![Sego lily cm-150](https://upload.wikimedia.org/wikipedia/commons/6/6d/Sego_lily_cm-150.jpg)
A compression artifact (or artefact) is a noticeable distortion of media (including
images
An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
,
audio
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
*Audio signal, an electrical representation of sound
*Audio frequency, a frequency in the audio spectrum
*Digital audio, representation of sound ...
, and
video
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) syste ...
) caused by the application of
lossy compression
In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
. Lossy
data compression
In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
involves discarding some of the media's data so that it becomes small enough to be stored within the desired
disk space
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a computer ...
or
transmitted (''streamed'') within the available
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
(known as the data rate or
bit rate
In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time.
The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
). If the compressor cannot store enough data in the compressed version, the result is a loss of quality, or introduction of artifacts. The
compression algorithm
In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compressio ...
may not be intelligent enough to discriminate between distortions of little subjective importance and those objectionable to the user.
The most common digital compression artifacts are DCT blocks, caused by the
discrete cosine transform (DCT) compression algorithm used in many
digital media
Digital media is any communication media that operate in conjunction with various encoded machine-readable data formats. Digital media can be created, viewed, distributed, modified, listened to, and preserved on a digital electronics device. ' ...
standards, such as
JPEG
JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...
,
MP3
MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio developed largely by the Fraunhofer Society in Germany, with support from other digital scientists in the United States and elsewhere. Origin ...
, and
MPEG
The Moving Picture Experts Group (MPEG) is an alliance of working groups established jointly by International Organization for Standardization, ISO and International Electrotechnical Commission, IEC that sets standards for media coding, includ ...
video file formats.
These compression artifacts appear when heavy compression is applied,
and occur often in common digital media, such as
DVD
The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any kind ...
s, common computer file formats such as JPEG, MP3 and MPEG files, and some alternatives to the
compact disc
The compact disc (CD) is a Digital media, digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then rele ...
, such as
Sony's MiniDisc
MiniDisc (MD) is an erasable magneto-optical disc-based data storage format offering a capacity of 60, 74, and later, 80 minutes of digitized audio.
Sony announced the MiniDisc in September 1992 and released it in November of that year fo ...
format. Uncompressed media (such as on
Laserdisc
The LaserDisc (LD) is a home video format and the first commercial optical disc storage medium, initially licensed, sold and marketed as DiscoVision, MCA DiscoVision (also known simply as "DiscoVision") in the United States in 1978. Its diam ...
s,
Audio CDs, and
WAV
Waveform Audio File Format (WAVE, or WAV due to its filename extension; pronounced "wave") is an audio file format standard, developed by IBM and Microsoft, for storing an audio bitstream on PCs. It is the main format used on Microsoft Wind ...
files) or
losslessly compressed media (such as
FLAC
FLAC (; Free Lossless Audio Codec) is an audio coding format for lossless compression of digital audio, developed by the Xiph.Org Foundation, and is also the name of the free software project producing the FLAC tools, the reference software p ...
or
PNG) do not suffer from compression artifacts.
The minimization of perceivable artifacts is a key goal in implementing a lossy compression algorithm. However, artifacts are occasionally ''intentionally'' produced for artistic purposes, a style known as
glitch art
Glitch art is the practice of using digital or analog errors for aesthetic purposes by either corrupting digital data or physically manipulating electronic devices. Glitches appear in visual art such as the film ''A Colour Box'' (1935) by Len Lye, ...
or datamoshing.
Technically speaking, a compression artifact is a particular class of data error that is usually the consequence of
quantization in lossy data compression. Where
transform coding
Transform coding is a type of data compression for "natural" data like audio signals or photographic images. The transformation is typically lossless (perfectly reversible) on its own but is used to enable better (more targeted) quantization, whi ...
is used, it typically assumes the form of one of the
basis function
In mathematics, a basis function is an element of a particular basis for a function space. Every function in the function space can be represented as a linear combination of basis functions, just as every vector in a vector space can be represen ...
s of the coder's transform space.
Images
When performing block-based
discrete cosine transform (DCT)
coding for
quantization, as in
JPEG
JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...
-compressed images, several types of artifacts can appear.
