Definition: Process of reducing size of digital files by changing the way the data is coded. * NB: It does not involve a physical change in the way the data itself is stored; there is no physical squeezing or squashing. * Uncompressed data may be re-coded in a more space-efficient form without any loss of original information (the compression is 'lossless') or it may be coded to lose specific parts of the data which are considered less important than other parts (the compression is 'lossy').
Data is generally not stored in compressed form in the first place because it is then not in a form optimised for actual use such as pixel editing; the main exception is music with compressed formats such as MiniDisc. When image editing, then, don't work in JPEG or GIF format: leave that for the final save. Note that compressed files take longer to open up (they must be uncompressed) and may trip up systems that expect 'pure' files e.g. RIPs (raster image processors) at the printers.
Related Terms: encoding
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