- 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
multimediadata (audio, video, still images), especially in applications such as streaming mediaand internet telephony. By contrast, lossless compressionis required for text and data files, such as bank records, text articles, etc. 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.
Information-theoretical foundations for lossy data compression are provided by
rate-distortion theory. Much like the use of probabilityin optimal coding theory, rate-distortion theory heavily draws on Bayesianestimation and decision theoryin order to model perceptual distortion and even aestheticjudgment.
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 versus 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.
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 (imperceptible), which can be verified via an
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
bitlevel while being indistinguishable to the human ear or eye for most practical purposes. Many compression methods focus on the idiosyncrasies of human physiology, taking into account, for instance, that the human eye can see only certain wavelengths of light. The psychoacoustic modeldescribes 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 artifacts.
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.
* Video can be compressed immensely (e.g. 300:1) with little visible quality loss;Fact|date=September 2008
* Audio can often be compressed at 10:1 with imperceptible loss of quality;Fact|date=September 2008
* Still images are often lossily compressed at 10:1, as with audio, but the quality loss is more noticeable, especially on closer inspection.Fact|date=September 2008
Transcoding and Editing
An important caveat about lossy compression is that converting (formally,
transcoding) or editing lossily compressed files causes digital generation lossfrom 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 formatinstead of JPEG.
Some lossless editing of lossily compressed files is possible, which consists of modifying the compressed data directly, without decoding and re-encoding.
The primary programs for lossless editing of JPEGs are
, and the derived
exiftran(which also preserves EXIF information), and [http://sylvana.net/jpegcrop/ Jpegcrop] (which provides a Windows interface).
[http://sylvana.net/jpegcrop/jpegjoin/ JPEGjoin] allows one to join different JPEG images (which have the same encoding), without re-encoding. (See also: [http://sylvana.net/jpegcrop/jpegtran/ New jpegtran features] .)
One can also make some changes to the compression without re-encoding:
* optimize the compression (so it takes less space),
* convert between progressive and non-progressive encoding,
There is also the freeware Windows-only
IrfanView, which has some lossless JPEG operations in its
;Splitting and joining:
Mp3spltand Mp3wrap(or AlbumWrap) allow one to split an MP3 file into pieces or join them loselessly. These are analogous to
cat. [Though the wrap programs do more, encoding the divisions between the original files.] ;Gain: Various
Replay Gainprograms such as MP3gainallow one to modify the gain(overall volume) of MP3 files losslessly.
One can generally modify or remove metadata, such as
ID3 tags, Vorbis comments, or EXIFinformation, without modifying the underlying media.
Downsampling / compressed representation scalability
One may wish to
downsampleor 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, 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 2000for still images and H.264/MPEG-4 AVCbased Scalable Video Codingfor video. Actually such schemes have also been standardized for older designs as well, such as JPEGimages with progressive encoding, and MPEG-2and MPEG-4 Part 2video, 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), thought this latter tends to cause digital generation loss.
On a related point, some audio formats feature a combination of a lossy format and a lossless correction; this allows stripping the correction to easily obtain a lossy file, though whether the lossy portion itself can be further stripped is a separate question. Such formats include
MPEG-4 SLS(Scalable to Lossless), WavPack, and OptimFROG DualStream.
Cartesian Perceptual Compression: Also known as CPC
* HAM, hardware compression of color information used in
ICER, used by the Mars Rovers: related to JPEG 2000in its use of wavelets
JPEG 2000, JPEG's successor format that uses wavelets.
* PGF, Progressive Graphics File (lossless or lossy compression)
S3TCtexture compression for 3D computer graphics hardware
Harmonic and Individual Lines and Noise(HILN)
* AMR (used by GSM cell carriers, such as T-Mobile)
Speex(noted for its lack of patent restrictions)
Researchers have (semi-seriously) performed lossy compression on text by either using a thesaurus to substitute short words for long ones, or generative text techniques [cite web|title=Semantic and Generative Models for Lossy Text Compression |url=http://compression.ru/download/articles/text/witten_1994cj_lossy_text_compression.pdf |author=I. H. WITTEN, et al. |publisher=The Computer Journal |accessdate=2007-10-13 |format=PDF] , although these sometimes fall into the related category of
lossy data conversion.
Lossless data compression
List of codecs
* [http://www.bobulous.org.uk/misc/lossy_audio_2006.html Lossy audio formats] , comparing the speed and compression strength of five lossy audio formats.
* [http://membled.com/work/apps/lossy_png/ lossy PNG image compression (research)]
* [http://www.websiteoptimization.com/speed/tweak/lossy/ using lossy GIF/PNG compression for the web (article)]
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Look at other dictionaries:
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