LZW Compression
   
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    GraphicsAcademy.com  >  Glossary  >  LZW Compression

       

     

    LZW Compression

    LZW is a method of compression named after Abraham Lempel, Jakob Ziv and Terry Welch, the scientists who developed this compressionalgorithm.

    In vastly simplified terms the way that LZW works is to build a dictionary of repeating terms, and to express these repeating terms as references to that dictionary. For example, in the sentence "the quick brown fox jumped over the lazy dog", the word "the" is a repeating pattern that might be encoded in this way.

    LZW is very effective (and fast) for data which contains lots of repeating patterns, for example images with solid color areas. However, LZW is really unsuited for images which contain a lot of noise, and in these cases may even increase file size.

    LZW is, or can be used in a number of file formats including GIF and TIFF.

    LZW is (or was) patented by Unisys. This resulted in a certain amount of controversy in the context of GIF files, when GIF files are compressed using an LZW compression scheme, which attracted a large amount of controversy when Unisys announced this algorithm was patented, and charged license fees for software developers using this algorithm. However, according to The GIF Controversy: A Software Developer's Perspective, these patents have now expired as of 20 June 2004

    A technical discussion of the LZW algorithm can be found in Chapters 1 and 7 of Graphics File Formats by David C. Kay and John R. Levine.

    Please Note:
    • We are not lawyers. This web site is not intended as legal advice, and should not be construed as such. Please consult an attorney should you require legal advice on copyrights, patents or any other matter.

    • GIF and 'Graphics Interchange Format' are trademarks of CompuServe Inc..

    • Unisys held patents on the LZW compression scheme, which according to The GIF Controversy: A Software Developer's Perspective these patents expired as of 20 June 2004.

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    Related Books:
    • Graphics File Formats

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      Graphics File Formats
       
        
      By David C. Kay

      Windcrest
      Paperback (496 pages)

      Product Description:
      Introduces graphics representation theory and image compression, covers advantages and disadvantages of each format, and provides format details and an overview of the file structure. This second edition contains expanded treatment of image compression and motion, and updated chapters on 30 formats, such as FLI/FLC, QuickTime Animation, RIB, TIFF, JPEG, Windows Bitmap, and PICT. Although it lacks image examples, graphics aficionados will appreciate the comprehensive nature of the text. Includes chapter bibliographies. Annotation copyright Book News, Inc. Portland, Or.

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    Related Web Sites:
    • LZW Data Compression

      A technical explanation (with sample code) by Mark Nelson, of LZW compression.




       
     
       
     
       
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