WebP
Encyclopedia
WebP is an image format
that employs lossy compression. It is developed by Google
, based on technology acquired with the purchase of On2 Technologies
.
As a derivative of the video format VP8
, it is a sister project to the multimedia container format WebM
. WebP related software is released under a BSD license
.
The format was first announced in 2010 and is supposed to be a new open standard
for lossily-compressed true-color graphics on the web, thereby being presented as a direct competitor to the older JPEG
scheme, to which it is meant to compare favorably with the production of smaller files for comparable image quality. On October 3, 2011 they announced WebP support for animation, ICC profile
, XMP
metadata and tiling
(compositing very large images from max. 16384×16384 tiles). On November 18, 2011 they introduced lossless compression and support for transparency
(alpha channel
) in both the lossless and lossy modes. According to their measurements, after PNG to WebP conversion images got an average 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.
coding of the VP8
video format and the classical 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 leads to an overhead
of only twenty bytes and can hold additional metadata
. The side length of WebP images is limited to 16383 pixels (14 bits).
WebP is based on block prediction. Each block is predicted on the values from three blocks above it and from one block 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 Discrete cosine transform
and (rare) 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 free
reference implementation
consists of a converter software in the form of a command-line
program for Linux (webpconv) 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.
and Opera
natively support WebP. (Support for incremental image decoding is scheduled for Google Chrome v12.) All WebM
-compatible browsers may also display WebP via a JavaScript shim. WebP can also be displayed in all major browsers using WebPJS JavaScript library (IE6+ support is achieved by using Flash
).
Amongst the graphics software, Pixelmator
, ImageMagick
, ReaConverter , Konvertor and GDAL
natively support WebP. Telegraphics has released a free plug-in that enables WebP support in Adobe Photoshop CS5
and earlier. Google has also released a plug-in for Microsoft Windows that enables WebP support in Windows Photo Viewer, Microsoft Office 2010, and any other application that uses Windows Imaging Component
.
Gmail
and Picasa Web Albums (both Google web application
s) support WebP. Support for WebP is also planned for Google App Engine
. The Instant Previews feature of Google Search
currently uses WebP internally to reduce disk space used by previews. Android 4.0 supports encoding and decoding WebP images (via bitmap and Skia
).
encoder, gave several points of criticism for WebP. Using a comparison of different encodings (JPEG, x264, Theora
, and WebP) of a reference image, he stated that the quality of the WebP-encoded result was the worst of the four, mostly because of blurriness on the image. His 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 psy[cho-visual] optimizations
are more important than anything else for compression." He also criticized Google's announcement, saying that it shouldn't have publicized the format before its results are better than JPEG's.
Image file formats
Image file formats are standardized means of organizing and storing digital images. Image files are composed of either pixels, vector data, or a combination of the two. Whatever the format, the files are rasterized to pixels when displayed on most graphic displays...
that employs lossy compression. It is developed by Google
Google
Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...
, based on technology acquired with the purchase of On2 Technologies
On2 Technologies
On2 Technologies , formerly known as The Duck Corporation, was a small publicly-traded company , headquartered in Clifton Park, New York, that designs video codec technology. They created a series of video codecs called TrueMotion...
.
As a derivative of the video format VP8
VP8
VP8 is an open video compression format released by Google, originally created by On2 Technologies.After purchasing On2 Technologies in early 2010, Google has provided an irrevocable patent promise for underlying patents for the VP8 format, and released a bitstream format specification under a...
, it is a sister project to the multimedia container format WebM
WebM
WebM is an audio-video format designed to provide a royalty-free, open video compression format for use with HTML5 video. The project's development is sponsored by Google....
. WebP related software is released under a BSD license
BSD licenses
BSD licenses are a family of permissive free software licenses. The original license was used for the Berkeley Software Distribution , a Unix-like operating system after which it is named....
.
The format was first announced in 2010 and is supposed to be a new open standard
Open standard
An open standard is a standard that is publicly available and has various rights to use associated with it, and may also have various properties of how it was designed . There is no single definition and interpretations vary with usage....
for lossily-compressed true-color graphics on the web, thereby being presented as a direct competitor to the older JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
scheme, to which it is meant to compare favorably with the production of smaller files for comparable image quality. On October 3, 2011 they announced WebP support for animation, ICC profile
ICC profile
In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium...
, XMP
XMP
XMP may refer to:In software and data standards:* Extensible Metadata Platform, Adobe's standard for processing and embedding metadata in various file formats*EXtreme Memory ProfileIn gaming:...
metadata and tiling
Tiling
Tiling may refer to:* The physical act of laying tiles* The mathematics of tessellations* The compiler optimization of loop tiling* In computing, a tiling window manager, where windows do not overlap...
