Lossy Compression - Important Concepts in Signal Processing, Image Processing and Data Compression - page 66

Lossy compression formats suffer from generation loss: repeatedly compressing and
decompressing the file will cause it to progressively lose quality. This is in contrast with
lossless data compression, where data will not be lost via the use of such a procedure.

Information-theoretical foundations for lossy data compression are provided by rate-
distortion theory. Much like the use of probability in optimal coding theory, rate-
distortion theory heavily draws on Bayesian estimation and decision theory in order to
model perceptual distortion and even aesthetic judgment.

Types

There are two basic lossy compression schemes:

In lossy transform codecs , samples of picture or sound are taken, chopped into
small segments, transformed into a new basis space, and quantized. The resulting
quantized values are then entropy coded.



In lossy predictive codecs , previous and/or subsequent decoded data is used to
predict the current sound sample or image frame. The error between the predicted
data and the real data, together with any extra information needed to reproduce
the prediction, is then quantized and coded.



In some systems the two techniques are combined, with transform codecs being used to
compress the error signals generated by the predictive stage.

Lossy versus lossless

The advantage of lossy methods over lossless methods is that in some cases a lossy
method can produce a much smaller compressed file than any lossless method, while still
meeting the requirements of the application.

Lossy methods are most often used for compressing sound, images or videos. This is
because these types of data are intended for human interpretation where the mind can
easily "fill in the blanks" or see past very minor errors or inconsistencies - ideally lossy
compression is transparent (imperceptible), which can be verified via an ABX test.

Transparency

When a user acquires a lossily compressed file, (for example, to reduce download time)
the retrieved file can be quite different from the original at the bit level while being
indistinguishable to the human ear or eye for most practical purposes. Many compression
methods focus on the idiosyncrasies of human physiology, taking into account, for
instance, that the human eye can see only certain wavelengths of light. The
psychoacoustic model describes how sound can be highly compressed without degrading