Editing Lossy compression (section)

==Types==
It is possible to compress many types of digital data in a way that reduces the size of a [[computer file]] needed to store it, or the [[Bandwidth (computing)|bandwidth]] needed to transmit it, with no loss of the full information contained in the original file. A picture, for example, is converted to a digital file by considering it to be an array of dots and specifying the color and brightness of each dot. If the picture contains an area of the same color, it can be compressed without loss by saying "200 red dots" instead of "red dot, red dot, ...(197 more times)..., red dot."

The original data contains a certain amount of information, and there is a lower bound to the size of a file that can still carry all the information. Basic [[information theory]] says that there is an absolute limit in reducing the size of this data. When data is compressed, its [[Entropy (information theory)|entropy]] increases, and it cannot increase indefinitely. For example, a compressed [[ZIP (file format)|ZIP]] file is smaller than its original, but repeatedly compressing the same file will not reduce the size to nothing. Most compression algorithms can recognize when further compression would be pointless and would in fact increase the size of the data.

In many cases, files or data streams contain more information than is needed. For example, a picture may have more detail than the eye can distinguish when reproduced at the largest size intended; likewise, an audio file does not need a lot of fine detail during a very loud passage. Developing lossy compression techniques as closely matched to human perception as possible is a complex task. Sometimes the ideal is a file that provides exactly the same perception as the original, with as much digital information as possible removed; other times, perceptible loss of quality is considered a valid tradeoff.

The terms "irreversible" and "reversible" are preferred over "lossy" and "lossless" respectively for some applications, such as medical image compression, to circumvent the negative implications of "loss". The type and amount of loss can affect the utility of the images. Artifacts or undesirable effects of compression may be clearly discernible yet the result still useful for the intended purpose. Or lossy compressed images may be '[[visually lossless]]', or in the case of medical images, so-called [[diagnostically acceptable irreversible compression]] (DAIC)<ref>{{cite journal|last=European Society of Radiology|title=Usability of irreversible image compression in radiological imaging. A position paper by the European Society of Radiology (ESR)|date=2011|pmc=3259360|pmid=22347940|doi=10.1007/s13244-011-0071-x|volume=2|issue=2|journal=Insights Imaging|pages=103–115}}</ref> may have been applied.