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In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size for storing, handling, and transmitting content. The different versions of the photo of the cat on this page show how higher degrees of approximation create coarser images as more details are removed. This is opposed to
lossless data compression Lossless compression is a class of data compression algorithms that allows the original data to be perfectly reconstructed from the compressed data. By contrast, lossy compression permits reconstruction only of an approximation of the original d ...
(reversible data compression) which does not degrade the data. The amount of data reduction possible using lossy compression is much higher than using lossless techniques. Well-designed lossy compression technology often reduces file sizes significantly before degradation is noticed by the end-user. Even when noticeable by the user, further data reduction may be desirable (e.g., for real-time communication or to reduce transmission times or storage needs). The most widely used lossy compression algorithm is the
discrete cosine transform A discrete cosine transform (DCT) expresses a finite sequence of data points In statistics Statistics is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics ...
(DCT), first published by Nasir Ahmed, T. Natarajan and
K. R. Rao Kamisetty Ramamohan Rao was an Indian-American electrical engineer. He was a professor of Electrical Engineering at the University of Texas at Arlington (UT Arlington). Academically known as K. R. Rao, he is credited with the co-invention of ...
in 1974. In 2019 a new family of sinusoidal-hyperbolic transform functions, which have comparable properties and performance with DCT, were proposed for lossy compression. Lossy compression is most commonly used to compress
multimedia Multimedia is a form of communication that combines different such as , , , , or into a single interactive presentation, in contrast to traditional mass media which featured little to no interaction fr ...

multimedia
data (
audio Audio most commonly refers to sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion ( ...
,
video Video is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active d ...

video
, and
image An image (from la, imago) is an artifact that depicts visual perception Visual perception is the ability to interpret the surrounding environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non- ...

image
s), especially in applications such as
streaming media Streaming media is multimedia that is delivered and consumed in a continuous manner from a source, with little or no intermediate storage in network elements. ''Streaming'' refers to the delivery method of content, rather than the content its ...
and
internet telephony Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia Multimedia is a form of communication that co ...
. By contrast, lossless compression is typically required for text and data files, such as bank records and text articles. It can be advantageous to make a master lossless file which can then be used to produce additional copies from. This allows one to avoid basing new compressed copies off of a lossy source file, which would yield additional artifacts and further unnecessary information loss.


Types

It is possible to compress many types of digital data in a way that reduces the size of a
computer file A computer file is a computer resource for recording data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sense, data are a set of values of qualitative property, qualitative or quant ...
needed to store it, or the
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 thro ...
needed to transmit it, with no loss of the full information contained in the original file. A picture, for example, is converted to a digital file by considering it to be an array of dots and specifying the color and brightness of each dot. If the picture contains an area of the same color, it can be compressed without loss by saying "200 red dots" instead of "red dot, red dot, ...(197 more times)..., red dot." The original data contains a certain amount of information, and there is a lower limit to the size of file that can carry all the information. Basic
information theory Information theory is the scientific study of the quantification (science), quantification, computer data storage, storage, and telecommunication, communication of Digital data, digital information. The field was fundamentally established by the ...
says that there is an absolute limit in reducing the size of this data. When data is compressed, its
entropy Entropy is a scientific concept as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics ...
increases, and it cannot increase indefinitely. For example, a compressed ZIP file is smaller than its original, but repeatedly compressing the same file will not reduce the size to nothing. Most compression algorithms can recognize when further compression would be pointless and would in fact increase the size of the data. In many cases, files or data streams contain more information than is needed. For example, a picture may have more detail than the eye can distinguish when reproduced at the largest size intended; likewise, an audio file does not need a lot of fine detail during a very loud passage. Developing lossy compression techniques as closely matched to human perception as possible is a complex task. Sometimes the ideal is a file that provides exactly the same perception as the original, with as much digital information as possible removed; other times, perceptible loss of quality is considered a valid tradeoff. The terms "irreversible" and "reversible" are preferred over "lossy" and "lossless" respectively for some applications, such as medical image compression, to circumvent the negative implications of "loss". The type and amount of loss can affect the utility of the images. Artifacts or undesirable effects of compression may be clearly discernible yet the result still useful for the intended purpose. Or lossy compressed images may be '
visually lossless In data compression and psychoacoustics, transparency is the result of lossy data compression accurate enough that the compressed result is perception, perceptually indistinguishable from the uncompressed input, i.e. perceptually lossless. In other ...
', or in the case of medical images, so-called
Diagnostically Acceptable Irreversible Compression (DAIC)Diagnostically acceptable irreversible compression (DAIC) is the amount of lossy compression which can be used on a medical imaging, medical image to produce a result that does not prevent the reader from using the image to make a medical diagnosis. ...
may have been applied.


