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Lossy data compression

From Academic Kids

A lossy data compression method is one where compressing a file and then decompressing it retrieves a file that may well be different to 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.

Contents

Types of lossy compression

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 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 or images. In these cases, the retrieved file can be quite different to 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

Missing image
Lena.jpg
Original Lena Image (12KB size)
Missing image
Lena30.jpg
Lena Image, Compressed (85% less information, 1.8KB)
Missing image
Lena5.jpg
Lena Image, Highly Compressed (96% less information, 0.56KB)

The above images show the use of lossless compression to reduce the file size of the image. The image is an excerpt of the image of Lena, which has an amusing history all on its own.

  • The first picture is 12,249 bytes.
  • The second picture has been compressed (JPEG quality 30) and is 85% smaller, at 1,869 bytes. Notice the loss of detail in the brim of the hat.
  • The third picture has been highly compressed (JPEG quality 5) and is 96% smaller, at 559 bytes. The compression artifacts are much more noticible.

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

Still image compression

Moving image compression

Sound compression

Music

Speech


Other types of data

es:Algoritmo de compresión con pérdida de:Verlustbehaftete Datenkompression ja:非可逆圧縮 pl:Kompresja stratna sv:Förstörande komprimering

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