*
Ringing
Ringing may mean:
Vibrations
* Ringing (signal), unwanted oscillation of a signal, leading to ringing artifacts
* Vibration of a harmonic oscillator
** Bell ringing
* Ringing (telephony), the sound of a telephone bell
* Ringing (medicine), a ri ...
* Contouring
*
Posterizing
Posterization or posterisation of an image is the conversion of a continuous gradation of tone to several regions of fewer tones, causing abrupt changes from one tone to another. This was originally done with photographic processes to create ...
* Staircase noise (
aliasing
In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when a ...
) along curving edges
* Blockiness in "busy" regions (block boundary artifacts, sometimes called macroblocking, quilting, or checkerboarding)
Other lossy algorithms, which use
pattern matching
In computer science, pattern matching is the act of checking a given sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually has to be exact: "either it will or will not be ...
to deduplicate similar symbols, are prone to introducing hard to detect errors in printed text. For example, the numbers "6" and "8" may get replaced. This has been observed to happen with
JBIG2
JBIG2 is an image compression standard for bi-level images, developed by the Joint Bi-level Image Experts Group. It is suitable for both lossless and lossy compression. According to a press release from the Group, in its lossless mode JBIG2 ty ...
in certain photocopier machines.
Block boundary artifacts
![The macroblocking effect (JPEG)](https://upload.wikimedia.org/wikipedia/commons/b/b1/The_macroblocking_effect_%28JPEG%29.png)
At low bit rates, any
lossy
In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
block-based coding scheme introduces visible artifacts in pixel blocks and at block boundaries. These boundaries can transform block boundaries, prediction block boundaries, or both, and may coincide with
macroblock The macroblock is a processing unit in image and video compression formats based on linear block transforms, typically the discrete cosine transform (DCT). A macroblock typically consists of 16×16 samples, and is further subdivided into transform ...
boundaries. The term ''macroblocking'' is commonly used regardless of the artifact's cause. Other names include tiling, mosaicing, pixelating, quilting, and checkerboarding.
Block-artifacts are a result of the very principle of
block transform coding. The transform (for example the discrete cosine transform) is applied to a block of pixels, and to achieve lossy compression, the transform coefficients of each block are
quantized. The lower the bit rate, the more coarsely the coefficients are represented and the more coefficients are quantized to zero. Statistically, images have more low-
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
than high-frequency content, so it is the low-frequency content that remains after quantization, which results in blurry, low-resolution blocks. In the most extreme case only the DC-coefficient, that is the coefficient which represents the average color of a block, is retained, and the transform block is only a single color after reconstruction.
Because this quantization process is applied individually in each block, neighboring blocks quantize coefficients differently. This leads to discontinuities at the block boundaries. These are most visible in flat areas, where there is little detail to mask the effect.
Image artifact reduction
Various approaches have been proposed to reduce image compression effects, but to use standardized compression/decompression techniques and retain the benefits of compression (for instance, lower transmission and storage costs), many of these methods focus on "post-processing"—that is, processing images when received or viewed. No post-processing technique has been shown to improve image quality in all cases; consequently, none has garnered widespread acceptance, though some have been implemented and are in use in proprietary systems. Many photo editing programs, for instance, have proprietary JPEG artifact reduction algorithms built-in. Consumer equipment often calls this post-processing "MPEG Noise Reduction".
Boundary artifact in JPEG can be turned into more pleasing "grains" not unlike those in high ISO photographic films. Instead of just multiplying the quantized coefficients with the quantisation step pertaining to the 2D-frequency, intelligent noise in the form of a random number in the interval can be added to the dequantized coefficient. This method can be added as an integral part to JPEG decompressors working on the trillions of existing and future JPEG images. As such it is not a "post-processing" technique.
The ringing issue can be reduced at encode time by overshooting the DCT values, clamping the rings away.
Posterization generally only happens at low quality, when the DC values are given too little importance. Tuning the quantization table helps.
Video
![Macroblocking error](https://upload.wikimedia.org/wikipedia/commons/f/ff/Macroblocking_error.png)
When motion prediction is used, as in
MPEG-1
MPEG-1 is a standard for lossy compression of video and audio. It is designed to compress VHS-quality raw digital video and CD audio down to about 1.5 Mbit/s (26:1 and 6:1 compression ratios respectively) without excessive quality loss, making ...
