GIF - Colour Tables

The access to colour information is made by RGB colour tables. These consist of a list describing for each index the composition of the colour in the RGB format.

Example: Structure of a GIF colour table

Index	Red	Green	Blue
0.	255	000	000
1.	000	255	000
...	...	...	...
255.	255	255	255

In the above example 8 bits per colour component are used (range of values: 0 to 255), i.e. 3 byte are necessary per index. Each table of 256 colours require 768 Byte in total. The lower effort for addressing however is more essential than the reduction of the colour table. A graphic file with two colours require only one bit to address the colour index, 8 bits are necessary addressing 256 colours.

Number of Colours	Adressing	Size of the Table	Coding 100 x 100 Pixel (uncompressed)
2	1 bit	6 byte	1,250 byte
4	2 bit	12 byte	2,500 byte
8	3 bit	24 byte	3,750 byte
16	4 bit	48 byte	5,000 byte
32	5 bit	96 byte	6,250 byte
64	6 bit	192 byte	7,500 byte
128	7 bit	384 byte	8,750 byte
256	8 bit	768 byte	10,000 byte

The required data volume shall serve only as an example to give an impression about the dimension. In a real GIF file the contents will become compressed. The differences of the real file sizes are usually smaller.

The examples shall demonstrate that colours reduction is worthwhile. The gain results directly in shorter waiting times and improved acceptance.

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