Google has introduced WebP, a new open-source image file format that promises an average 39 percent reduction in file size compared to JPEG, while offering a comparable image quality.
To improve on the compression that JPEG provides, we used an image compressor based on the VP8 codec that Google open-sourced in May 2010. We applied the techniques from VP8 video intra frame coding to push the envelope in still image coding. We also adapted a very lightweight container based on RIFF. While this container format contributes a minimal overhead of only 20 bytes per image, it is extensible to allow authors to save meta-data they would like to store.
While the benefits of a VP8 based image format were clear in theory, we needed to test them in the real world. In order to gauge the effectiveness of our efforts, we randomly picked about 1,000,000 images from the web (mostly JPEGs and some PNGs and GIFs) and re-encoded them to WebP without perceptibly compromising visual quality. This resulted in an average 39% reduction in file size. We expect that developers will achieve in practice even better file size reduction with WebP when starting from an uncompressed image.
More details can be found on the company's Chromium blog. A zip file with ten samples (JPEG vs WebP stored as lossless PNG) can be downloaded over here. Most of the images look pretty much the same, but in some cases (like photo 7) the difference is pretty big.