,
MPEG-2
MPEG-2 (a.k.a. H.222/H.262 as was defined by the ITU) is a standard for "the generic video coding format, coding of moving pictures and associated audio information". It describes a combination of Lossy compression, lossy video compression and ...
or
MPEG-4
MPEG-4 is a group of international standards for the compression of digital audio and visual data, multimedia systems, and file storage formats. It was originally introduced in late 1998 as a group of audio and video coding formats and related tec ...
, compression artifacts tend to remain on several generations of decompressed frames, and move with the
optic flow of the image, leading to a peculiar effect, part way between a painting effect and "grime" that moves with objects in the scene.
Data errors in the compressed bit-stream, possibly due to transmission errors, can lead to errors similar to large quantization errors, or can disrupt the parsing of the data stream entirely for a short time, leading to "break-up" of the picture. Where gross errors have occurred in the bit-stream, decoders continue to apply updates to the damaged picture for a short interval, creating a "ghost image" effect, until receiving the next independently compressed frame. In MPEG picture coding, these are known as "
I-frame
In the field of video compression a video frame is compressed using different algorithms with different advantages and disadvantages, centered mainly around amount of data compression. These different algorithms for video frames are called pict ...
s", with the 'I' standing for "intra". Until the next I-frame arrives, the decoder can perform
error concealment Error concealment is a technique used in signal processing that aims to minimize the deterioration of signals caused by missing data, called packet loss. A signal is a message sent from a transmitter to a receiver in multiple small packets. Packet ...
.
Motion compensation block boundary artifacts
Block boundary discontinuities can occur at edges of
motion compensation
Motion compensation in computing, is an algorithmic technique used to predict a frame in a video, given the previous and/or future frames by accounting for motion of the camera and/or objects in the video. It is employed in the encoding of video d ...
prediction blocks. In motion compensated video compression, the current picture is predicted by shifting blocks (macroblocks, partitions, or prediction units) of pixels from previously decoded frames. If two neighboring blocks use different motion vectors, there will be a discontinuity at the edge between the blocks.
Mosquito noise
Video compression artifacts include cumulative results of compression of the comprising still images, for instance
ringing
Ringing may mean:
Vibrations
* Ringing (signal), unwanted oscillation of a signal, leading to ringing artifacts
* Vibration of a harmonic oscillator
** Bell ringing
* Ringing (telephony), the sound of a telephone bell
* Ringing (medicine), a ri ...
or other edge busyness in successive still images appear in sequence as a shimmering blur of dots around edges, called mosquito noise, as they resemble mosquitoes swarming around the object.
The so-called "mosquito noise" is caused by the block-based
discrete cosine transform (DCT) compression algorithm used in most
video coding standards
A video coding format (or sometimes video compression format) is a content representation format for storage or transmission of digital video content (such as in a data file or bitstream). It typically uses a standardized video compression algo ...
, such as the
MPEG
The Moving Picture Experts Group (MPEG) is an alliance of working groups established jointly by International Organization for Standardization, ISO and International Electrotechnical Commission, IEC that sets standards for media coding, includ ...
formats.
Video artifact reduction
The artifacts at block boundaries can be reduced by applying a
deblocking filter
A deblocking filter is a video filter applied to decoded compressed video to improve visual quality and prediction performance by smoothing the sharp edges which can form between macroblocks when block coding techniques are used. The filter aims ...
. As in still image coding, it is possible to apply a deblocking filter to the decoder output as post-processing.
In motion-predicted video coding with a closed prediction loop, the encoder uses the decoder output as the prediction reference from which future frames are predicted. To that end, the encoder conceptually integrates a decoder. If this "decoder" performs a deblocking, the deblocked picture is then used as a reference picture for motion compensation, which improves coding efficiency by preventing a propagation of block artifacts across frames. This is referred to as an in-loop deblocking filter. Standards which specify an in-loop deblocking filter include
VC-1
SMPTE 421, informally known as VC-1, is a video coding format. Most of it was initially developed as Microsoft's proprietary video format Windows Media Video 9 in 2003. With some enhancements including the development of a new Advanced Profile, ...
