Lossless and Transparency Encoding in WebP

Thursday, November 17, 2011

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In September 2010 we announced the WebP image format with lossy compression. WebP was proposed as an alternative to JPEG, with 25–34% better compression compared to JPEG images at equivalent SSIM index. We received lots of feedback, and have been busy improving the format. Last month we announced WebP support for animation, ICC profile, XMP metadata and tiling. Today, we introduce a new mode in WebP to compress images losslessly, and support for transparency – also known as alpha channel – in both the lossless and lossy modes.

With these new modes, you can now use WebP to better compress all types of images on the web. Photographic images typically encoded as JPEG can be encoded in WebP lossy mode to achieve smaller file size. Icons and graphics can be encoded better in WebP lossless mode than in PNG. WebP lossy with alpha can be used to create transparent images that have minimal visual degradation, yet are much smaller in file size. Animations compressed as GIFs can use animation support in WebP.

New lossless mode

Our main focus for lossless mode has been in compression density and simplicity in decoding. On average, we get a 45% reduction in size when starting with PNGs found on the web, and a 28% reduction in size compared to PNGs that are re-compressed with pngcrush and pngout. Smaller images on the page mean faster page loads.

New transparency mode

Today, webmasters who need transparency must encode images losslessly in PNG, leading to a significant size bloat. WebP alpha encodes images with low bits-per-pixel and provides an effective way to reduce the size of such images. Lossless compression of the alpha channel adds just 22% bytes over lossy (quality 90) WebP encoding. Smaller alpha overhead means richer images on webpages.

You can find a more detailed compression study for these modes here and sample images in the WebP-Gallery. The bit stream specification has not been finalized, and the encoding and decoding implementations have not yet been optimized for processing speed. We encourage you to try it out on your favorite set of images, check out the code, and provide feedback. We hope WebP will now handle all your needs for web images, and we're working to get WebP supported in more browsers.

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