Information about Image Compression
Image compression is the application of Data compression on digital images. In effect, the objective is to reduce redundancy of the image data in order to be able to store or transmit data in an efficient form.
Image compression can be lossy or lossless. Lossless compression is sometimes preferred for artificial images such as technical drawings, icons or comics. This is because lossy compression methods, especially when used at low bit rates, introduce compression artifacts. Lossless compression methods may also be preferred for high value content, such as medical imagery or image scans made for archival purposes. Lossy methods are especially suitable for natural images such as photos in applications where minor (sometimes imperceptible) loss of fidelity is acceptable to achieve a substantial reduction in bit rate.
Methods for lossless image compression are:
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 can also 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:
Meta information. Compressed data can contain information about the image which can be used to categorize, search or browse images. Such information can include color and texture statistics, small preview images and author/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 is often 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 judgement of the viewer is also regarded as an important, perhaps the most important measure.
(See for formats and for codecs)
Image compression can be lossy or lossless. Lossless compression is sometimes preferred for artificial images such as technical drawings, icons or comics. This is because lossy compression methods, especially when used at low bit rates, introduce compression artifacts. Lossless compression methods may also be preferred for high value content, such as medical imagery or image scans made for archival purposes. Lossy methods are especially suitable for natural images such as photos in applications where minor (sometimes imperceptible) loss of fidelity is acceptable to achieve a substantial reduction in bit rate.
Methods for lossless image compression are:
- Run-length encoding – used as default method in PCX and as one of possible in BMP, TGA, TIFF
- Entropy coding
- Adaptive dictionary algorithms such as LZW – used in GIF and TIFF
- Deflation – used in PNG, MNG and TIFF
- Reducing the color space to the most common colors in the image. The selected colors are specified in the color palette 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.
- Chroma subsampling. This takes advantage of the fact that the eye perceives brightness more sharply than color, by dropping half or more of the chrominance information in the image.
- Transform coding. This is the most commonly used method. A Fourier-related transform such as DCT or the wavelet transform are applied, followed by quantization and entropy coding.
- Fractal compression.
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 can also 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; then color.
Meta information. Compressed data can contain information about the image which can be used to categorize, search or browse images. Such information can include color and texture statistics, small preview images and author/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 is often 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 judgement of the viewer is also regarded as an important, perhaps the most important measure.
See also
- Graphics file formats
- Digital signal processing
- Image processing
- Computer graphics
- Lenna
- Standard test image
- Image Quality
- PSNR
- SSIM
External links
- MIT Linear Algebra Lecture on Image Compression at Google Video, from MIT OpenCourseWare
- Image Coding Fundamentals
- A study about image compression (Image compression basics and comparing different compression methods like JPEG2000, JPEG and HD Photo)
| Lossless compression methods | ||||
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| Audio compression methods |
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| Image compression methods |
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| Video compression |
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| Timeline of information theory, data compression, and error-correcting codes | ||||
Multimedia compression formats | ||||
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| Video compression formats |
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| Audio compression formats | ||||
| Image compression formats |
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| Media container formats |
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| See for methods and for codecs | ||||
data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an un-encoded representation would use through use of specific encoding schemes.
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A digital image is a representation of a two-dimensional image as a finite set of digital values, called picture elements or pixels. The digital image contains a fixed number of rows and columns of pixels.
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Data transmission is the conveyance of any kind of information from one space to another. Historically this could be done by courier, a chain of bonfires or semaphores, and later by Morse code over copper wires.
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lossy compression method is one where compressing data and then decompressing it retrieves data that may well be different from the original, but is close enough to be useful in some way.
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Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. This can be contrasted to lossy data compression, which does not allow the exact original data to be reconstructed from the
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bitrate (sometimes written bit rate, data rate or as a variable R or fb) is the number of bits that are conveyed or processed per unit of time. Bit rate is synonymous to data rate and digital bandwidth.
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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
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Run-length encoding (RLE) is a very simple form of data compression in which runs of data (that is, 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 run.
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PCX
File extension:
Developed by: ZSoft Corporation
Type of format: lossless bitmap image format
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PCX
File extension:
.pcxDeveloped by: ZSoft Corporation
Type of format: lossless bitmap image format
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Windows Bitmap
File extension:
MIME type:
Type code: 'BMP '
Uniform Type Identifier: com.microsoft.
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File extension:
.bmp or .dibMIME type:
image/x-ms-bmp (unofficial)Type code: 'BMP '
Uniform Type Identifier: com.microsoft.
