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Image compression is a type of data compression applied to
digital image 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 ...
s, to reduce their cost for storage or transmission.
Algorithm In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...
s may take advantage of
visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum ref ...
and the
statistical Statistics (from German: '' Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industr ...
properties of image data to provide superior results compared with generic data compression 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 methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data si ...
or lossless. Lossless compression is preferred for archival purposes and often for medical imaging, technical drawings,
clip art Clip art (also clipart, clip-art) is a type of graphic art. Pieces are pre-made images used to illustrate any medium. Today, clip art is used extensively and comes in many forms, both electronic and printed. However, most clip art today is creat ...
, 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 bit per second (symbol: bit/s), often in conjunction ...
s, introduce
compression artifact 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 beco ...
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 methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data si ...
: *
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, ...
– This is the most commonly used method. ** Discrete Cosine Transform (DCT) – The most widely used form of lossy compression. It is a type of Fourier-related transform, 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 di ...
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). It is generally the most efficient form of image compression. *** DCT is used 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 ...
, the most popular lossy format, and the more recent HEIF. ** The more recently developed wavelet transform is also used extensively, followed by quantization and entropy coding. * Color quantization - 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 colorwhether such representation entails an analog or a digital represen ...
to a few "representative" 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 knife, an implement for painting * Palette (company) ...
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 to avoid posterization. ** Whole-image palette, typically 256 colors, used in GIF and PNG file formats. ** block palette, typically 2 or 4 colors for each block of 4x4 pixels, used in BTC, CCC, S2TC, and S3TC. * Chroma subsampling. 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. Methods for
lossless compression Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statisti ...
: *
Run-length encoding Run-length encoding (RLE) is a form of lossless data compression in which ''runs'' of data (sequences in which the same data value occurs in many consecutive data elements) are stored as a single data value and count, rather than as the original ...
– used in default method in
PCX PCX, standing for ''PiCture eXchange'', was an image file format developed by the now-defunct ZSoft Corporation of Marietta, Georgia, United States. It was the native file format for PC Paintbrush and became one of the first widely accepted DOS ...
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 scanning, faxing, word process ...
* 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 (PCM) but adds some functionalities based on the prediction of the samples of the signal. The input can be an analog signal or a digital ...
* Entropy encoding – the two most common entropy encoding techniques are arithmetic coding and Huffman coding * Adaptive dictionary algorithms such as LZW – used in GIF 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 scanning, faxing, word process ...
* DEFLATE – used in PNG, MNG, 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 scanning, faxing, word process ...
* 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 bit per second (symbol: bit/s), often in conjunction ...
) 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 images, and author or copyright information. Processing power. Compression algorithms require different amounts of processing power 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. 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 started in the 1940s with the introduction of
Shannon–Fano coding In the field of data compression, Shannon–Fano coding, named after Claude Shannon and Robert Fano, is a name given to two different but related techniques for constructing a prefix code based on a set of symbols and their probabilities (estimat ...
, the basis for Huffman coding which was developed in 1950.
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, ...
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 ...
(FFT) coding in 1968 and the Hadamard transform 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 was the discrete cosine transform (DCT), a
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 si ...
technique first proposed 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 di ...
in 1973.
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 ...
was introduced by the Joint Photographic Experts Group (JPEG) in 1992. JPEG compresses images down to much smaller file sizes, and has become the most widely used image file format. JPEG was largely responsible for the wide proliferation of
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 ...
and
digital photo Digital photography uses cameras containing arrays of electronic photodetectors interfaced to an analog-to-digital converter (ADC) to produce images focused by a lens, as opposed to an exposure on photographic film. The digitized image i ...
s, with several billion JPEG images produced every day as of 2015. Lempel–Ziv–Welch (LZW) is a
lossless compression Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statisti ...
algorithm developed by
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 compression algorithms. Biography Lempel was born on 10 February 1936 in Lwów, Poland (n ...
,
Jacob Ziv Jacob Ziv ( he, יעקב זיו; born 1931) is an Israeli electrical engineer who, along with Abraham Lempel, developed the LZ family of lossless data compression algorithms. Biography Ziv was born in Tiberias, British mandate Palestine, on 27 ...
and
Terry Welch Terry Archer Welch was an American computer scientist. Along with Abraham Lempel and Jacob Ziv, he developed the lossless Lempel–Ziv–Welch (LZW) compression algorithm, which was published in 1984. Education Welch received a B.S., M.S. and Ph. ...
in 1984. It is used in the 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 for data compression, and the author of PKZIP, a program for creating zip files that ran under DOS. A ...
and specified in 1996, is used in the
Portable Network Graphics Portable Network Graphics (PNG, officially pronounced , colloquially pronounced ) is a raster-graphics file format that supports lossless data compression. PNG was developed as an improved, non-patented replacement for Graphics Interchange ...
(PNG) format. The
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 th ...
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 In numerical analysis and functional analysis, a discrete wavelet transform (DWT) is any wavelet transform for which the wavelets are discretely sampled. As with other wavelet transforms, a key advantage it has over Fourier transforms is temporal ...
(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 and mathematician. She is best known for her work with wavelets in image compression. Daubechies is recognized for her study of the mathematical methods that enhance ...
in 1992) for its lossy compression algorithm, and the Le Gall–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 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 th ...
technology, which includes the Motion JPEG 2000 extension, was selected as the video coding standard for digital cinema in 2004.


Notes and references


External links


Image compression
lecture from MIT OpenCourseWare
Image Compressor tool online



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
Advantages And Disadvantages of Image Compressor
{{Compression formats