,
H.263 Annex J,
H.264/AVC, and
H.265/HEVC.
Audio
Lossy audio compression typically works with a psychoacoustic model—a model of human hearing perception. Lossy audio formats typically involve the use of a time/frequency domain transform, such as a
modified discrete cosine transform
The modified discrete cosine transform (MDCT) is a transform based on the type-IV discrete cosine transform (DCT-IV), with the additional property of being lapped transform, lapped: it is designed to be performed on consecutive blocks of a larger ...
. With the psychoacoustic model, masking effects such as frequency masking and temporal masking are exploited, so that sounds that should be imperceptible are not recorded. For example, in general, human beings are unable to perceive a quiet tone played simultaneously with a similar but louder tone. A lossy compression technique might identify this quiet tone and attempt to remove it. Also, quantization noise can be "hidden" where they would be masked by more prominent sounds. With low compression, a conservative psy-model is used with small block sizes.
When the psychoacoustic model is inaccurate, when the transform block size is restrained, or when aggressive compression is used, this may result in compression artifacts. Compression artifacts in compressed audio typically show up as ringing,
pre-echo
In audio signal processing, pre-echo, sometimes called a '' forward echo'', (not to be confused with reverse echo) is a digital audio compression artifact where a sound is heard before it occurs (hence the name). It is most noticeable in impulsiv ...
, "birdie artifacts", drop-outs, rattling, warbling, metallic ringing, an underwater feeling, hissing, or "graininess".
An example of compression artifacts in audio is applause in a relatively highly compressed audio file (e.g. 96 kbit/sec MP3). In general, musical tones have repeating waveforms and more predictable variations in volume, whereas applause is essentially random, therefore hard to compress. A highly compressed track of applause may have "metallic ringing" and other compression artifacts.
Artistic use
Compression artifacts may intentionally be used as a visual style, sometimes known as ''
glitch art
Glitch art is the practice of using digital or analog errors for aesthetic purposes by either corrupting digital data or physically manipulating electronic devices. Glitches appear in visual art such as the film ''A Colour Box'' (1935) by Len Lye, ...
''.
Rosa Menkman's glitch art makes use of
compression artifacts
A compression artifact (or artefact) is a noticeable distortion of media (including images, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the media's data so that it bec ...
,
particularly the
discrete cosine transform blocks (DCT blocks) found in most
digital media
Digital media is any communication media that operate in conjunction with various encoded machine-readable data formats. Digital media can be created, viewed, distributed, modified, listened to, and preserved on a digital electronics device. ' ...
data compression
In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
formats such as
JPEG
JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...
digital images
A digital image is an image composed of picture elements, also known as ''pixels'', each with ''finite'', '' discrete quantities'' of numeric representation for its intensity or gray level that is an output from its two-dimensional functions f ...
and
MP3
MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio developed largely by the Fraunhofer Society in Germany, with support from other digital scientists in the United States and elsewhere. Origin ...
digital audio
Digital audio is a representation of sound recorded in, or converted into, digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical samples in a continuous sequence. For example, in CD audio, sa ...
.
In still images, an example is ''Jpegs'' by German photographer
Thomas Ruff
Thomas Ruff (born 10 February 1958) is a German photographer who lives and works in Düsseldorf, Germany. He has been described as "a master of edited and reimagined images".
Ruff shares a studio on Düsseldorf's Hansaallee, with fellow German ...
, which uses intentional JPEG artifacts as the basis of the picture's style.
In
video art
Video art is an art form which relies on using video technology as a visual and audio medium. Video art emerged during the late 1960s as new consumer video technology such as video tape recorders became available outside corporate broadcasting ...
, one technique used is ''datamoshing'', where two videos are interleaved so intermediate frames are interpolated from two separate sources. Another technique involves simply transcoding from one lossy video format to another, which exploits the difference in how the separate video codecs process motion and color information. The technique was pioneered by artists
Bertrand Planes in collaboration with Christian Jacquemin in 2006 with DivXPrime, Sven König,
Takeshi Murata
Takeshi Murata is an American contemporary artist who creates digital media artworks using video and computer animation techniques. In 2007 he had a solo exhibition, ''Black Box: Takeshi Murata'', at the Hirshhorn Museum and Sculpture Garden i ...
