Data Compression


Criteria

Survey Formats

Basics

Compression Methods

Data Formats


Run Length Encoding

General Principle

Control Characters

RLE for Image Data

Pixel Coding

Sequential Processing

Line Coding

Cross-Coding

End of Line Marker

Repetition of Lines

Lossy RLE

Implementations


Glossary

Index


Download


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RLE for Image Data


Traditionally run length encoding is of importance for the compression of image data. The reason is the internal structure of certain images on the one hand and the low complexity of the RLE implementation on the other hand.


Especially technical drawings, sketches and diagrams contain a large proportion of single coloured areas, i.e. the data consist of long sequential parts represented by a constant value. The RLE coding scheme is totally reversible and does not generate compression artefacts, which would particularly affect the mentioned type of contents.


The relatively small requirements were decisive for the early use of RLE because the performance of the workstations was not sufficient for more complex procedures, especially for high resolution images.


To utilize RLE for image data a variety of extensions was introduced to provide specific functionalities. Among others, RLE coding schemes were part of certain subformats within the BMP, PCX and TGA specification.


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Run Length Encoding (RLE) 7 Bit RLE Pixel Coding