Run length encoding

Run length encoding

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  RUN LENGTH ENCODING Praseetha S Nair M.Tech(CS)
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  COMPRESSION  Data compressedby eliminating redundant elements.  Purpose of compression:- -save storage space -reduce communications capacity requirements  Some code must be substituted for eliminated element.  Based on this code receiver can reconstruct original data.
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   Categories:- 1.lossless compression -no information lost -decompressed data are identical to original uncompressed data 2.lossy compression -decompressed data may be an approximation to the original uncompressed data
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  Null Suppression  Oldestand simplest  Long string of blanks , nulls etc cause redundancy.  At transmitter: -scans data for string of blanks. -substitute two character code for any string that is encountered.
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   Example : the string XYZbbbbbQRX is replaced by XYZSC5QRX where Sc is a special compression indication character b is symbol for a blank space
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   At receiver: -scan incoming character for special indication character -reconstruct original data
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  Run-Length Encoding  Compressany type of repeating data sequence  At transmitter: -identify repeating characters to replace -if found, eliminate and replace by three- character code
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  Sc X Cc Sc-Special character indicating compression follows X-Any repeated data character Cc-Character count (a) Compression format Original data string Encoded data string $******55.72 $c*655.72 --------- Sc-9 GunsScb9Butter (b)Examples Fig: Run-Length Encoding
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   Efficiency dependson -number of repeated character occurrences in data to be compressed -average repeated character length.  Compression ratio=length of uncompressed data length of compressed data  Any compression scheme will have variable performance as the content of the input varies.
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   It isused as an element in more complex image compression technique.  For run-length encoding on an image, transmission of digital line scan is replaced by transmission of a quantity count of each of successive run of black or white scanned picture element.
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  0000000000 0000000000 0001111000 0001001000 0001111000 0000001000 0000001000 0001111000 0000000000 0000000000 Length:100 bit 23W 4B 6W 1B 2W 1B 6W 4B 9W 1B 9W 1B 6W 4B 23W (a)Image or: Fig: Run-Length Code for a 100-pixel 23 4 6 1 6 4 9 1 916 4 image 23 Length:15characters=1 20bit
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   Application ofrun length encoding to a 10X10 image.  10X10 representation is converted to a 100- bit code.  Each pixel is represented by a single bit indicate black or white.  Run-length code consist of length of alternate black or white sequence.  Encoded data stream is a string of number that indicate length of alternate black or white runs.
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  THANK YOU