Transform coding

Some forms of lossy compression can be thought of as an application of
transform coding Transform coding is a type of data compression In signal processing, 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 e ...
, which is a type of data compression used for
digital images Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electronics comprises the physics, engineering, technology a ...
,
digital audio Digital audio is a representation of sound recorded in, or converted into, Digital signal (signal processing), digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical Sampling (signal processing), s ...
signals, and
digital video Digital video is an electronic representation of moving visual images (video) in the form of encoded digital data. This is in contrast to analog video, which represents moving visual images in the form of analog signals. Digital video comprises ...
. The transformation is typically used to enable better (more targeted) quantization. Knowledge of the application is used to choose information to discard, thereby lowering its
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 thro ...
. The remaining information can then be compressed via a variety of methods. When the output is decoded, the result may not be identical to the original input, but is expected to be close enough for the purpose of the application. The most common form of lossy compression is a transform coding method, the
discrete cosine transform A discrete cosine transform (DCT) expresses a finite sequence of data points In statistics Statistics is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics ...
(DCT), which was first published by Nasir Ahmed, T. Natarajan and
K. R. Rao Kamisetty Ramamohan Rao was an Indian-American electrical engineer. He was a professor of Electrical Engineering at the University of Texas at Arlington (UT Arlington). Academically known as K. R. Rao, he is credited with the co-invention of ...
in 1974. DCT is the most widely used form of lossy compression, for popular
image compression Image compression is a type of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal processing, sound, image proc ...
formats (such as
JPEG JPEG ( ) is a commonly used method of lossy compression In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represe ...

JPEG
),
video coding standards A video coding format (or sometimes video compression format) is a Content format, content representation format for storage or transmission of Digital data, digital video, video content (such as in a data file or Bitstream format, bitstream). It ...
(such as
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 ...

MPEG
and H.264/AVC) and audio compression formats (such as
MP3 MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio Digital audio is a representation of sound recorded in, or converted into, Digital signal (signal processing), digital form. In digital a ...

MP3
and
AAC AAC may refer to: Aviation * Advanced Aircraft Advanced Aircraft Corporation is an aircraft manufacturer based in Carlsbad, California. History AAC bought out Riley Aircraft in 1983 in aviation, 1983. Products The firm has specialised in conve ...
). In the case of audio data, a popular form of transform coding is
perceptual coding Psychoacoustics is the branch of psychophysics Psychophysics quantitatively investigates the relationship between physical stimulus (physiology), stimuli and the sensation (psychology), sensations and perceptions they produce. Psychophysics has be ...
, which transforms the raw data to a domain that more accurately reflects the information content. For example, rather than expressing a sound file as the amplitude levels over time, one may express it as the frequency spectrum over time, which corresponds more accurately to human audio perception. While data reduction (compression, be it lossy or lossless) is a main goal of transform coding, it also allows other goals: one may represent data more accurately for the original amount of space – for example, in principle, if one starts with an analog or high-resolution
digital master A digital master is an image An SAR radar imaging, radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour shows the Teide volcano. The city of Santa Cruz de Tenerife is visible as the purple and white area on the ...
, an
MP3 MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio Digital audio is a representation of sound recorded in, or converted into, Digital signal (signal processing), digital form. In digital a ...

