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compression,external stress applied to an object or substance, tending to cause a decrease in volume (see pressurepressure,
in mechanics, ratio of the force acting on a surface to the area of the surface; it is thus distinct from the total force acting on a surface. A force can be applied to and sustained by a single point on a solid.
..... Click the link for more information. ). Gases can be compressed easily, solids and liquids to a very small degree if at all. Water, for example, is practically incompressible, thus making it especially useful for hydraulic machineshydraulic machine,
machine that derives its power from the motion or pressure of water or some other liquid. Hydraulic Engines
Water falling from one level to a lower one is used to drive machines like the water wheel and the turbine.
..... Click the link for more information. . According to the kinetic-molecular theory of gaseskinetic-molecular theory of gases,
physical theory that explains the behavior of gases on the basis of the following assumptions: (1) Any gas is composed of a very large number of very tiny particles called molecules; (2) The molecules are very far apart compared to their sizes,
..... Click the link for more information. , when the molecules of a gas are brought close enough together by compression, the gas (under certain conditions of temperature) undergoes liquefactionliquefaction,
change of a substance from the solid or the gaseous state to the liquid state. Since the different states of matter correspond to different amounts of energy of the molecules making up the substance, energy in the form of heat must either be supplied to a substance
..... Click the link for more information. . This principle is applied commercially to several gases, including liquid oxygen and the so-called bottled gas (a mixture of hydrocarbons) used as a fuel. Boyle's law deals with the decrease in the volume of a gas in relation to the increase of pressure upon it (see gas lawsgas laws,
physical laws describing the behavior of a gas under various conditions of pressure, volume, and temperature. Experimental results indicate that all real gases behave in approximately the same manner, having their volume reduced by about the same proportion of the
..... Click the link for more information. ). The ability or the degree to which an internal-combustion engine reduces the volume of its fuel mixture preparatory to firing is called its compression. Also, a region of high pressure in a fluid is called a compression; thus sound waves are said to propagate at compressions and rarefactions (regions of low pressure) of their medium, such as air.
(in Russian, kompressiia), a force effect on a gaseous body that reduces the volume occupied by the gas and increases its pressure and temperature. Compression is achieved by means of compressors and during the operation of internal combustion engines and other equipment.
Russian technical literature usually uses the word szhatie instead of kompressiia, although the meaning of the former is more general, since it also covers solids and volume reduction upon cooling.
The standard Unix compression utilty is called compress though GNU's superior gzip has largely replaced it. Other compression utilties include pack, zip and PKZIP.
When compressing several similar files, it is usually better to join the files together into an archive of some kind (using tar for example) and then compress them, rather than to join together individually compressed files. This is because some common compression algorithms build up tables based on the data from their current input which they have already compressed. They then use this table to compress subsequent data more efficiently.
See also TIFF, JPEG, MPEG, Lempel-Ziv Welch, "lossy", "lossless".
Web Content Compression FAQ.
Usenet newsgroups: news:comp.compression, news:comp.compression.research.
archive programSoftware that compresses multiple files and folders into a single file (the "archive"). If a folder or a hierarchy of folders is compressed, the file/folder relationship is maintained within the archived file, and it is restored when the archive is decompressed back to its original state. Although there are numerous compression formats, many archive programs (archivers) support the very popular ZIP format. See archive, archive formats, ZIP file and data compression.
data compressionThere are two categories of data compression. The first reduces the size of a single file to save storage space and transmit faster. The second is for storage and transmission convenience.
#1 - Compressing a Single File
The JPEG image, MPEG video, MP3 audio and G.7xx voice formats are widely used "lossy" methods that analyze which pixels, video frames or sound waves can be removed forever without the average person noticing (see lossy compression). GIF images have no loss of pixels but may have a loss of colors (see GIF).
JPEG files can be reduced as much as 80%; MPEG enables a two-hour HD movie to fit on a single disc, and MP3 sparked a revolution by reducing CD music 90%. For a list of compression methods, see codec examples. See JPEG, GIF, MPEG, MP3, G.7xx and interframe coding.
#2 - Compressing a Group of Files (Archiving)
The second "lossless" category compresses and restores data without the loss of a single bit. Although this is widely used for documents, this method is not aware of the content's purpose. It merely looks for repeatable patterns of 0s and 1s, and the more patterns, the higher the compression ratio. Text documents compress the most, while binary and already-compressed files (JPEG, MPEG, etc.) compress the least.
Although lossless methods such as the ZIP format are used to reduce the size of a single, huge file, they are widely used to compress several files into one "archive." It is convenient to store and considerably more convenient to transmit a single file than to keep track of multiple files. See lossless compression, archive, archive formats and capacity optimization.
Lossless Methods (Dictionary and Statistical)
The widely used dictionary method creates a list of repeatable phrases. For example, GIF images and ZIP and JAR archives are compressed with this method (see LZW). The statistical method converts characters into variable length strings of bits based on frequency of use (see Huffman coding).