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GIF: (Graphics Interchange Format)
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Limited to 256 colors and less.
Recommended - 72 dpi.
8-bit color planes
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Originally designed by Compuserve. June 1987
It used a compression scheme called LZW.
Uses a compression method which uses a particular algorithm. This algorithm is copyrighted by Unisys. Any software which supports the format must obtain the rights to use the format but all users are free to use it.
Because of this issue..., a new format called PNG (Portable Network Graphics) may, in time, replace the GIF.
How it compresses...
- It compresses repeated patterns of pixels in an image.
- The more repeated patterns there are in an image, the more it can be compressed. When the image is decompressed it is
exactly the same as the original image.
- Example:
If the code looks like this - (@ 42-bytes)
1 5 6 4 6 7 9 1 2 5 6 9 8 4 5 8 9 2 5 6 9 8 5 6 7 2 5 6 9 6 1 5 6 4 6 7 9 7 8 2 5 6
The conversion looks for repeating strings of more than 3 numbers -
1 5 6 4 6 7 9 1 2 5 6 9 8 4 5 8 9 2 5 6 9 8 5 6 7 2 5 6 9 6 1 5 6 4 6 7 9 7 8 2 5 6
It then replaces the strings with a "token". It refers to which number it repeates and for how many characters -
1 5 6 4 6 7 9 1 2 5 6 9 8 4 5 8 9 [9,5] 5 6 7 [9,4] 6 [1,7] 7 8 [9,3]
- Each "token" takes 2 bytes. having eliminated several repeating characters, our code is now only 31-bytes
Points to note:
- GIF uses Lossless, Pattern Matching Compression
- It compresses repeated patterns of pixels in an image.
- The more repeated patterns there are in an image, the more it can be compressed.
- When the image is decompressed it is exactly the same as the original image
Summary:
When you convert images to the GIF format you first must reduce the number of colors to 256 or less, (this process looses information). The fewer the colors, the smaller the file. But when the image is compressed, no image detail is lost.
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