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TGA may refer to:
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- Tandy Graphic Adaptor, the graphics system of the Tandy 1000 computer
- Tennessee Governor's Academy for Math and Science
- The Gersh Agency
- The Glasgow Academy, Scotland, UK
- Therapeutic Goods Administration
- Thermogravimetric Analysis
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Tagged Image File Format
File extension:
MIME type:
Type code: TIFF
Uniform Type Identifier: public.
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File extension:
.tiff, .tifMIME type:
image/tiff, image/tiff-fxType code: TIFF
Uniform Type Identifier: public.
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In information theory an entropy encoding is a lossless data compression scheme that is independent of the media’s specific characteristics.
One of the main types of entropy coding assigns codes to symbols so as to match code lengths with the probabilities of the
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One of the main types of entropy coding assigns codes to symbols so as to match code lengths with the probabilities of the
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Lempel-Ziv-Welch (LZW) is a universal lossless data compression algorithm created by Abraham Lempel, Jacob Ziv, and Terry Welch. It was published by Welch in 1984 as an improved implementation of the LZ78 algorithm published by Lempel and Ziv in 1978.
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Graphics Interchange Format
A rotating globe in GIF format. The gradient blue areas of this image transition choppily, a common artifact produced when dithering is not employed.
File extension:
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A rotating globe in GIF format. The gradient blue areas of this image transition choppily, a common artifact produced when dithering is not employed.
File extension:
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Tagged Image File Format
File extension:
MIME type:
Type code: TIFF
Uniform Type Identifier: public.
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File extension:
.tiff, .tifMIME type:
image/tiff, image/tiff-fxType code: TIFF
Uniform Type Identifier: public.
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DEFLATE is a lossless data compression algorithm that uses a combination of the LZ77 algorithm and Huffman coding. It was originally defined by Phil Katz for version 2 of his PKZIP archiving tool, and was later specified in RFC 1951.
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PNG may stand for:
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- Papua New Guinea, a country in Oceania, occupying the eastern half of the island of New Guinea and numerous off-shore islands.
- Portable Network Graphics, a bitmapped image format that employs lossless data compression.
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Multiple-image Network Graphics
File extension:
MIME type:
Type of format: computer animation
Container for: PNG, JNG
Extended from: PNG
Multiple-image Network Graphics (MNG
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File extension:
.mngMIME type:
video/x-mng (unofficial)Type of format: computer animation
Container for: PNG, JNG
Extended from: PNG
Multiple-image Network Graphics (MNG
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Tagged Image File Format
File extension:
MIME type:
Type code: TIFF
Uniform Type Identifier: public.
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File extension:
.tiff, .tifMIME type:
image/tiff, image/tiff-fxType code: TIFF
Uniform Type Identifier: public.
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color space. For example, Adobe RGB and sRGB are two different absolute color spaces, both based on the RGB model.
In the most generic sense of the definition above, color spaces can be defined without the use of a color model.
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In the most generic sense of the definition above, color spaces can be defined without the use of a color model.
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Dither is an intentionally applied form of noise, used to randomize quantization error, thereby preventing large-scale patterns such as contouring that are more objectionable than uncorrelated noise.
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Posterization of a photographic image being developed occurs when a region of an image with a continuous gradation of tone is replaced with several regions of fewer tones, resulting in an abrupt change from one tone to another.
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Chroma subsampling is the practice of implementing more resolution for the (quantity representative of) luminance than the (quantity representative of) color. It is used in many video encoding schemes (both analog and digital) and also in JPEG encoding.
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Transform coding is a type of data compression for "natural" data like audio signals or photographic images. The transformation is typically "lossy", resulting in a lower quality copy of the original input.
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This is a list of linear transformations of functions related to Fourier analysis. Such transformations map a function to a set of coefficients of basis functions, where the basis functions are sinusoidal and are therefore strongly localized in the frequency spectrum.
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discrete cosine transform (DCT) is a Fourier-related transform similar to the discrete Fourier transform (DFT), but using only real numbers. DCTs are equivalent to DFTs of roughly twice the length, operating on real data with even symmetry (since the Fourier transform of a real and
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In mathematics, a wavelet series is a representation of a square-integrable (real- or complex-valued) function by a certain orthonormal series generated by a wavelet. This article provides a formal, mathematical definition of an orthonormal wavelet and of the
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Quantization is the procedure of constraining something from a continuous set of values (such as the real numbers) to a discrete set (such as the integers). Quantization in specific domains is discussed in:
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In information theory an entropy encoding is a lossless data compression scheme that is independent of the media’s specific characteristics.
One of the main types of entropy coding assigns codes to symbols so as to match code lengths with the probabilities of the
..... Click the link for more information.
One of the main types of entropy coding assigns codes to symbols so as to match code lengths with the probabilities of the
..... Click the link for more information.
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