how lossy, lossless compression algorithms work

        
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definition of lossy compression, lossless compression

PeaZip is a free cross-platform file compression utility that provides an unified portable GUI for many Open Source technologies like 7-Zip, FreeArc, PAQ, UPX... free alternative to WinRar, WinZip and similar proprietary software.
- Create 7Z, ARC, BZ2, GZ, *PAQ, PEA, QUAD/BALZ, TAR, UPX, WIM, XZ, ZIP files
- Open and extract ACE, ARJ, CAB, DMG, ISO, LHA, RAR, UDF, ZIPX files and more,
over 180 archive types supported
Features of PeaZip includes:
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Lossy and lossless compression algorithms



file compression algorithms types



What is file compression, types of algorithms


File compression
is the process of creating a smaller output from a larger input, in order to sparing storage space, or saving bandwidth to share the data, or reduce the input to a suitable size for a media of distribution (CD, DVD, mail attachment size limit...).

How lossy and lossless compression works


Data compression can be defined lossy or lossless, in terms of reversibility of the compression process due loss (or preservation) of original information in the process. The two types of algorithms have different pros and cons, and different field of application.

Lossless compression definition, file archiving


Lossless compression uses statistical models to map the input to a smaller output eliminating redundancy in the data.
In this way the output carry exactly all the information featured by the input in less bytes, and can be expanded when needed to a 1:1 copy of the original data (restoring exactly the original content), which is a fundamental property for storing some types of data - i.e. a software, a database.

For this reason lossless compression algorithms are used for data backup and for archive file formats used in general purpose archive manager utilities, like 7Z, RAR, and ZIP, where an exact and reversible image of the original data must be saved.
Examples of lossless compression algorithms are Deflate (used i.e. for ZIP and GZ formats), BZip2 (used in BZ2 format), PPMd (RAR, 7Z formats), LZMA / LZMA2 (7Z / XZ format).

what is lossless compression

Lossless compression is fully invertible, as 1:1 copy of original content input is stored in the smaller, efficiently encoded output, so it is usually suitable for backup, file archiving and other applications where any loss of information is not tolerable.

reversible compression algorithms

Some graphic file fomats (notably, PNG files and deflated TIFF) uses lossless compression, which usually results in less compression but no image quality degradation after multiple cycles of modification and saving of the picture, making this kind of image format suitable as intermediate save files for image editing tools.

Lossy compression definition, multimedia data compression


Lossy compression, instead, works identifying unnecessary or less relevant information (not just redundant data) and removing it.

The loss of information / content is irreversible, and depending from the nature of the algorithm, will likely happen each time the content is modified and saved to a lossy file format - e.g. when editing a lossy jpeg images, and saving it multiple times to intermediate work files.

In this way data compression is improved but at the cost of making lossy compression a non reversible process - as it comes at the cost of losing part of the information - and making it a suitable choice only when it is not intended, by design, to restore the original content again.

Lossy compression is consequently not suitable for general purpose file archiving (as in example losing a single byte of an executable file would make it not working), but  it works very well when loss of less relevant information is acceptable, as for graphic and multimedia files compression - in example for MP3 losing audio information below the audibility threshold, or losing not visible details in JPEG images, or both in compressed video formats such as MPEG (MPG, MP4...).

what is lossy compression

Information loss is destructive for the ability of 1:1 reversal of the algorithm (the information is permanently lost), but it is not prejudicial for the ability of end users to receive meaningful information - intelligible audio, clear picture or video.

non reversible compression algorithms

Most common lossy compression algorithms are consequently usually fine tuned for the specific pattern of a multimedia data type.

Due the lossy nature of those compression schemes, however, usually professional editing work is performed on non compressed data (i.e. WAV audio, or TIFF images) or data compressed in a lossless way (i.e. FLAC audio, or PNG images) every time it is feasible so saving the work in progress multiple times does not result in losing bits of the information each time, with progressive degradation of quality - usually reserving use of lossy compression to final step for creating a reasonably sized output to distribute for media consumption.

Read lossless compressionoptimize data compression and lossy compressionoptimize picture compression definitions on Wikipedia.


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FAQ > Archiving > What are lossy & lossless compression

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