(compositing very large images from max. 16384×16384 tiles). On November 18, 2011 they introduced lossless compression and support for transparency
Transparency (graphic)
Transparency is possible in a number of graphics file formats. The term transparency is used in various ways by different people, but at its simplest there is "full transparency" i.e. something that is completely invisible. Of course, only part of a graphic should be fully transparent, or there...
(alpha channel
Alpha compositing
In computer graphics, alpha compositing is the process of combining an image with a background to create the appearance of partial or full transparency. It is often useful to render image elements in separate passes, and then combine the resulting multiple 2D images into a single, final image in a...
) in both the lossless and lossy modes. According to their measurements, after PNG to WebP conversion images got an average 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.
Technology
WebP's lossy compression algorithm is based on the intra-frameIntra-frame
Intra-frame coding is used in video coding . It is part of group of pictures with inter frames.The term intra-frame coding refers to the fact that the various lossless and lossy compression techniques are performed relative to information that is contained only within the current frame, and not...
coding of the VP8
VP8
VP8 is an open video compression format released by Google, originally created by On2 Technologies.After purchasing On2 Technologies in early 2010, Google has provided an irrevocable patent promise for underlying patents for the VP8 format, and released a bitstream format specification under a...
video format and the classical Resource Interchange File Format (RIFF) as a container format. As such it is a block-based transformation scheme with eight bits of color depth
Color depth
In computer graphics, color depth or bit depth is the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel , particularly when specified along with the number of bits used...
and a luminance-chrominance model with chroma subsampling
Chroma subsampling
Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance....
by a ratio of 1:2 (YCbCr
YCbCr
YCbCr or Y′CbCr, sometimes written or , is a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Y′ is the luma component and CB and CR are the blue-difference and red-difference chroma components...
4:2:0). Without further content the mandatory RIFF container leads to an overhead
Computational overhead
In computer science, overhead is generally considered any combination of excess or indirect computation time, memory, bandwidth, or other resources that are required to attain a particular goal...
of only twenty bytes and can hold additional metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
. The side length of WebP images is limited to 16383 pixels (14 bits).
WebP is based on block prediction. Each block is predicted on the values from three blocks above it and from one block 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 Discrete cosine transform
Discrete cosine transform
A discrete cosine transform expresses a sequence of finitely many data points in terms of a sum of cosine functions oscillating at different frequencies. DCTs are important to numerous applications in science and engineering, from lossy compression of audio and images A discrete cosine transform...
and (rare) Walsh–Hadamard transform. Both transforms are done with fixed-point arithmetic
Fixed-point arithmetic
In computing, a fixed-point number representation is a real data type for a number that has a fixed number of digits after the radix point...
to avoid rounding errors. The output is compressed with entropy encoding
Entropy encoding
In information theory an entropy encoding is a lossless data compression scheme that is independent of the specific characteristics of the medium....
. WebP also has explicit support for parallel decoding.
The free
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
reference implementation
Reference implementation
In the software development process, a reference implementation is the standard from which all other implementations, with their attendant customizations, are measured, and to which all improvements are added...
consists of a converter software in the form of a command-line
Command-line interface
A command-line interface is a mechanism for interacting with a computer operating system or software by typing commands to perform specific tasks...
program for Linux (webpconv) 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.
Support
Amongst web browsers, Google ChromeGoogle Chrome
Google Chrome is a web browser developed by Google that uses the WebKit layout engine. It was first released as a beta version for Microsoft Windows on September 2, 2008, and the public stable release was on December 11, 2008. The name is derived from the graphical user interface frame, or...
and Opera
Opera (web browser)
Opera is a web browser and Internet suite developed by Opera Software with over 200 million users worldwide. The browser handles common Internet-related tasks such as displaying web sites, sending and receiving e-mail messages, managing contacts, chatting on IRC, downloading files via BitTorrent,...
natively support WebP. (Support for incremental image decoding is scheduled for Google Chrome v12.) All WebM
WebM
WebM is an audio-video format designed to provide a royalty-free, open video compression format for use with HTML5 video. The project's development is sponsored by Google....
-compatible browsers may also display WebP via a JavaScript shim. WebP can also be displayed in all major browsers using WebPJS JavaScript library (IE6+ support is achieved by using Flash
Adobe Flash
Adobe Flash is a multimedia platform used to add animation, video, and interactivity to web pages. Flash is frequently used for advertisements, games and flash animations for broadcast...
).
