A new image format for the Web

最新推荐文章于 2018-11-26 10:30:35 发布

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from: https://developers.google.com/speed/webp/

WebP

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A new image format for the Web

WebP is a new image format that provides lossless and lossy compression forimages on the web. WebP lossless images are 26% smaller in size comparedto PNGs. WebP lossy images are 25-34% smaller in size compared to JPEGimages at equivalent SSIM index. WebP supports lossless transparency (alsoknown as alpha channel) with just 22% additional bytes. Transparency isalso supported with lossy compression and typically provides 3x smaller filesizes compared to PNG when lossy compression is acceptable for thered/green/blue color channels.

Webmasters and web developers can use the WebP image format to create smallerand richer images that can help make the web faster.

How WebP Works

Lossy WebP compression uses predictive coding to encode an image, the samemethodology used by the VP8 video codec to compress keyframes in videos.Predictive coding uses the values in neighboring blocks of pixels to predictthe values in a block, and then encodes only the difference (residual) betweenthe actual values and the prediction.

The residuals typically contain many zero values, which can be compressed muchmore effectively. The residuals are then transformed, quantized and entropy-coded as usual. WebP also uses variable block sizes.

Lossless WebP compression uses already seen image fragments in order toexactly reconstruct new pixels. It can also use a local palette if nointeresting match is found. This palette is continuously updated to re-userecent colors. This compression mode is named "VP8L" and shares some commonfeatures with the so-called LZ77 compression algorithm.

A WebP file consists of VP8 or VP8L image data, and a containerbased on RIFF. The standalone libwebp library serves as a referenceimplementation for the WebP specification, and is available fromour git repository and as a tarball.

WebP Support

WebP is natively supported in Google Chrome and the Opera browser, and bymany other tools and software libraries.

Developers have also added support to a variety of image editing tools. Thisrelease also provides the lightweight encoding and decoding librarylibwebp and the command line tools cwebp and dwebp forconverting images to and from the WebP format. The full source code isavailable on the download page.