Data Compression


Criteria

Survey Formats

Basics

Basic Terms

Symbol

Set of Symbols

Alphabet

Code

Coding

Redundancy

Information Theory

Message

Probability

Information

Entropy

Redundancy Reduction

Irrelevance Reduction

Entropy Coding

Variable Length Codes

Code Trees

Compression Methods

Data Formats


Glossary

Index


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Coding, Encoding, Decoding


In general the term coding describes the procedure defining the transformation of symbols from one set of symbols to another one. For that a coding specification or code will be applied.


The steps for processing some original data can be differentiated into encoding and decoding. The encoding process denotes the coding into a particular destination format, e.g. converting a bitmap graphic into a JPEG file.


The decoding process denotes the reverse process related to this format; for example the conversion of the JPEG file back into a proper format. Encoding and decoding always refer to their common denominator, in this case the JPEG format.


For lossless data compresion both the original and the decoded data are identical; the coding process is called reversible. That is not neccessarily the case for any compression procedure. GIF for example is only able to represent 256 different colours. Any further colour information will be lost during encoding.


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