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Image compression is a type 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 equipment, devices, and systems which use electricity, electro ...
applied to
digital image A digital image is an composed of s, also known as ''pixels'', each with ', ' of numeric representation for its or that is an output from its fed as input by its denoted with ''x'', ''y'' on the x-axis and y-axis, respectively. Depending on ...
s, to reduce their cost for storage or
transmission Transmission may refer to: Science and technology * Power transmissionPower transmission is the movement of energy from its place of generation to a location where it is applied to perform useful Mechanical work, work. Power (physics), Power is d ...

transmission
.
Algorithm In and , an algorithm () is a finite sequence of , computer-implementable instructions, typically to solve a class of problems or to perform a computation. Algorithms are always and are used as specifications for performing s, , , and other ...

Algorithm
s may take advantage of
visual perception Visual perception is the ability to interpret the surrounding environment (biophysical), environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in ...
and the
statistical Statistics is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data Data are units of information Information can be thought of as the resolution of uncertainty; it answers th ...
properties of image data to provide superior results compared with generic
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 equipment, devices, and systems which use electricity, electro ...
methods which are used for other digital data.


Lossy and lossless image compression

Image compression may be
lossy In information technology, lossy compression or irreversible compression is the class of data compression, data encoding methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to r ...
or
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 ...
. Lossless compression is preferred for archival purposes and often for medical imaging, technical drawings,
clip art Clip art (also clipart, clip-art), in the graphic arts, is pre-made images used to illustrate any medium. Today, clip art is used extensively. Clip art comes in many forms, both electronic and printed. However, most clip art today is created, di ...
, or comics. Lossy compression methods, especially when used at low
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' ...
s, introduce
compression artifact A compression artifact (or artefact) is a noticeable distortion of media (including images File:TEIDE.JPG, An Synthetic aperture radar, SAR radar imaging, radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour ...
s. Lossy methods are especially suitable for natural images such as photographs in applications where minor (sometimes imperceptible) loss of fidelity is acceptable to achieve a substantial reduction in bit rate. Lossy compression that produces negligible differences may be called visually lossless. Methods for
lossy compression In information technology, lossy compression or irreversible compression is the class of data compression, data encoding methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to r ...
: *
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 ...
– This is the most commonly used method. **
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) – The most widely used form of lossy compression. It is a type of
Fourier-related transform This is a list of linear transformation In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, chan ...
, and was originally developed 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. The DCT is sometimes referred to as "DCT-II" in the context of a family of discrete cosine transforms (see
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 ...
). It is generally the most efficient form of image compression. *** DCT is used in
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
, the most popular lossy format, and the more recent
HEIF High Efficiency Image File Format (HEIF) is a container format for storing individual images and image sequences. The standard covers multimedia files that can also include other media streams, such as timed text, audio and video. HEIF can st ...
. ** The more recently developed
wavelet transform In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
is also used extensively, followed by quantization and
entropy coding In 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 fundamenta ...
. * Reducing 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 color -- whether such representation entails an analog signal, analog or a ...
to the most common colors in the image. The selected colors are specified in the colour
palette Palette may refer to: * Cosmetic palette, an archaeological form * Palette, another name for a color scheme * Palette (painting), a wooden board used for mixing colors for a painting * Palette (company), a Japanese visual novel studio (video game c ...
in the header of the compressed image. Each pixel just references the index of a color in the color palette, this method can be combined with
dithering image represented in 1-bit black-and-white space with dithering Dither is an intentionally applied form of image noise, noise used to randomize quantization error Quantization, in mathematics and digital signal processing, is the proces ...
to avoid
posterization format (24-bit color or 16.7 million colors) before posterization, contrasting the result of saving to GIF format (256 colors). Posterization occurs across the image, but is most obvious in areas of subtle variation in tone. Image:Lucidity I.jpg, ...
. *
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 ...
. This takes advantage of the fact that the human eye perceives spatial changes of brightness more sharply than those of color, by averaging or dropping some of the chrominance information in the image. *
Fractal compression Fractal compression is a 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 represent the content. Thes ...
. Methods for
lossless compression 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 ...
: *
Run-length encoding Run-length encoding (RLE) is a form of 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 ...
– used in default method in
PCX PCX, standing for ''PiCture eXchange'', is an image file format Image file formats are standardized means of organizing and storing digital images. An image file format may store data in an uncompressed format, a compressed format (which may b ...
and as one of possible in BMP, TGA,
TIFF Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by image scanner, scanning, FAX, faxi ...

