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


Download


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GIF - Colours


The GIF standard was developed under the premises of the resources at that time; i.e. the available data transfer rate and in particular the efficiency of the graphic devices suggest to reduce the number of colours. Therefore an absolute limitation on 256 colours is standardized. This restriction cannot be exceeded within a picture segment.


However only the number of different colours is restricted to 256. The real colour representation is defined by colour tables using an RGB colour system with 8 bit resolution per primary colour. In total 256 out of 16.7 million colours are available.


  Red  x   Green  x  Blue

  28   x   28     x   28   = 224
                           = 16.777.216

Generally a GIF picture may consist of several picture segments. Each segment is arranged in a logical picture, which is then projected into the bitmap intended for displaying.


For colour allocation GIF provides two different table types, which have either a global or local character. The global table is accessible for each segment, but can be replaced by a local table. Thus, the limitation to 256 colours may be avoided, at least theoretically.


Beside the largest tables with 256 colours also smaller tables may be used. With those tables a clear reduction is achievable, if the picture's contents is well-suited for that. The table sizes can be varied between 1 bit (monochrome, e.g. b/w) and 8 bits (256 colours).


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GIF - Graphic Interchange Format GIF - Graphic Interchange Format GIF - Data Compression