Compression: An Overview
Lossless compression techniques, as their name implies,
involve no loss of information. If data have been losslessly compressed, the original data
can be recovered exactly from the compressed data after a compress/expand cycle. Lossless
compression is generally used for so-called "discrete" data, such as database
records, spreadsheets, word-processing files, and even some kinds of image and video
Text compression is a significant area for lossless compression. It is very important
that the reconstruction is identical to the text original, as very small differences can
result in statements with very different meanings. Consider the sentences "Do not
send money" and "Do now send money." A similar argument holds for
computer files and for certain types of data such as bank records.
Furthermore, if data of any kind are to be processed or "enhanced" later to
yield more information, it is important that the integrity be preserved. For example,
suppose we compressed a radiological image in a lossy fashion, and the difference between
the reconstruction Y and the original X was visually undetectable. If this image
was later enhanced, the previously undetectable differences may cause the appearance of
anomalies that could potentially mislead the radiologist. Because the price for this kind
of mishap may be a human life, it makes sense to be very careful about using a compression
scheme that generates a reconstruction that is different from the original.
In essence, lossless compression algorithms are needed in cases that require
compression where we want the reconstruction to be identical to the original.