TIFF
* Area image compression * Predictive coding – used in
DPCM Differential pulse-code modulation (DPCM) is a signal encoder that uses the baseline of pulse-code modulation Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signal An analog signal is any continuous ...

DPCM
*
Entropy encoding In information theory, an entropy coding (or entropy encoding) is a lossless compression , lossless data compression scheme that is independent of the specific characteristics of the medium. One of the main types of entropy coding creates and assi ...
– the two most common entropy encoding techniques are
arithmetic coding Arithmetic coding is a form of entropy encoding In information theory, an entropy coding (or entropy encoding) is a lossless compression , lossless data compression scheme that is independent of the specific characteristics of the medium. One of ...
and
Huffman coding In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algor ...
* Adaptive dictionary algorithms such as LZW – used in
GIF The Graphics Interchange Format (GIF; or , see pronunciation) is a bitmap In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algor ...
and
TIFF Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by image scanner, scanning, FAX, faxi ...

TIFF
* DEFLATE – used in PNG,
MNG Multiple-image Network Graphics (MNG) is a graphics file format Image file formats are standardized means of organizing and storing digital image A digital image is an image An SAR radar imaging, radar image acquired by the SIR-C/ ...
, and
TIFF Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by image scanner, scanning, FAX, faxi ...

TIFF
* Chain codes


Other properties

The best image quality at a given compression 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 Data rate units, bit per second unit (symbol: ''bit/s' ...
) is the main goal of image compression, however, there are other important properties of image compression schemes: Scalability generally refers to a quality reduction achieved by manipulation of the bitstream or file (without decompression and re-compression). Other names for scalability are ''progressive coding'' or ''embedded bitstreams''. Despite its contrary nature, scalability also may be found in lossless codecs, usually in form of coarse-to-fine pixel scans. Scalability is especially useful for previewing images while downloading them (e.g., in a web browser) or for providing variable quality access to e.g., databases. There are several types of scalability: * Quality progressive or layer progressive: The bitstream successively refines the reconstructed image. * Resolution progressive: First encode a lower image resolution; then encode the difference to higher resolutions. * Component progressive: First encode grey-scale version; then adding full color. Region of interest coding. Certain parts of the image are encoded with higher quality than others. This may be combined with scalability (encode these parts first, others later). Meta information. Compressed data may contain information about the image which may be used to categorize, search, or browse images. Such information may include color and texture statistics, small
preview Preview may refer to: Theatre, film, television * Preview (subscription service), an early subscription television service in the United States * Preview (theatre), a public performance of a theatrical show before the official opening * Preview sc ...
images, and author or copyright information. Processing power. Compression algorithms require different amounts of
processing power In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwar ...
to encode and decode. Some high compression algorithms require high processing power. The quality of a compression method often is measured by the
peak signal-to-noise ratio Peak signal-to-noise ratio (PSNR) is an engineering term for the ratio between the maximum possible power of a signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and ...
. It measures the amount of noise introduced through a lossy compression of the image, however, the subjective judgment of the viewer also is regarded as an important measure, perhaps, being the most important measure.


