Data Compression


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Basics

Compression Methods

Data Formats


Run Length Encoding

General Principle

Control Characters

8 Bit Control Character

1 Bit RLE

7 Bit RLE

RLE for Image Data

Implementations


Glossary

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7 Bit RLE


Originally for coding of pure text files 7 bit codes were used. According to the ASCII character code all English letters and most of the punctuators were coded within a range from 0 to 127. The range of values from 128 to 255 remained unused. The most significant bit had to be left free e.g. due to the conception of traditional email systems.


From this a special coding scheme was developed using this redundancy. Values below 128 were interpreted as uncoded text and all values larger or equal 128 as counters for the character following.


7 Bit Coding Scheme:



Example:


uncoded  b   c   d   d   d   d   d   e
ASCII   98  99 100 100 100 100 100 101  (decimal)
coded   98  99 131 100             101  (decimal)

Nowadays this procedure has no practical meaning anymore because it refers to a data type, that is no longer relevant.


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Versionen mit Steuerzeichen 1 Bit RLE RLE for Image Data