announcement post, "Guetzli is a JPEG encoder for digital images and web graphics that can enable faster online experiences by producing smaller JPEG files while still maintaining compatibility with existing browsers, image processing applications and the JPEG standard". The company bills this as a new (albeit slow at the moment) way for websites to crunch their images, reducing file size for faster page loading without sacrificing quality. "Guetzli strikes a balance between minimal loss and file size by employing a search algorithm that tries to overcome the difference between the psycho-visual modelling of JPEG's format, and Guetzli's psycho visual model, which approximates colour perception and visual masking in a more thorough and detailed way than what is achievable by simpler colour transforms and the discrete cosine transform".
To accomplish this, Guetzli is using psychovisual research based on HVS or the human visual system model. Higher-quality images packed into smaller file sizes will be particularly useful on the web. Guetzli works by producing multiple candidates of compressed JPEGs and then comparing them to see which is best. 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. Right, Guetzli compression. Credit: Google. Guetzli (on the right) shows less ringing artefacts than libjpeg (middle) without requiring a larger file size. For example, a multi-megabyte image stored in BMP or PNG format can look nearly exactly the same converted to a JPEG that's only a few hundred kilobytes in size. The team also suggests that the psychovisual approach adopted by the project will inspire further research into the way our eyes perceive compressed images and video.