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A compression artifact (or artefact) is the result of an aggressive data compression scheme applied to an image, audio, or video that discards some data which is determined by an algorithm to be of lesser importance to the overall content but which is nonetheless discernible and objectionable to the user. Artifacts in time-dependent data such as audio or video are often a result of the latent error in lossy data compression. Image File history File links Compression-artifacts. ...
Image File history File links Compression-artifacts. ...
In computer science, data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than a more obvious representation would use, through use of specific encoding schemes. ...
See also: Photograph In common usage, an image (from Latin imago) or picture is an artifact that reproduces the likeness of some subject—usually a physical object or a person. ...
Audio can mean: Sound that can be heard. ...
Look up Video on Wiktionary, the free dictionary Video (from Latin, I see) is the technology of processing electronic signals for representing moving pictures. ...
Flowcharts are often used to represent algorithms. ...
A lossy data compression method is one where compressing data and then decompressing it retrieves data that may well be different to the original, but is close enough to be useful in some way. ...
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 is used, they typically assume the form of one of the basis functions of the coder's transform space. Generally, quantization is the state of being constrained to a set of discrete values, rather than varying continuously. ...
Transform coding is a type of data compression for natural data like audio signals or photographic images. ...
In functional analysis and its applications, a function space can be viewed as a vector space of infinite dimension whose basis vectors are functions not vectors. ...
Compression artifacts occur in many common media such as DVDs and also common computer file formats such as JPEG, MP3, or MPEG files. Uncompressed media (such as on Laserdiscs, Audio CDs, and WAV files) or losslessly compressed media (FLAC, PNG, etc.) do not suffer from compression artifacts. DVD is an optical disc storage media format that can be used for data storage, including movies with high video and sound quality. ...
A photo of a flower compressed with successively higher compression ratios from left to right. ...
MP3 is a popular digital audio encoding and lossy compression format invented and standardised in 1991 by a team of engineers working in the framework of the ISO/IEC MPEG audio committee under the chairmanship of Professor Hans Musmann (University of Hannover - Germany). ...
The Moving Picture Experts Group or MPEG is a working group of ISO/IEC charged with the development of video and audio encoding standards. ...
Pioneers LaserDisc Logo The laserdisc (LD) was the first commercial optical disc storage medium, and was used primarily for the presentation of movies. ...
CD re-directs here; see Cd for other meanings of CD. Image of a compact disc (pencil included for scale) A compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. ...
WAV (or WAVE), short for WAVE form audio format, is a Microsoft and IBM audio file format standard for storing audio on PCs. ...
Lossless data compression is a class of data compression algorithms that allows the original data to be reconstructed exactly from the compressed data. ...
The Free Lossless Audio Codec (FLAC) Logo. ...
A PNG image with an 8-bit transparency layer (top), overlayed onto a chequered background (bottom). ...
Compression artifacts in picture coding
When using the Discrete Cosine Transform for block-based coding, as in JPEG-compressed images, several types of artifacts can appear, including contouring in smooth regions, staircase noise along curving edges, mosquito noise around edges, and/or checker boarding in "busy" regions. The discrete cosine transform (DCT) is a Fourier-related transform similar to the discrete Fourier transform (DFT), but using only real numbers. ...
Where predictive coding of motion pictures is used, as in MPEG-1, compression artifacts tend to remain on several generations of decompressed frames, leading to a "painting" effect being seen, as if the picture were being painted by an unseen artist's paint-brush. MPEG-1 is the designation for a group of audio and video coding standards agreed upon by MPEG (Moving Pictures Experts Group). ...
Where motion prediction is used, as in MPEG-2 or MPEG-4, compression artifacts tend to 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. MPEG-2 (1994) is the designation for a group of coding standards for digital audio and video, agreed upon by MPEG (Moving Pictures Experts Group), and published as the ISO/IEC 13818 international standard. ...
MPEG-4, introduced in late 1998, is the designation for a group of audio and video coding standards and related technology agreed upon by the ISO/IEC Moving Picture Experts Group (MPEG). ...
Optic flow is the perceived visual motion of objects as the observer moves relative to them. ...
Errors in the bit-stream 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, it is not unknown for decoders to continue to apply 'painting' updates to the damaged picture, creating "ghost image" effects.
To stop the build-up of compression artifacts, most compression systems occasionally send an entire compressed frame without prediction or differencing, usually at the start of a shot and at regular intervals thereafter. In MPEG picture coding, these are known as "I-frames", with the 'I' standing for "intraframe compression." B pictures (often called B frames) are one of the three major picture types found in typical video compression designs. ...
