Lzw contribution of terry welsh
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LZW: Contribution of TERRY WELSH. Further improvement is to encode singles instead of pairs to transmit. We simply send an index, but first initialize the dictionary . UNIX compress, GIF , facsimile V.42 standard. LZW. wabba_wabba_wabba_wabba_woo_woo_woo.

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LZW: Contribution of TERRY WELSH

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LZW: Contribution of TERRY WELSH

Further improvement is to encode singles instead of pairs to transmit

We simply send an index, but first initialize the dictionary

UNIX compress, GIF , facsimile V.42 standard


LZW

wabba_wabba_wabba_wabba_woo_woo_woo

Initialize the dictionary for all symbols in the alphabet

Example: for the source

A={ space ,a,b,o,w}, the initial dictionary


LZW

  • Each time you code the symbol/pattern that in the dictionary add a new pattern formed by concatenation of the existing symbol/pattern with the next symbol in the sequence.


wabba wabba wabba wabba woo woo woo

Output:5 2 3 3 2 1 6 8 10 12 9 11 7 16 5 4 4 11 21 23 4


Decodehaving the initial table and adding new codewords following the same procedureas encoding


UNIX compress

  • LZW +adaptive dictionary size

  • The dictionary is progressively doubled as it fills up.

  • When the largest allowed dictionary fills up

    • If the compression ratio is less than threshold, delete the dictionary and start with a new dictionary

      • The dictionary always reflects the local characteristics of the source.

  • Otherwise, static-dictionary method.


UNIX- compress: Example

  • Start with 29=512 entries

  • The first 256 of them already filled up

  • When the dictionary is full, it is doubled 210 entries, then…. Up to 216

  • If the compression ratio is less than threshold, delete the dictionary and start a new 512-entry dictionary


GIF (Graphics Interchange Format)

  • GIF is a graphics file format that uses a variant of LZW to compress the graphics data

  • The dictionary size is set initially to 2n+1 entries, where n is a number of bits per pixel in the original image.

  • If compression ratio is less than the threshold, the encoder sends the value 2n as the clear code to notify the decoder to discard the dictionary.


GIF

  • The dictionary is progressively doubled as it fills-up, until the maximum dictionary size of 212=4096 is reached

  • At this point the algorithm behaves as a static dictionary coding

  • The codewords stored in the blocks maximum 255 bits long. Each block has a header indicating the size of the block, and the terminator (eight zeros) afterwards.


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