MP3
file of a given size should provide a better representation than a raw uncompressed audio in
WAV Waveform Audio File Format (WAVE, or WAV due to its filename extension A filename extension, file extension or file type is an identifier specified as a suffix to the name A name is a term used for identification by an external observer. T ...

WAV
or
AIFF Audio Interchange File Format (AIFF) is an audio file format An audio file format is a file format for storing digital audio data on a computer system. The bit layout of the audio data (excluding metadata) is called the audio coding format and can ...
file of the same size. This is because uncompressed audio can only reduce file size by lowering bit rate or depth, whereas compressing audio can reduce size while maintaining bit rate and depth. This compression becomes a selective loss of the least significant data, rather than losing data across the board. Further, a transform coding may provide a better domain for manipulating or otherwise editing the data – for example, equalization of audio is most naturally expressed in the frequency domain (boost the bass, for instance) rather than in the raw time domain. From this point of view, perceptual encoding is not essentially about ''discarding'' data, but rather about a ''better representation'' of data. Another use is for
backward compatibility Backward or Backwards is a relative direction Body relative directions (also known as egocentric coordinates) are orientation (geometry), geometrical orientations relative to a body such as a human person's. The most common ones are: left ...
and
graceful degradation Fault tolerance is the property that enables a system to continue operating properly in the event of the failure of one or more faults within some of its components. If its operating quality decreases at all, the decrease is proportional to the s ...
: in color television, encoding color via a
luminance Luminance is a photometricPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an ...
-
chrominance Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture, separately from the accompanying Luma (video), luma signal (or Y' for short). Chrominance is usually represented as two ...
transform domain (such as
YUV YUV is a color encoding system typically used as part of a color image pipelineAn image pipeline or video pipeline is the set of components commonly used between an image source (such as a camera, a scanner, or the rendering engine in a compute ...
) means that black-and-white sets display the luminance, while ignoring the color information. Another example is
chroma subsampling Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information Information is processed, organised and structured data. It provides context for data and enables decision making process. For examp ...
: the use of
color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of color -- whether such representation entails an analog signal, analog or a ...
s such as
YIQ An image along with its Y, I, and Q components. YIQ is the color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of co ...
, used in
NTSC The National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12-19, with Some supplementary references cited in the Reports, and the Petition for adoption of transm ...
, allow one to reduce the resolution on the components to accord with human perception – humans have highest resolution for black-and-white (luma), lower resolution for mid-spectrum colors like yellow and green, and lowest for red and blues – thus NTSC displays approximately 350 pixels of luma per
scanline A scan line (also scanline) is one line, or row, in a raster scan A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics file:Rgb-raster ...
, 150 pixels of yellow vs. green, and 50 pixels of blue vs. red, which are proportional to human sensitivity to each component.


Information loss

Lossy compression formats suffer from
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 consi ...
: repeatedly compressing and decompressing the file will cause it to progressively lose quality. This is in contrast with
lossless data compression Lossless compression is a class of data compression In signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipme ...
, where data will not be lost via the use of such a procedure. Information-theoretical foundations for lossy data compression are provided by rate-distortion theory. Much like the use of
probability Probability is the branch of mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained ...

probability
in optimal coding theory, rate-distortion theory heavily draws on
Bayesian Thomas Bayes (/beɪz/; c. 1701 – 1761) was an English statistician, philosopher, and Presbyterian minister. Bayesian () refers to a range of concepts and approaches that are ultimately based on a degree-of-belief interpretation of probability, ...
estimation Estimation (or estimating) is the process of finding an estimate, or approximation An approximation is anything that is intentionally similar but not exactly equal to something else. Etymology and usage The word ''approximation'' is derived ...
and
decision theory Decision theory (or the theory of choice not to be confused with choice theory) is the study of an agent's choices. Decision theory can be broken into two branches: normative Normative generally means relating to an evaluative standard. Normativi ...
in order to model perceptual distortion and even
aesthetic Aesthetics, or esthetics (), is a branch of philosophy that deals with the nature of beauty and taste (sociology), taste, as well as the philosophy of art (its own area of philosophy that comes out of aesthetics). It examines subjective and ...