Image artifact reduction Various approaches have been proposed to reduce the effects of image compression, but in order to utilize standardized compression/decompression techniques and to retain the benefits of the compression (for instance, lower transmission and storage costs), many of these methods have focused on "post-processing" — that is, processing the images when they are received or viewed. No post-processing technique has been shown to improve image quality in all cases because, and 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.
Compression artifacts in audio coding One technique is to use a lower bitrate by resampling the audio. By reducing the sample rate, higher frequencies must be removed to conform to the Nyquist-Shannon sampling theorem. If the anti-aliasing filter works imperfectly, digital distortion or aliasing will be heard in the form of inharmonic frequencies reflected around the Nyquist frequency. (e.g. a 22.85 kHz tone processed with a Nyquist frequency of 22.05 kHz will result in a tone of 22.05 - (22.85 - 22.05) = 21.25 kHz. This can be generalised to outputF = NF x 2 - inputF) This may be subtle, but more severe levels of distortion can sound similar to ring modulation. (see Aliasing#An_audio_example) Lowering the amount of data (bits) captured per sample can result in loss of detail and dynamic range in the audio. The loss of quality in both methods will be uniform across the recording. In telecommunications and computing, bit rate (sometimes written bitrate) is the frequency at which bits are passing a given (physical or metaphorical) point. It is quantified using the bit per second (bit/s) unit. ...
The sampling frequency or sampling rate defines the number of samples per second taken from a continuous signal to make a discrete signal. ...
The Nyquist-Shannon sampling theorem is the fundamental theorem in the field of information theory, in particular telecommunications. ...
An anti-aliasing filter is commonly used in conjuction with digital signal processing and is a filter to restrict the bandwidth to approximately satisfy the Shannon-Nyquist sampling theorem. ...
In statistics, signal processing, and related disciplines, aliasing is an effect that causes different continuous signals to become indistinguishable (or aliases of one another) when sampled. ...
In music, inharmonic refers to the degree to which the frequencies of the overtones of a fundamental differ from whole number multiples of the fundamentals frequency. ...
The Nyquist frequency, named after the Nyquist-Shannon sampling theorem, is half the sampling frequency for a signal. ...
Ring modulation is an audio effect performed by multiplying two audio signals, where one is typically a sine-wave or another simple waveform. ...
In statistics, signal processing, and related disciplines, aliasing is an effect that causes different continuous signals to become indistinguishable (or aliases of one another) when sampled. ...
Dynamic range is a term used frequently in numerous fields to describe the ratio between the smallest and largest possible values of a changeable quantity. ...
Another technique is to attempt to remove sounds that typical human hearing cannot perceive. As a human being cannot perceive the difference, the resulting data will be simpler (and thus compress better using lossless techniques). For example, in general human beings are unable to perceive a quiet tone simultaneously with a similar, but louder tone. A lossy compression technique might identify this quiet tone and attempt to remove it. As no algorithm is perfect and tradeoffs can be made to throw away additional data to reduce data rate, this will occasionally lead to perceivable sounds being discarded. As these sounds are, ideally, hard to perceive anyway, the result will generally be of flattening complexity, or muddying the sound.
Many systems attempt to replace the series of samples of audio with other representations. Typically these representations make it easier to attempt to eliminate non-perceivable sounds and make it easier to compress the data using traditional lossless techniques. One common technique is to represent the audio as the sum of a series of sine waves. The representation may not be perfect; in exchange for a more easily compressed description, accuracy may be sacrificed.
Many audio compression systems endeavor to match a target data rate, typically expressed in bits of data per second of audio. When using a constant data rate, simple portions of the recording (a simple tone or silence) will be easily compressed to the target rate; the resulting playback will be highly similar to the source audio. As more complex sections are recorded, the system will be forced to seriously reduce quality to meet the target rate; the resulting playback will display more artifacts. Many audio compression systems support Variable Bit Rate encoding, varying the target rate in an attempt to maintain a constant quality of reproduction. Variable bit rate (VBR) is a term used in telecommunications, relating to the quality of service. ...
See also In data compression or psychoacoustics, transparency is the ideal result of lossy data compression. ...
A lossy data compression method is one where compressing data and then decompressing it retrieves data that may well be different to the original, but is close enough to be useful in some way. ...
External links - ff123's Audio Artifact Training Page
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