SLR09 - Compression encryption & hashing

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6 Terms

1
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Purpose of compression

  • ↓ size file

  • ↓ download time

  • ↓ storage requirements

  • make best use of bandwidth

2
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Lossy compression

  • Some data lost

  • Slightly ↓ quality

  • Significantly ↓ file size

  • Suitable for: image, audio, video

3
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Lossless compression

  • None orig data lost

  • Orig file can be recreated when it is uncompressed

  • Suitable for: executable files & documents

4
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Methods of lossless compression

  • Dictionary coding: text based documents

  • Run-length coding: images

5
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Dictionary coding

  • Builds index using every data item/entry in file 

  • File is now made up of:

    • Dictionary index

    • Sequence of occurrences needed to recreate orig file

<ul><li><p>Builds index using every data item/entry in file&nbsp;</p></li><li><p>File is now made up of:</p><ul><li><p>Dictionary index</p></li><li><p>Sequence of occurrences needed to recreate orig file</p></li></ul></li></ul><p></p>
6
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Run length coding

  • ideal for bitmap images

  • There may be continuous pixels of the same colour 

  • So instead of storing each one individually, information stored in frequency/data pairs:

    • 22 0: 22 continuous white pixels

    • 6 1: 6 continuous black pixels