Amongst the graphics software, Pixelmator
Pixelmator
Pixelmator is a graphic editor developed for Mac OS X, by Pixelmator Team Ltd built upon a combination of open source and Mac OS X technologies...
, ImageMagick
ImageMagick
ImageMagick is an open source software suite for displaying, converting, and editing raster image files. It can read and write over 100 image file formats. ImageMagick is licensed under the Apache 2.0 license.- Features and capabilities:...
, ReaConverter , Konvertor and GDAL
GDAL
GDAL is a library for reading and writing raster geospatial data formats, and is released under the permissive X/MIT style free software license by the Open Source Geospatial Foundation. As a library, it presents a single abstract data model to the calling application for all supported formats...
natively support WebP. Telegraphics has released a free plug-in that enables WebP support in Adobe Photoshop CS5
Adobe Photoshop
Adobe Photoshop is a graphics editing program developed and published by Adobe Systems Incorporated.Adobe's 2003 "Creative Suite" rebranding led to Adobe Photoshop 8's renaming to Adobe Photoshop CS. Thus, Adobe Photoshop CS5 is the 12th major release of Adobe Photoshop...
and earlier. Google has also released a plug-in for Microsoft Windows that enables WebP support in Windows Photo Viewer, Microsoft Office 2010, and any other application that uses Windows Imaging Component
Windows Imaging Component
The Windows Imaging Component is a Component Object Model based imaging codec framework introduced in Windows Vista and Windows XP Service Pack 3 for working with and processing digital images and image metadata...
.
Gmail
Gmail
Gmail is a free, advertising-supported email service provided by Google. Users may access Gmail as secure webmail, as well via POP3 or IMAP protocols. Gmail was launched as an invitation-only beta release on April 1, 2004 and it became available to the general public on February 7, 2007, though...
and Picasa Web Albums (both Google web application
Web application
A web application is an application that is accessed over a network such as the Internet or an intranet. The term may also mean a computer software application that is coded in a browser-supported language and reliant on a common web browser to render the application executable.Web applications are...
s) support WebP. Support for WebP is also planned for Google App Engine
Google App Engine
Google App Engine is a platform as a service cloud computing platform for developing and hosting web applications in Google-managed data centers. It virtualizes applications across multiple servers,...
. The Instant Previews feature of Google Search
Google search
Google or Google Web Search is a web search engine owned by Google Inc. Google Search is the most-used search engine on the World Wide Web, receiving several hundred million queries each day through its various services....
currently uses WebP internally to reduce disk space used by previews. Android 4.0 supports encoding and decoding WebP images (via bitmap and Skia
Skia Graphics Engine
The Skia Graphics Engine is a compact open source graphics library written in C++. It was originally developed by Skia Inc., which was subsequently acquired by Google in 2005 , who then released the software as open source licensed under the Apache License 2.0 free software license.Now known as...
).
Criticism
Jason Garrett-Glaser, a developer of the x264X264
x264 is a free software library for encoding video streams into the H.264/MPEG-4 AVC format. It is released under the terms of the GNU General Public License.-History:...
encoder, gave several points of criticism for WebP. Using a comparison of different encodings (JPEG, x264, Theora
Theora
Theora is a free lossy video compression format. It is developed by the Xiph.Org Foundation and distributed without licensing fees alongside their other free and open media projects, including the Vorbis audio format and the Ogg container....
, and WebP) of a reference image, he stated that the quality of the WebP-encoded result was the worst of the four, mostly because of blurriness on the image. His 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
The phrase peak signal-to-noise ratio, often abbreviated PSNR, is an engineering term for the ratio between the maximum possible power of a signal and the power of corrupting noise that affects the fidelity of its representation...
" (peak signal-to-noise ratio), arguing instead that "good psy[cho-visual] optimizations
Subjective video quality
Subjective video quality is a subjective characteristic of video quality. It is concerned with how video is perceived by a viewer and designates his or her opinion on a particular video sequence...
are more important than anything else for compression." He also criticized Google's announcement, saying that it shouldn't have publicized the format before its results are better than JPEG's.
See also
- WebMWebMWebM is an audio-video format designed to provide a royalty-free, open video compression format for use with HTML5 video. The project's development is sponsored by Google....
, a multimedia container format introduced by Google earlier in 2010, on which WebP is based - JPEG 2000JPEG 2000JPEG 2000 is an image compression standard and coding system. It was created by the Joint Photographic Experts Group committee in 2000 with the intention of superseding their original discrete cosine transform-based JPEG standard with a newly designed, wavelet-based method...
, an improvement intended to replace the older JPEG by the JPEG committee, introduced in 2000 - JPEG XR, a computationally lightweight alternative to JPEG 2000, introduced in 2009