|Internet media type|
|Initial release||30 September 2010|
|Type of format|
|Contained by||Resource Interchange File Format (RIFF)|
1.0.3 / 4 July 2019
The format was first announced on September 30 in 2010 as a new open standard for lossy compressed true-color graphics on the web, producing smaller files of comparable image quality to the older JPEG scheme. On October 3, 2011, Google announced WebP support for animation, ICC profile, XMP metadata, and tiling (compositing very large images from maximum 16384×16384 tiles).
On November 18, 2011, Google began to experiment with lossless compression and support for transparency (alpha channel) in both lossless and lossy modes; support has been enabled by default in libwebp 0.2.0 (August 16, 2012). According to Google's measurements, a conversion from PNG to WebP results in a 45% reduction in file size when starting with PNGs found on the web, and a 28% reduction compared to PNGs that are recompressed with pngcrush and PNGOUT.
||VP8 key frame|
|pad||? (even length)|
WebP's lossy compression algorithm is based on the intra-frame coding of the VP8 video format and the Resource Interchange File Format (RIFF) as a container format. As such, it is a block-based transformation scheme with eight bits of color depth and a luminance-chrominance model with chroma subsampling by a ratio of 1:2 (YCbCr 4:2:0). Without further content, the mandatory RIFF container has an overhead of only twenty bytes, though it can also hold additional metadata. The side length of WebP images is limited to 16383 pixels.
WebP is based on block prediction. Each block is predicted on the values from the three blocks above it and from one block to the left of it (block decoding is done in raster-scan order: left to right and top to bottom). There are four basic modes of block prediction: horizontal, vertical, DC (one color), and TrueMotion. Mispredicted data and non-predicted blocks are compressed in a 4×4 pixel sub-block with a discrete cosine transform or a Walsh–Hadamard transform. Both transforms are done with fixed-point arithmetic to avoid rounding errors. The output is compressed with entropy encoding. WebP also has explicit support for parallel decoding.
The reference implementation consists of converter software in the form of a command-line program for Linux (cwebp) and a programming library for the decoding, the same as for WebM. The open source community quickly managed to port the converter to other platforms, such as Windows.
WebP's lossless compression, a new format unrelated to VP8, was designed by Google software engineer Jyrki Alakuijala. It uses 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. This format uses recursive definition: all of the control images such as the local entropy code selection are encoded the same way as the whole image itself.
Google has proposed using WebP for animated images as an alternative to the popular GIF format, citing the advantages of 24-bit color with transparency, combining frames with lossy and lossless compression in the same animation, and as well as support for seeking to specific frames. Google reports a 64% reduction in file size for images converted from animated GIFs to lossy WebP, and a 19% reduction when converted to lossless WebP.
Among web browsers, Google Chrome, Firefox, Opera, GNOME Web, Midori, Falkon, Pale Moon, and Waterfox natively support WebP. EdgeHTML based versions of Microsoft Edge supports WebP through a platform extension (installed by default). EdgeHTML based versions of Microsoft Edge doesn’t support platform extensions, including the WebP image format extension, when running in the security hardened “Application Guard” mode. Since versions of Microsoft Edge released after January 2020 are based on the Chromium browser, WebP support is built-in without the need of platform extensions.
Amongst graphics software, Picasa (from version 3.9), PhotoLine, Pixelmator, ImageMagick, XnView, IrfanView, GDAL, Aseprite, Paint.NET, (from version 4.2.5) and GIMP (from version 2.10) all natively support WebP. In 2019, Google released a free plug-in that enables WebP support in Adobe Photoshop. Before that, free Photoshop plug-ins were released by Telegraphics and fnordware. Imagine supports WebP via a plugin. GIMP up to version 2.8 also supported WebP via a plugin, later this plugin was shipped in GIMP 2.9 branch, and received multiple improvements. Google has also released a plug-in for Microsoft Windows  that enables WebP support in Windows Photo Viewer, Microsoft Office 2010, FastPictureViewer, and any other application that uses Windows Imaging Component.
FFmpeg linked with the VP8/VP9 reference codec library libvpx can extract VP8 key frames from WebM media and a script can then add the WebP RIFF header and the NUL pad byte for odd frame lengths. Meanwhile, FFmpeg supports libwebp directly.
Gmail and Picasa Web Albums (both Google web applications) support WebP. Support for WebP is also planned for Google App Engine. The Instant Previews feature of Google Search uses WebP internally to reduce disk space used by previews. Android 4.0 supports encoding and decoding WebP images (via bitmap and Skia). SDL_image supports the format since 1.2.11.
In early beta versions of macOS Sierra and iOS 10, Apple added WebP support, but was later removed GM seed versions of iOS 10 and macOS Sierra released 7 September 2016. WebP is unsupported in macOS and iOS Safari browser as of version 13.
Content management systems usually do not support WebP natively or by default. However, for most popular CMS, extensions are available for automated conversion from other image formats to WebP and delivering WebP images to compatible browsers.
Like VP8 on which it is based, former lossy WebP only supports 8-bit YUV 4:2:0 format, which may cause color loss on images with thin contrast elements (such as in pixel art and computer graphics) and ghosting in anaglyph. To overcome this restriction, new lossless WebP supports VP8L encoding that works exclusively with 8-bit ARGB color space.
In September 2010, Fiona Glaser, a developer of the x264 encoder, wrote a very early critique of WebP. Comparing different encodings (JPEG, x264, and WebP) of a reference image, she stated that the quality of the WebP-encoded result was the worst of the three, mostly because of blurriness on the image. Her main remark was that "libvpx, a much more powerful encoder than ffmpeg's jpeg encoder, loses because it tries too hard to optimize for PSNR" (peak signal-to-noise ratio), arguing instead that "good psycho-visual optimizations are more important than anything else for compression."
In October 2013, Josh Aas from Mozilla Research published a comprehensive study of current lossy encoding techniques and was not able to conclude WebP outperformed JPEG by any significant margin.
- FLIF, a work-in-progress lossless image format which claims to outperform PNG, lossless WebP, lossless BPG and lossless JPEG2000 in terms of compression ratio, introduced in 2015
- BPG, an image format intended to be a more compression-efficient replacement for the JPEG image format, based on the intra-frame encoding of the High Efficiency Video Coding (HEVC) video compression standard, introduced in 2014
- HEIF, another image format based on HEVC
- JPEG XR, an alternative to JPEG 2000 supporting HDR and wide gamut color spaces, introduced in 2009
- JPEG 2000, an improvement intended to replace the older JPEG by the JPEG committee, introduced in 2000
- MNG and APNG, PNG-based animated image formats, supporting lossless 24-bit RGB color and 8-bit alpha channel
- AV1 Still Image File Format, a container format based on the AV1 video codec
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