Data Compression


Criteria

Survey Formats

Basics

Compression Methods

Data Formats

Files

Audio Data

Image Data

GIF

Colours

Data Compression

Characteristics

GIF87a

GIF89a

Patents

Optimizations

Colour Tables

Optimized Tables

Standard Tables

Raster

Anti-Aliasing

Examples

Drawings

JPEG

Video Data


Glossary

Index


Download


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GIF - Raster


In this context the term raster describes the attempt to represent a colour that is not part of the current colour table. I.e. the raster algorithm arranges the available colours in a way providing the best possible approximation to the impression of the missing colour.


A raster process is unavoidable whenever the colour table is not sufficient to represent the original data. That is always the case if photos shall be encoded in GIF.


The raster procedure should be usually done with an optimized color table. Standard tables commonly result in worse quality.


In principle a raster reduces the subjective resolution of the pictures because it takes several pixels to represent a single one. As the monitor resolution is strictly limited, clearly visible effects impair the impression.


Whether and to which extent such a procedure is successful, depends on the quality of the encoder, the structure of the original data and the target resolution. Finally one must judge the result and vary the adjustments oneself.


Actually GIF is not the best format if data must be rastered. The maximum number of colours is exceeded and the compression algorithm becomes ineffective. A file being too large and providing less quality would be the result. Other formats like JPEG presumably would be better suited.


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 JPEG 

GIF - Optimizations GIF - Standard Colour Tables GIF - Anti-Aliasing