Unit 3 Dictionary based Compression Techniques
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This document discusses dictionary-based compression techniques, specifically LZ77. It explains that LZ77 uses a sliding window approach with a search buffer and look-ahead buffer. To encode a sequence, it searches the search buffer for a match to the look-ahead buffer and outputs a triplet with the offset, length and next symbol. An example demonstrates encoding a message using this approach by outputting triplets.
Unit 3 Dictionary based Compression Techniques
- 1. Lecture Notes on Dictionary Based Compression Techniques for Open Educational Resource on Data Compression(CA209) by Dr. Piyush Charan Assistant Professor Department of Electronics and Communication Engg. Integral University, Lucknow This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
- 2. • Dictionary-based compression algorithms usually create a dictionary (a pattern of characters) in memory as data is scanned looking for repeated information (some implementations use a static dictionary so it does have to be built dynamically). 4/22/2021 2 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 3. LZ77/LZ1/ Sliding Window • In many applications, the output of the source consists of recurring patterns. • A very reasonable approach to encode such sources is to keep a list, or dictionary, of frequently occurring patterns. • The input is split into two classes, frequently occurring and infrequently occurring patterns. • There are static and adaptive dictionary techniques. Most adaptive techniques have their roots in two papers by Ziv and Lempel in 1977 (LZ77) and 1978 (LZ78) 4/22/2021 3 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 4. LZ77 Approach • LZ77 is a Dynamic Adaptive Dictionary Technique that consists of a Sliding Window. • The widow consists of two parts: – Search Buffer (SB) – Look Ahead Buffer (LAB) • The size of the sliding window is given as: • Window Size= SB+LAB 1 2 3 4 5 6 7 8 9 10 11 12 13 Search Buffer Look Ahead Buffer 4/22/2021 4 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 5. • Search Buffer: A Search Buffer that contains a portion of the recently encoded sequence. • Look Ahead Buffer: A Look - Ahead Buffer that contains the next portion of the sequence. • To encode the sequence in look-ahead buffer, the encoder moves a search pointer back through the search buffer until it encounters a match to the first symbol in the look-ahead buffer. • Any two of the three must be given in the problem to encode given sequence of text. 4/22/2021 5 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 6. Process of LZ77 Compression • Lets see the process below: • Triplets: <o, l, c> c a b r a c a d a b r a Window Size=13 r r a …… Search Buffer Look-Ahead Buffer Offset Length of match codeword 4/22/2021 6 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 7. • Offset (o): The distance between the search pointer and the look-ahead buffer is called the offset. • Length of match (l): The number of consecutive symbols in the search buffer that match the consecutive symbols in the look-ahead buffer, starting with the first symbol, is called the length of match. • Codeword (c): It is the codeword corresponding to the symbol in the look-ahead buffer that follows the match. 4/22/2021 7 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 8. LZ77 Example • Encode the message- c a b r a c a d a b r a r r a r r a d • Here Window Size =13 • And Size of Look Ahead Buffer =6 c a b r a c a d a b r a r r a r r a d 4/22/2021 8 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 9. LZ77 Example contd.. c a b r a c a c a b r a c c a b r a c a d a d a…… d a b…… a b r…… <0,0,c(c)> <0,0,c(a)> Search buffer Look Ahead Buffer <0,0,c(b)> 4/22/2021 9 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 10. LZ77 Example contd.. c a b r a c a d a b c a b r a c a d a c a b r a c a d a b r a b r a…… r a r…… r r a…… <0,0,c(r)> <3,1,c(c)> Search buffer Look Ahead Buffer <2,1,c(d)> 4/22/2021 10 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 11. LZ77 Example contd.. a d a b r a r r a r r a d a b r a c a d a b r a r r a r r…… <7,4,c(r)> <3,5,c(d)> Search buffer Look Ahead Buffer c c a b r a c 4/22/2021 11 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow
- 12. LZ77 Example contd.. • The encoded message in the form of triplets are as follows: <0, 0, c(c)>,<0, 0, c(a)>,<0, 0, c(b)>,<0, 0, c(r)> <3, 1, c(c)>,<2, 1, c(d)>,<7, 4, c(r)>,<3, 5, c(d)> 4/22/2021 12 Dr. Piyush, Charan Dept. of ECE, Integral University, Lucknow