Data Compression


Criteria

Survey Formats

Basics

Compression Methods

Data Formats


Run Length Encoding

General Principle

Control Characters

RLE for Image Data

Implementations

MS-Windows .bmp RLE8

Encoded Mode

Absolute Mode

Marker

Example "B"

MS-Windows .bmp RLE4


Glossary

Index


Download


www.BinaryEssence.com

Absolute Mode


The absolute mode is provided for sequences with varying contents. The minimum number of pixels in such a sequence is 3, the maximum 255. The absolute mode is indicated by an intial 0 followed by a value greater or equal than 3.


Coding scheme absolute mode:


Sequences providing an odd length will be treated in a special way. In this case the colour indices have to be terminated with 0, a value that is not part of the contents and will not be written to decoded data. This extension result in a word alignment as used in the entire scheme.


Example:

 encoded             decoded
 00 03 00 01 02 00   00 01 02
 00 04 00 01 02 03   00 01 02 03

Single or sequences of two pixels cannot be coded in absolute mode. Instead a run with the counter value 1 has to be used. This does not decrease compression efficiency because both the single pixel and the sequence of two pixels would require 4 byte coding in absolute mode.


Example:

 encoded             decoded
 01 00 01 01         00 01

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MS-Windows .bmp RLE8 Encoded Mode Marker