LZW compression is an acronym for Lemple-Ziv-Welch compression.
This is a classical, lossless dictionary-based compression system.
Prior, to compression any GIF image must be reduced to 256
colors. This can be done using quantization followed by dithering
as discussed earlier. This is the where the loss occurs in GIF compression.
After the image is quantized then LZW compression is applied
to reduce the image size. LZW compression starts with a dictionary that contains all possible
symbols (in our case numbers from 0 to 255). For the sake of illustration,
I will use characters in this explanation.
Basically, build up a dictionary of longer and longer strings
based on the input stream. The encoder compares the incoming characters for the
longest possible match and returns the index for that dictionary entry. It then
then adds a new dictionary entry with the current character concatenated to
the longest match.
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LZW Compression Algorithm:
w = NIL;
while ( read a character k )
{
if wk exists in the dictionary
w = wk;
else
add wk to the dictionary;
output the code for w;
w = k;
}
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