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Lossy data compression

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A lossy data 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. This type of compression is used frequently on the Internet and especially in streaming media and telephony applications. These methods are typically referred to as codecs in this context. This is in contrast with lossless data compression. Depending on the design of the format, lossy data compression suffers from generation loss: compressing and decompressing the file will cause it to progressively lose quality.

Contents
  • 5 See also
  • 6 External links

    • Types of lossy compression

    There are two basic lossy compression schemes:

    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 Compression

    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. The compression ratio (that is, the size of the compressed file compared to that of the uncompressed file) of lossy video codecs are nearly always far superior to those of the audio and still-image equivalents. Audio can be compressed at 10:1 with no noticeable loss of quality, video can be compressed immensely with little visible quality loss, eg 300:1. 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 anatomy, taking into account, for example, that the human eye can see only certain frequencies 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.

    Example of Lossy Compression

    Lenna Image, Highly Compressed (96% less information, 0.56KB)
    Lenna Image, Highly Compressed (96% less information, 0.56KB)

    The above images show the use of lossy compression to reduce the file size of the image. The image is an excerpt of the image of Lenna, a de facto industry-standard test image. Even though the third image has high distortion, the face is still recognizable. Good lossy compression algorithms are able to throw away "less important" information and still retain the "essential" information.

    Lossy compression methods

    Sound compression

    Music compression

    Other types of data

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

    See also

    External links

     

    From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
    All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

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