3. Lossy compression

If file size is an issue, then it may be acceptable to discard some of the original information. That way, less data needs to be stored. This is called 'lossy compression'.

Lossy compression depends on patterns being present within the information, as mentioned on the previous page. The algorithm looks at the data, tries to identify the patterns and decides how much it can throw away without noticably affecting the quality of the data.

Lossy compression is irreversible. Once the data bytes are thrown away, they cannot be recovered. Lossy compression always involves loss of quality. If you re-compress the same file more than once, more and more information will be lost.

For example if a raw image was converted to a lossy jpeg file, that file should not be opened and re-compressed again as the degradation becomes noticable.