aesthetic
judgment. There are two basic lossy compression schemes: * In ''lossy transform
codec A codec is a device or computer program In imperative programming, a computer program is a sequence of instructions in a programming language that a computer can execute or interpret. In declarative programming, a ''computer program'' is a Set ...
s'', samples of picture or sound are taken, chopped into small segments, transformed into a new basis space, and quantized. The resulting quantized values are then entropy coded. * In ''lossy predictive codecs'', previous and/or subsequent decoded data is used to predict the current sound sample or image frame. The error between the predicted data and the real data, together with any extra information needed to reproduce the prediction, is then quantized and coded. In some systems the two techniques are combined, with transform codecs being used to compress the error signals generated by the predictive stage.


Comparison

The advantage of lossy methods over
lossless Lossless compression is a class of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal processing, sound, image ...
methods is that in some cases a lossy method can produce a much smaller compressed file than any lossless method, while still meeting the requirements of the application. Lossy methods are most often used for compressing sound, images or videos. This is because these types of data are intended for human interpretation where the mind can easily "fill in the blanks" or see past very minor errors or inconsistencies – ideally lossy compression is
transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * Transparency (photography), a sti ...
(imperceptible), which can be verified via an
ABX testAn ABX test is a method of comparing two choices of sensory stimuli to identify detectable differences between them. A subject is presented with two known samples (sample , the first reference, and sample , the second reference) followed by one unkno ...
. Data files using lossy compression are smaller in size and thus cost less to store and to transmit over the Internet, a crucial consideration for
streaming video Streaming media is multimedia Multimedia is a form of communication that combines different such as , , , , or into a single interactive presentation, in contrast to traditional mass media which ...
services such as
Netflix Netflix, Inc. is an American subscription The subscription business model is a business model in which a customer In sales Sales are activities related to selling or the number of goods sold in a given targeted time period. Th ...

Netflix
and
streaming audio Streaming media is multimedia that is constantly received by and presented to an end user while being delivered by a provider over the Internet. The verb ''to stream'' refers to the process of delivering or obtaining media in a continuous mann ...
services such as
Spotify Spotify (Help:IPA/English, /ˈspɒtɪfaɪ/; Swedish: Help:IPA/Swedish, spɔ̂tːɪfaj, is a Sweden, Swedish Streaming media, audio streaming and media services provider, founded in 2006 by Daniel Ek. Spotify is incorporated in Luxembourg a ...

Spotify
.


Emotional effects

A study conducted by the Audio Engineering Library concluded that lower
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 Data rate units, bit per second unit (symbol: ''bit/s' ...
(112 kbps) lossy compression formats such as
MP3 MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio Digital audio is a representation of sound recorded in, or converted into, Digital signal (signal processing), digital form. In digital a ...

MP3
s have distinct effects on timbral and emotional characteristics, tending to strengthen negative emotional qualities and weaken positive ones. The study further noted that the
trumpet The trumpet is a brass instrument A brass instrument is a that produces sound by of air in a tubular in sympathy with the vibration of the player's lips. Brass instruments are also called labrosones or labrophones, from Latin and Greek ...

trumpet
is the instrument most affected by compression, while the
horn Horn usually refers to: *Horn (acoustic), a conical or bell shaped aperture used to guide sound ** Horn (instrument), collective name for tube-shaped wind musical instruments *Horn (anatomy), a pointed, bony projection on the head of various anima ...

horn
is least.


Transparency

When a user acquires a lossily compressed file, (for example, to reduce download time) the retrieved file can be quite different from the original at the
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algo ...
level while being indistinguishable to the human ear or eye for most practical purposes. Many compression methods focus on the idiosyncrasies of
human physiology The human body is the structure of a human being Humans (''Homo sapiens'') are the most populous and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an ...

human physiology
, taking into account, for instance, that the human eye can see only certain wavelengths of light. The
psychoacoustic model Psychoacoustics is the branch of psychophysics Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they produce. Psychophysics has been described as "the scientific study of the ...

psychoacoustic model
describes how sound can be highly compressed without degrading perceived quality. Flaws caused by lossy compression that are noticeable to the human eye or ear are known as
compression artifact A compression artifact (or artefact) is a noticeable distortion of media (including Image, images, Sound recording, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the medi ...
s.


