Data Compression


Criteria

Survey Formats

Basics

Compression Methods

Data Formats

Files

Audio Data

Image Data

GIF

Colours

Data Compression

Characteristics

GIF87a

GIF89a

Patents

Optimizations

Examples

Drawings

JPEG

Video Data


Glossary

Index


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GIF - Data Compression


Apart from reducing the number of colours, GIF uses common entropy coding to get further compression. A slightly modified version of the LZW algorithm will be applied.


LZW is a lossless compression method suitable for arbitrary data. The abbreviation stands for Lempel-Ziv-Welch designating the inventor Terry Welch and the basic work of Abraham Lempel and Jacob Ziv.


The used data compression algorithm is characterised by its ability to compress recurring sequences of pixels effectively.


At run-time a sequence table is developed reflecting the already encoded part of the picture. Instead of the colour information, indices to this table will be used. The compression effect arises if longer sequences are already contained in the table.


This algorithm is particularly effective with simple graphics, technical drawings, sketches or diagrams. Photo-realistic representations or high definition pictures are not suited for this format.


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 Lempel-Ziv-Welch (LZW) 

GIF - Graphic Interchange Format GIF - Colours GIF - Characteristics