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

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 contribution of terry welsh

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 contribution of terry welsh

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

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


Decode having the initial table and adding new codewords following the same procedure as encoding

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


Unix compress

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

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 (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.


Lzw contribution of terry welsh

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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