Compression ratio

The
compression ratio The compression ratio is the ratio between the volume of the cylinder A cylinder (from ) has traditionally been a Solid geometry, three-dimensional solid, one of the most basic of curvilinear geometric shapes. Geometrically, it can be consi ...
(that is, the size of the compressed file compared to that of the uncompressed file) of lossy video codecs is nearly always far superior to that of the audio and still-image equivalents. * Video can be compressed immensely (e.g., 100:1) with little visible quality loss * Audio can often be compressed at 10:1 with almost imperceptible loss of quality * Still images are often lossily compressed at 10:1, as with audio, but the quality loss is more noticeable, especially on closer inspection.


Transcoding and editing

An important caveat about lossy compression (formally transcoding), is that editing lossily compressed files causes digital generation loss from the re-encoding. This can be avoided by only producing lossy files from (lossless) originals and only editing (copies of) original files, such as images in
raw image format A camera raw image file contains minimally processed data from the image sensor of either a digital camera, a motion picture film scanner, or other image scanner. Raw files are named so because they are not yet processed and therefore are not ...
instead of
JPEG JPEG ( ) is a commonly used method of lossy compression In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represe ...

JPEG
. If data which has been compressed lossily is decoded and compressed losslessly, the size of the result can be comparable with the size of the data before lossy compression, but the data already lost cannot be recovered. When deciding to use lossy conversion without keeping the original, format conversion may be needed in the future to achieve compatibility with software or devices (
format shiftingFormat shifting is the conversion of media files into different file format ogg-file: 154 kilobytes. A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode informa ...
), or to avoid paying patent royalties for decoding or distribution of compressed files.


Editing of lossy files

By modifying the compressed data directly without decoding and re-encoding, some editing of lossily compressed files without degradation of quality is possible. Editing which reduces the file size as if it had been compressed to a greater degree, but without more loss than this, is sometimes also possible.


JPEG

The primary programs for lossless editing of JPEGs are jpegtran, and the derived exiftran (which also preserves
Exif Exchangeable image file format (officially Exif, according to JEIDA/JEITA/CIPA specifications) is a standard that specifies the formats for image An image (from la, imago) is an artifact that depicts visual perception Visual pe ...
information), an
Jpegcrop
(which provides a Windows interface). These allow the image to be cropped, rotated, flipped, and flopped, or even converted to
grayscale In digital photography Digital photography uses cameras containing arrays of electronics, electronic photodetectors to produce images focused by a lens (optics), lens, as opposed to an exposure on photographic film. The captured images a ...

grayscale
(by dropping the
chrominance Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture, separately from the accompanying Luma (video), luma signal (or Y' for short). Chrominance is usually represented as two ...
channel). While unwanted information is destroyed, the quality of the remaining portion is unchanged. Some other transforms are possible to some extent, such as joining images with the same encoding (composing side by side, as on a grid) or pasting images such as logos onto existing images (both vi
Jpegjoin
, or scaling. Some changes can be made to the compression without re-encoding: * Optimizing the compression (to reduce size without change to the decoded image) * Converting between progressive and non-progressive encoding. The freeware Windows-only
IrfanView IrfanView () is an image viewer, editor, organiser and converter program for Microsoft Windows. It can also play video and audio files, and has some image creation and painting capabilities. IrfanView is free for non-commercial use; commercial u ...

IrfanView
has some lossless JPEG operations in its JPG_TRANSFORM plugin.


Metadata

Metadata, such as
ID3 tag ID3 is a metadata Metadata is "data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sense, data are a set of values of qualitative property, qualitative or quantity, quantitative ...
s,
Vorbis comment A Vorbis comment is a metadata container used in the Vorbis, FLAC, Theora, Speex and Opus (audio format), Opus file formats. It allows information such as the title, artist, album, track number or other information about the file to be added to the ...
s, or
Exif Exchangeable image file format (officially Exif, according to JEIDA/JEITA/CIPA specifications) is a standard that specifies the formats for image An image (from la, imago) is an artifact that depicts visual perception Visual pe ...
information, can usually be modified or removed without modifying the underlying data.


