Multimedia Systems WS 2010/2011 31.01.2011 M. Rahamatullah - - PowerPoint PPT Presentation

University of Kaiserslautern Department of Computer Science Integrated Communication Systems ICSY http://www.icsy.de

Multimedia Systems

WS 2010/2011 31.01.2011

• M. Rahamatullah Khondoker (Room # 36/410 )

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• M. Rahamatullah Khondoker, University of Kaiserslautern

Outline

 Review of Compression: Dictionary Methods

 LZ77  LZ78  LZW

Abraham Lempel and Jacob Ziv

Dictionary-based Compression

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• M. Rahamatullah Khondoker, University of Kaiserslautern

Dictionary-based Compression

 Introduction

 Huffman algorithm depends on the statistical model of the data  Dictionary-based methods

• Do not use statistical model of the data
• Do not employ variable-length codes
• Are based on the assumption that a typical data file has redundancies

in the form of patterns and repetitions of data symbols

• Selects strings of symbols from the input and employs a dictionary

(static or dynamic) to encode each string as a token

• Achieve compression when there are many repetitions in the input file

and the size of the token is smaller than the matched string

• Add the symbols in the raw form if not found in the dictionary
• Write the raw items and tokens on the output such that the decoder

will be able to distinguish them

• Take into account what to do when the dictionary is filled up

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• M. Rahamatullah Khondoker, University of Kaiserslautern

Dictionary-based Compression Methods

 Several dictionary based compression methods exists. Among them, we will discuss

 LZ77

• Is simple but not efficient because its output tokens are triplets and

therefore large

 LZ78

• Generates tokens that are pairs. Therefore, more efficient than LZ77

 LZW

• Outputs single-item tokens. Therefore, most efficient

LZ77 method

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• M. Rahamatullah Khondoker, University of Kaiserslautern

LZ77 (sliding window)

 LZ77 (LZ1) method

 Uses part of the previously-seen input stream as dictionary  Maintains a window to the input stream and shifts the input in that window from right to left as strings of symbols are being encoded  Based on sliding window  Window has two parts: search buffer (thousands of bytes long) and look-ahead buffer (tens of bytes long)  Search buffer contains the current dictionary and includes symbols that have recently been input and encoded  Look-ahead buffer contains text to be encoded next  The encoder search for e in the search buffer and prepares the token (offset, length, next symbol in the look-ahead buffer) = (16,3,e)

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• M. Rahamatullah Khondoker, University of Kaiserslautern

LZ77 (sliding window)

 Example string: “sir sid eastman easily teases sea sick seals….”  The first five steps of the encoding is shown below

LZ78 method

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• M. Rahamatullah Khondoker, University of Kaiserslautern

LZ78

 LZ78 (LZ2) method

 Uses no search buffer, look-ahead buffer or sliding window  Uses a dictionary of previously encountered strings which is empty in the beginning and its size depends on the size of the available memory  Outputs two-field tokens (index, symbol). The index is a pointer to the dictionary and the symbol is the code of a symbol  Does not delete any entry from the dictionary

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• M. Rahamatullah Khondoker, University of Kaiserslautern

LZ78

 Example string: sir sid eastman easily teases sea sick seals….” The following table shows some steps of encoding the string

LZW method

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• M. Rahamatullah Khondoker, University of Kaiserslautern

LZW

 LZW method

 A popular variant of LZ78, developed by Terry Welch in 1984  Its main feature is to eliminate the second field of the token. Thus the token consists of just a pointer to the dictionary  It starts by initializing the dictionary to all the symbols in the alphabet  Because the dictionary is initialized, the next input character will always be found in the dictionary

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• M. Rahamatullah Khondoker, University of Kaiserslautern

LZW

 Example of LZW method

I In dict? New entry? Output s Y si N 256-si 115 (s) i Y ir N 257-ir 105 (i) r Y r¶ N 258-r¶ 114 (r) ¶ Y ¶s N 259-¶s 32 (¶) …

The above table is constructed from the string “sir sid eastman…”

Index Entry NULL 1 SOH … 32 SP … 97 a 98 b 99 c … 107 k 108 l … 255 255

Thanks for your attention Any questions, comments or concerns?

Integrated Communication Systems ICSY University of Kaiserslautern Department of Computer Science P.O. Box 3049 D-67653 Kaiserslautern

• M. Rahamatullah Khondoker, M.Sc.

Phone: +49 (0)631 205-26 43 Fax: +49 (0)631 205-30 56 Email: khondoker@informatik.uni-kl.de Internet: http://www.icsy.de