How does File Compression work?

Because raster files contain a larger amount of information compared to vector files, "compression" of the file is often used to reduce its size. The most commonly known type of compression is CCITT Group 4 (TIFF Group 4). This type of compression is used in the files produced by fax machines and you may recall fax machines being classified as either "Group 3" or "Group 4". The CCITT compression works by identifying the start and end of say, "a line of uninterrupted white space" and simply removes the white space between start and end and marks its position, start and finish, with markers. It then reinstates the white space when the file is opened as part of the de-compression process.

Compression has developed significantly and to enable colour photographs and films to be transferred digitally. The most common type of compression used for colour images is known as "JPEG". This compression reduces the size of files quite significantly, often in the order of 10-20 times. However, some data is lost each time the file is compressed, dependant upon the extent of compression chosen. The JEPG compression technique works by discarding adjacent pixels with small colour differences.