Compression Types

Popular formats for lossless compression include Zip, which runs on PCs under the Windows operating system; Stuffit which runs under the Apple operating system, and gzip which runs on UNIX type systems. Graphic files can be compressed by using the GIFs format, which reduces the number of colours to a maximum of 256, whilst otherwise being a lossless system; and JPEG compression, which uses a combination of both lossy and lossless compaction in order to achieve the greatest possible reduction in size.

Obviously, the greatest drawback to lossy compression is that detail can be lost. Pictures are composed of blocks of pixels, each one of which contains information which indicates whether the picture has a colour greyscale appearance. When two adjacent pixels are of a reasonably similar colour it is possible for them to share the same information and thus become the same colour. Some degree of definition is obviously a loss when this occurs but the human eye is not an absolutely perfect instrument and the knack is to reduce the size of the file as far as possible without reducing the perceived quality of the image excessively. Similar techniques can be used for reducing the size of sound files and it is indeed possible to reduce these, in many cases, to as little as 5% of their original size while still retaining sufficient clarity for most purposes.

There is of course a downside to compression, in that when the text or image is displayed on a screen or the sound is broadcast through the speakers it first of all has to be decompressed, and this takes time and processing power. A happy medium has to be found between the advantages of compression, in that a compressed file will load into memory more quickly, and the disadvantage that it will then have to be uncompressed. Text files can very often the losslessly reduced to around a half or even a third of their original sizes, while images whether static or video can very often be reduced by a factor of around 20 or even more by using lossy compression.

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