Lossless and Transparency Modes in WebP

AUG 30, 2012
Author Picture By Jyrki Alakuijala, WebP Team

At Google, we are constantly looking at ways to make web pages load faster. One way to do this is by making web images smaller. This is especially important for mobile devices where smaller images save both bandwidth and battery life. Earlier this month, we released version 0.2 of the WebP library that adds support for lossless and transparency modes to compress images. This version provides CPU and memory performance comparable to or better than PNG, yet results in 26% smaller files.

WebP’s improved compression comes from advanced techniques such as dedicated entropy codes for different color channels, exploiting 2D locality of backward reference distances and a color cache of recently used colors. This complements basic techniques such as dictionary coding, Huffman coding and color indexing transform. We think that we've only scratched the surface in improving compression. Our newly added support for alpha transparency with lossy images promises additional gains in this space, helping make WebP an efficient replacement for PNG.

The new WebP modes are supported natively in the latest Beta version of Chrome. The bit stream specification for these new WebP modes has been finalized and the container specification has been updated. We thank the community for their valuable feedback and for helping us evolve WebP as a new image compression format for the web. We encourage you to try these new compression methods on your favorite set of images, check out the code, and continue to provide feedback.

Dr. Jyrki Alakuijala is a Software Engineer with a special interest in data compression. He is a father of five daughters, and sings in the Finnish Choir in Zürich. Before joining Google, Jyrki worked in neurosurgical and radiotherapy development.

Posted by Ashleigh Rentz, Editor Emerita