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Runlength encoding

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• Runlength encoding
  • Run:

    Run = a sequence of characters that are all equal Example: aaaaaabbbbbbbcccccccaaaaa ^ ^ | | +-----+ a run

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  • Runlength:

    Runlength = the length of a run

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  • Runlength encoding:

    Runlength encoding = a very simple form of data compression in which runs of data are stored as a single data value + a count

    Example:

    aaaaaabbbbbbbcccccccaaaaa Runlength encoding: a6b7c7a5

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• Application of runlength encoding
  • Fax machine:

    a sheet of paper contains plain black text on a solid white background. There will be: many long runs of white pixels in the blank space on the paper, and many short runs of black pixels within the text. Example:

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  • Fact:

    The runlength encoding is used in fax machines to compress the data before it is transmitted Note: The compression ratio depends highly on the amount of text written on the sheet of paper You can actually witness the compress ratio by observing the transmission speed of the fac machine: When scanning an area with few text, the paper in the fax machine will move faster through the fax machine (due to good compression ratio, so very little data needs to be transmitted) When scanning an area with a lot of text, the paper in the fax machine will move very slowly through the fax machine (due to poor compression ratio, so a lot data needs to be transmitted and the fax machine can't go fast....)