Downsampling/compressed representation scalability

One may wish to downsample or otherwise decrease the resolution of the represented source signal and the quantity of data used for its compressed representation without re-encoding, as in
bitrate peeling Bitrate Peeling is a technique used in Ogg Ogg is a free, open Open or OPEN may refer to: citizen * Open (band), Australian pop/rock band * The Open (band), English indie rock band * ''Open'' (Blues Image album), 1969 * ''Open'' (Gotthar ...
, but this functionality is not supported in all designs, as not all codecs encode data in a form that allows less important detail to simply be dropped. Some well-known designs that have this capability include
JPEG 2000 JPEG 2000 (JP2) is an image compression Image compression is a type of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such a ...
for still images and H.264/MPEG-4 AVC based Scalable Video Coding for video. Such schemes have also been standardized for older designs as well, such as
JPEG JPEG ( ) is a commonly used method of lossy compression In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represe ...

JPEG
images with progressive encoding, and
MPEG-2 MPEG-2 (a.k.a. H.222/H.262 as was defined by the ITU 260px, ITU Monument, Bern The International Telecommunication Union is a specialized agency of the United Nations responsible for all matters related to information and communicati ...
and
MPEG-4 Part 2 MPEG-4 Part 2, MPEG-4 Visual (formally ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or re ...
video, although those prior schemes had limited success in terms of adoption into real-world common usage. Without this capacity, which is often the case in practice, to produce a representation with lower resolution or lower fidelity than a given one, one needs to start with the original source signal and encode, or start with a compressed representation and then decompress and re-encode it (
transcoding Transcoding is the direct digital-to-digital conversion of one encoding In communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (philosophy), entities or Org ...
), though the latter tends to cause digital generation loss. Another approach is to encode the original signal at several different bitrates, and then either choose which to use (as when streaming over the internet – as in
RealNetworks RealNetworks, Inc. is a provider of artificial intelligence and computer vision based products. RealNetworks was a pioneer in Internet streaming media delivery software and services. They are based in Seattle, Washington (state), Washington, U ...
' " SureStream" – or offering varying downloads, as at Apple's iTunes Store), or broadcast several, where the best that is successfully received is used, as in various implementations of hierarchical modulation. Similar techniques are used in mipmaps, pyramid (image processing), pyramid representations, and more sophisticated scale space methods. Some audio formats feature a combination of a lossy format and a lossless correction which when combined reproduce the original signal; the correction can be stripped, leaving a smaller, lossily compressed, file. Such formats include MPEG-4 SLS (Scalable to Lossless), WavPack, OptimFROG DualStream, and DTS-HD Master Audio, DTS-HD Master Audio in lossless (XLL) mode).


Methods


Graphics


Image

* Discrete cosine transform (DCT) **
JPEG JPEG ( ) is a commonly used method of lossy compression In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represe ...

JPEG
** WebP (high-density lossless or lossy compression of RGB and RGBA images) ** High Efficiency Image Format (HEIF) ** Better Portable Graphics (BPG) (lossless or lossy compression) ** JPEG XR, a successor of JPEG with support for high-dynamic-range imaging, high-dynamic range, wide gamut pixel formats (lossless or lossy compression) * Wavelet compression **
JPEG 2000 JPEG 2000 (JP2) is an image compression Image compression is a type of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such a ...
, JPEG's successor format that uses wavelets (lossless or lossy compression) ** DjVu ** ICER, used by the Mars Rovers, related to
JPEG 2000 JPEG 2000 (JP2) is an image compression Image compression is a type of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such a ...
in its use of wavelets ** Progressive Graphics File, PGF, Progressive Graphics File (lossless or lossy compression) * Cartesian Perceptual Compression, also known as CPC * Fractal compression * JBIG2 (lossless or lossy compression) * S3TC Texture mapping, texture compression for GPU, 3D computer graphics hardware


