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Information about Lossy Compression

A 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. Lossy compression is most commonly used to compress multimedia data (audio, video, still images), especially in applications such as streaming media and internet telephony. On the other hand lossless compression is preferred for text and data files, such as bank records, text articles, etc.

Most lossy compression formats suffer from generation loss: repeatedly compressing and decompressing the file will cause it to progressively lose quality. This is in contrast with lossless data compression.

Types

There are two basic lossy compression schemes:
  • In lossy transform codecs, 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.

Lossy vs. lossless

The advantage of lossy methods over lossless methods is that in some cases a lossy method can produce a much smaller compressed file than any known lossless method, while still meeting the requirements of the application. It is important to note all media storage methods are lossy when a live event has been recorded. Current recording methods only capture a small fraction of the total performance.

Lossy methods are most often used for compressing sound, images or videos. The compression ratio (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. Audio can often be compressed at 10:1 with imperceptible loss of quality, and video can be compressed immensely (e.g. 300:1) with little visible quality loss. Lossily compressed still images are often compressed to 1/10th their original size, as with audio, but the quality loss is more noticeable, especially on closer inspection.

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 level while being indistinguishable to the human ear or eye for most practical purposes. Many methods focus on the idiosyncrasies of the human physiology, taking into account, for example, that the human eye can see only certain wavelengths of light. The psychoacoustic model describes how sound can be highly compressed without degrading the perceived quality of the sound. Flaws caused by lossy compression that are noticeable to the human eye or ear are known as compression artifacts.

Methods

Graphics

Image

Video

Audio

Music

Speech

Other data

Technically, reducing text size by removing all vowels can be considered a lossy compression as well. The text is usually still readable from the context given by the consonants. Researchers have also (half-jokingly) performed lossy compression on text by either using a thesaurus to substitute short words for long ones, or generative text techniques [1], although these sometimes fall into the related category of lossy data conversion.

See also

Notes

1. ^ I. H. WITTEN, et al.. Semantic and Generative Models for Lossy Text Compression (PDF). The Computer Journal. Retrieved on 2007-10-13.

External links

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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Multimedia (Lat. Multum + Medium) is media that uses multiple forms of information content and information processing (e.g. text, audio, graphics, animation, video, interactivity) to inform or entertain the (user) audience.
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Sound recording and reproduction is the electrical or mechanical inscription and re-creation of sound waves, usually used for the voice or for music.

The two main classes of sound recording technology are analog recording and digital recording.
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Video (Latin for "I see", first person singular present, indicative of videre, "to see") is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.
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This article relies largely or entirely upon a .
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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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Generation loss refers to the loss of quality between subsequent copies 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 considered a form of generation loss.
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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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codec, see Codec (disambiguation).


A codec is a device or program capable of performing encoding and decoding on a digital data stream or signal. The word codec may be a combination of any of the following: 'Compressor-Dec
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quantization is the process of approximating a continuous range of values (or a very large set of possible discrete values) by a relatively-small set of discrete symbols or integer values.
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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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quantization is the process of approximating a continuous range of values (or a very large set of possible discrete values) by a relatively-small set of discrete symbols or integer values.
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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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BIT is an acronym for:
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Human physiology is the science of the mechanical, physical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. The principal level of focus of physiology is at the level of organs and systems.
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Psychoacoustics is the study of subjective human perception of sounds. Alternatively it can be described as the study of the psychological correlates of the physical parameters of acoustics.
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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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Cartesian Perceptual Compression (abbreviated CPC) is a file format specifically designed for the compression of black-and-white raster images in document image storage and transmission systems.
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DjVu

File extension: .djvu, .djv
MIME type: image/vnd.djvu
Type code: DJVU
Developed by: ATT Research
Type of format: Image file formats

DjVu
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JPEG 2000

Comparison of JPEG 2000 with the original JPEG format.
File extension: .jp2, .j2k
MIME type: image/jp2
Developed by: Joint Photographic Experts Group
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JPEG

A photo of a flower compressed with successively more lossy compression ratios from left to right.
File extension: .jpeg, .jpg, .jpe
.jfif, .jfi, .
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JPEG 2000

Comparison of JPEG 2000 with the original JPEG format.
File extension: .jp2, .j2k
MIME type: image/jp2
Developed by: Joint Photographic Experts Group
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JBIG2 is an image compression standard for bi-level images, developed by the Joint Bi-level Image Experts Group. It is suitable for both lossless and lossy compression. According to a press release[1]
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PGF

File extension: .pgf
Developed by: xeraina GmbH
Type of format: wavelet-based bitmapped image format

PGF (Progressive Graphics File
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Wavelet compression is a form of data compression well suited for image compression (sometimes also video compression and audio compression). The goal is to store image data in as little space as possible in a file.
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