Databases Reference
In-Depth Information
x
c
x
y
σιναννοψανσενα
οψτυνκεϖενελιφ
δερινυλασ
φυσυνφυνδαφιγεν
ταηιρυλκερ
σιναννοψανσενα
οψτυνκεϖενελιφ
δερινυλασ
φυσυνφυνδαφιγεν
ταηιρυλκερ
Original
Reconstructed
F I GU R E 1 . 1
Compression and reconstruction.
Based on the requirements of reconstruction, data compression schemes can be divided
into two broad classes:
, and
lossy
compression schemes, which generally provide much higher compression than lossless
compression but allow
lossless
compression schemes, in which
Y
is identical to
X
Y
to be different from
X
.
1.1.1 Lossless Compression
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. Lossless compression is generally used for applications that cannot tolerate
any difference between the original and reconstructed data.
Text compression is an important area for lossless compression. It is very important that the
reconstruction is identical to the original text, 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.
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 artifacts that could seriously 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.