,
Jacques Perconte and Paul B. Davis in collaboration with
Paperrad, and more recently used by
David OReilly and within
music video
A music video is a video of variable duration, that integrates a music song or a music album with imagery that is produced for promotion (marketing), promotional or musical artistic purposes. Modern music videos are primarily made and used as a m ...
s for
Chairlift
An elevated passenger ropeway, or chairlift, is a type of aerial lift, which consists of a continuously circulating steel wire rope loop strung between two end terminals and usually over intermediate towers, carrying a series of chairs. They ...
and by
Nabil Elderkin
Nabil Elderkin is an American-born Australian film and music video director and photographer, who is of Iranian and American descent. He is mononymously credited as Nabil in his videos. Elderkin has directed videos for musical artists such as: ...
in the "
Welcome to Heartbreak
"Welcome to Heartbreak" is a song by American record producer and vocalist Kanye West from his fourth studio album, ''808s & Heartbreak'' (2008). The song features a guest appearance from Kid Cudi on his debut collaboration with West, as well a ...
" music video for
Kanye West
Ye ( ; born Kanye Omari West ; June 8, 1977) is an American rapper, singer, songwriter, record producer, and fashion designer.
Born in Atlanta and raised in Chicago, West gained recognition as a producer for Roc-A-Fella Records in the ea ...
.
There is also a genre of
internet meme
An Internet meme, commonly known simply as a meme ( ), is an idea, behavior, style, or image that is spread via the Internet, often through social media platforms. What is considered a meme may vary across different communities on the Internet ...
s where often nonsensical images are purposefully heavily compressed sometimes multiple times for comedic effect. Images created using this technique are often referred to as "deep fried."
See also
*
Artifact (error)
In natural science and signal processing, an artifact or artefact is any error
An error (from the Latin ''error'', meaning "wandering") is an action which is inaccurate or incorrect. In some usages, an error is synonymous with a mistake. Th ...
*
Databending
Databending (or data bending) is the process of manipulating a media file of a certain format, using software designed to edit files of another format. Distortions in the medium typically occur as a result, and the process is frequently employed in ...
*
Digital artifact
Digital artifact in information science, is any undesired or unintended alteration in data introduced in a digital process by an involved technique and/or technology.
Digital artifact can be of any content types including text, audio, video, ...
*
Generation loss
Generation loss is the loss of quality between subsequent copies or transcodes of data. Anything that reduces the quality of the representation when copying, and would cause further reduction in quality on making a copy of the copy, can be consid ...
*
JPEG
JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...
*
JPEG 2000
JPEG 2000 (JP2) is an image compression standard and coding system. It was developed from 1997 to 2000 by a Joint Photographic Experts Group committee chaired by Touradj Ebrahimi (later the JPEG president), with the intention of superseding the ...
*
Lossy compression
In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
*
Noise print A noise print is part of a technique used in noise reduction. A noise print is commonly used in audio mastering to help reduce the effects of unwanted noise from a piece of audio. In this case, the noise print would be a recording of the ambient no ...
*
Ringing artifacts
In signal processing, particularly digital image processing, ringing artifacts are artifacts that appear as spurious signals near sharp transitions in a signal. Visually, they appear as bands or "ghosts" near edges; audibly, they appear as "ec ...
*
Transparency (data compression)
In data compression and psychoacoustics, transparency is the result of lossy data compression accurate enough that the compressed result is perceptually indistinguishable from the uncompressed input, i.e. perceptually lossless.
A transparency t ...
References
External links
DivXPrime First known experiments of datamoshing video software by Bertrand Planes & Christian Jacquemin (based on Xvid algorithm)
Teaser for "Sonic birth"a short movie directed by Jérome Blanquet, datamoshing effect by David Olivari, produced by
etronomic Full movie
"Sonic birth"datamosherA GPL video datamoshing software.
Example of heavy video compression artifacts
JPEG Tutor an interactive applet allowing you to investigate the effects of changing the quantization matrix.
JPEG deringing and deblocking: Matlab software and Photoshop plug-in
{{DEFAULTSORT:Compression Artifact
Audio engineering
Computer graphic artifacts
Data compression
Articles containing video clips