History

Entropy coding In 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 fundamenta ...
started in the 1940s with the introduction of Shannon–Fano coding, the basis for
Huffman coding In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algor ...
which was developed in 1950.
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 ...
dates back to the late 1960s, with the introduction of
fast Fourier transform A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in t ...
(FFT) coding in 1968 and the
Hadamard transform 300px, Fast Walsh–Hadamard transform, a faster way to calculate the Walsh spectrum of (1, 0, 1, 0, 0, 1, 1, 0). The Hadamard transform (also known as the Walsh–Hadamard transform, Hadamard–Rademacher–Walsh transform, Walsh transform, o ...
in 1969.William K. Pratt, Julius Kane, Harry C. Andrews:
Hadamard transform image coding
, in Proceedings of the IEEE 57.1 (1969): Seiten 58–68
An important development in image
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 equipment, devices, and systems which use electricity, electro ...
was 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), a
lossy compression In information technology, lossy compression or irreversible compression is the class of data compression, data encoding methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to r ...
technique first proposed by Nasir Ahmed in 1972. DCT compression became the basis for
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
, which was introduced by the
Joint Photographic Experts Group The Joint Photographic Experts Group (JPEG) is the joint committee between ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an ...
(JPEG) in 1992. JPEG compresses images down to much smaller file sizes, and has become the most widely used
image file format Image file formats are standardized means of organizing and storing digital image A digital image is an composed of s, also known as ''pixels'', each with ', ' of numeric representation for its or that is an output from its fed as input ...
. Its highly efficient DCT compression algorithm was largely responsible for the wide proliferation of
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 ...
and
digital photo Digital photography uses cameras A camera is an optical Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the ...
s, with several billion JPEG images produced every day as of 2015.
Lempel–Ziv–Welch Lempel–Ziv–Welch (LZW) is a universal lossless data compression Lossless compression is a class of data compression In signal processing Signal processing is an electrical engineering Electrical engineering is an engin ...
(LZW) is a
lossless compression 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 ...
algorithm developed by
Abraham Lempel Abraham Lempel ( he, אברהם למפל, born 10 February 1936) is an Israeli computer science, computer scientist and one of the fathers of the LZ77 and LZ78, LZ family of lossless data compression algorithms. Biography Lempel was born on 10 Fe ...
,
Jacob Ziv Yaakov Ziv ( he, יעקב זיו; born 1931) is an Israeli electrical engineer who, along with Abraham Lempel, developed the LZ family of lossless data compression Lossless compression is a class of data compression In signal processing, data ...
and
Terry Welch Terry Archer Welch was an American computer scientist. Along with Abraham Lempel Abraham Lempel ( he, אברהם למפל, born 10 February 1936) is an Israeli computer scientist and one of the fathers of the LZ family of lossless data compress ...
in 1984. It is used in the
GIF The Graphics Interchange Format (GIF; or , see pronunciation) is a bitmap In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algor ...

GIF
format, introduced in 1987. DEFLATE, a lossless compression algorithm developed by
Phil Katz Phillip Walter Katz (November 3, 1962 – April 14, 2000) was a computer programmer best known as the co-creator of the Zip (file format), Zip file format for data compression, and the author of PKZIP, a program for creating zip files that r ...
and specified in 1996, is used in the
Portable Network Graphics Portable Network Graphics (PNG, officially pronounced , sometimes pronounced ) is a raster graphics, raster-graphics file graphics file format, format that supports lossless data compression. PNG was developed as an improved, non-patented repl ...
(PNG) format.
Wavelet A wavelet is a wave In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion a ...

Wavelet
coding, the use of wavelet transforms in image compression, began after the development of DCT coding. The introduction of the DCT led to the development of wavelet coding, a variant of DCT coding that uses wavelets instead of DCT's block-based algorithm. The
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 ...
standard was developed from 1997 to 2000 by a JPEG committee chaired by Touradj Ebrahimi (later the JPEG president). In contrast to the DCT algorithm used by the original JPEG format, JPEG 2000 instead uses
discrete wavelet transform . The original image is high-pass filtered, yielding the three large images, each describing local changes in brightness (details) in the original image. It is then low-pass filtered and downscaled, yielding an approximation image; this image is hig ...
(DWT) algorithms. It uses the CDF 9/7 wavelet transform (developed by
Ingrid Daubechies Baroness Ingrid Daubechies ( ; ; born 17 August 1954) is a Belgian physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of in ...
in 1992) for its lossy compression algorithm, and the LeGall-Tabatabai (LGT) 5/3 wavelet transform (developed by Didier Le Gall and Ali J. Tabatabai in 1988) for its lossless compression algorithm.
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 ...
technology, which includes the
Motion JPEG 2000 Motion JPEG 2000 (MJ2 or MJP2) is a file format for motion sequences of JPEG 2000 JPEG 2000 (JP2) is an image compression Image compression is a type of data compression In signal processing, data compression, source coding, or bit-rate r ...
extension, was selected as the
video coding standard 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 ...
for
digital cinema Digital cinema refers to adoption of digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electronics c ...
in 2004.


Notes and references


External links


Image compression
lecture from MIT OpenCourseWare


A study about image compression
with basics, comparing different compression methods like JPEG2000, JPEG and JPEG XR / HD Photo
Data Compression Basics
includes comparison of PNG, JPEG and JPEG-2000 formats
FAQ:What is the state of the art in lossless image compression?
from ews://comp.compression comp.compression
IPRG
an open group related to image processing research resources {{Compression formats