3D computer graphics

* glTF


Video

* Discrete cosine transform (DCT) ** H.261 ** Motion JPEG ** MPEG-1 Part 2K. R. Rao and J. J. Hwang, ''Techniques and Standards for Image, Video, and Audio Coding'', Prentice Hall, 1996; JPEG: Chapter 8; H.261: Chapter 9; MPEG-1: Chapter 10; MPEG-2: Chapter 11. ** MPEG-2 Part 2 (H.262) **
MPEG-4 Part 2 MPEG-4 Part 2, MPEG-4 Visual (formally ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or re ...
(H.263) ** Advanced Video Coding (AVC / H.264 / MPEG-4 AVC) (may also be lossless, even in certain video sections) ** High Efficiency Video Coding (HEVC / H.265) ** Ogg Theora (noted for its lack of patent restrictions) ** VC-1 * Wavelet compression ** Motion JPEG 2000 ** Dirac codec, Dirac * Sorenson codec, Sorenson video codec


Audio


General

* Modified discrete cosine transform (MDCT) ** Dolby Digital (AC-3) ** Adaptive Transform Acoustic Coding (ATRAC) ** MPEG Layer III (MP3) ** Advanced Audio Coding (AAC / MP4 Audio) ** Vorbis ** Windows Media Audio (WMA) (Standard and Pro profiles are lossy. WMA Lossless is also available.) ** LDAC (codec), LDAC ** Opus (codec), Opus (Notable for lack of patent restrictions, low delay, and high quality speech and general audio.) * Adaptive differential pulse-code modulation (ADPCM) ** Master Quality Authenticated (MQA) * MPEG-1 Audio Layer II (MP2) * Musepack (based on Musicam) * AptX, aptX/ aptX-HD


Speech

* Linear predictive coding (LPC) ** Adaptive predictive coding (APC) ** Code-excited linear prediction (CELP) ** Algebraic code-excited linear prediction (ACELP) ** Relaxed code-excited linear prediction (RCELP) ** Low-delay CELP (LD-CELP) ** Adaptive Multi-Rate audio codec, Adaptive Multi-Rate (used in GSM and 3GPP) ** Codec2 (noted for its lack of patent restrictions) ** Speex (noted for its lack of patent restrictions) * Modified discrete cosine transform (MDCT) ** AAC-LD ** Constrained Energy Lapped Transform (CELT) ** Opus (codec), Opus (mostly for real-time applications)


Other data

Researchers have (semi-seriously) performed lossy compression on text by either using a thesaurus to substitute short words for long ones, or Natural language generation, generative text techniques, although these sometimes fall into the related category of lossy data conversion.


Lowering resolution

A general kind of lossy compression is to lower the resolution of an image, as in image scaling, particularly Decimation (signal processing), decimation. One may also remove less "lower information" parts of an image, such as by seam carving. Many media transforms, such as Gaussian blur, are, like lossy compression, irreversible: the original signal cannot be reconstructed from the transformed signal. However, in general these will have the same size as the original, and are not a form of compression. Lowering resolution has practical uses, as the NASA New Horizons craft transmitted thumbnails of its encounter with Pluto-Charon before it sent the higher resolution images. Another solution for slow connections is the usage of Interlacing (bitmaps), Image interlacing which progressively defines the image. Thus a partial transmission is enough to preview the final image, in a lower resolution version, without creating a scaled and a full version too.


See also

* Data compression * Lossless compression * Compression artifact * Rate–distortion theory * List of codecs * Lenna * Image scaling * Seam carving * Transcoding


Notes


External links


Lossy audio formats
comparing the speed and compression strength of five lossy audio formats.
Data compression basics
including chapters on lossy compression of images, audio and video.
Lossy PNG image compression (research)
(Wayback Machine copy)
Using lossy GIF/PNG compression for the web (article)


comparing the suitability of JPG and lossless compression for image archives {{DEFAULTSORT:Lossy Compression Data compression Lossy compression algorithms, Lossy compression algorithms fr:Compression de données#Compression avec pertes