lzw compression

1. LZW Encoding Algorithm
By
Israa A. Basheer
Rasha A. Yousef
Maalem A. Atiyaa
Supervised By
Assist. Prof. Wisam D. Abdullah
TIKRIT UNIVERSITY
COLLEGE OF COMPUTERSCIENCE
ANDMATHEMATICS
POSTGRADUATE

2. Introduction To LZW Compression Algorithm
History of LZW algorithm
developed by Abraham Lempel and Jacob Ziv ,
Lempel and Ziv published a series of papers describing
various compression algorithms. Their first algorithm was
published in 1977, hence its name: LZ77. This compression
algorithm maintains its dictionary within the data
themselves.
In 1978, Lempel and Ziv published a second paper outlining
a similar algorithm that is now referred to as LZ78. This
algorithm maintains a separate dictionary.
In 1984 Terry Welch published an improved version of the
LZ78 dictionary coding algorithm developed by Lempel and
Ziv.
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LZW (Lampel–Ziv-Welch) Definition
It is a lossless ‘dictionary based’ compression algorithm in
which longer fragments of the input text are replaced by
much shorter references to code words stored in the special
set called dictionary.
The Idea relies on reoccurring patterns to save data space.
LZW is the foremost technique for general purpose data
compression due to its simplicity and versatility. It is the
basis of many PC utilities that claim to “double the capacity
of your hard drive”.
It used to reduce the size of files, e.g. for archival or
transmission.
It used by a number of formats , including
GIF, TIFF, PostScript, PDF, Unix Compress, and V.42bis.

3. Working of the LZW
Algorithm
- The Algorithm is broken down into two parts, the encoding algorithm which
converts the strings into integer codes, and the decoding algorithm which does vice
versa.
- Both the encoder and decoder algorithm has a default table or a code-string pair
dataset that acts as the initial model for both the encoder and the decoder. And
when the algorithm goes on the new integer codes for the various string patterns
get added to this table.
- The Encoding works by reading a sequence of symbols, grouping the symbols into
strings, and converting the strings into codes. Because the codes take up less
space than the strings they replace, we get compression.
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4. Continue…
Algorithm
- LZW compression uses a code table, with 4096 as a common choice for the
number of table entries. Codes 0-255 in the code table are always assigned to
represent single bytes from the input file.
- When encoding begins the code table contains only the first 256 entries, with the
remainder of the table being blanks. Compression is achieved by using codes
256 through 4095 to represent sequences of bytes.
- As the encoding continues, LZW identifies repeated sequences in the data, and
adds them to the code table.
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5. Implementation :
The idea of the compression algorithm is the
following:
As the input data is being processed, a
dictionary keeps a correspondence between the
longest encountered words and a list of code
values. The words are replaced by their
corresponding codes and so the input file is
compressed. Therefore, the efficiency of the
algorithm increases as the number of long,
repetitive words in the input data increases.
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* PSEUDOCODE (For Viewing):
1 Initialize table with single character
strings
2 P = first input character
3 WHILE not end of input stream
4 C = next input character
5 IF P + C is in the string table
6 P = P + C
7 ELSE
8 output the code for P
9 add P + C to the string table
10 P = C
11 END WHILE
12 output code for P

6. Use the LZW algorithm to compress
the string : BABAABAAA
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Example Of LZW Compression

7. Example of LZW Compressiom : Step 1
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ENCODER OUTPUT STRING TABLE
Output code Representing Codeword String
66 B 256 BA
BABAABAAA P=empty
C=B
P=p+c
P=c

8. Example of LZW Compressiom : Step 2
8
ENCODER OUTPUT STRING TABLE
Output code Representing Codeword String
66 B 256 BA
65 A 257 AB
BABAABAAA P=B
C=A

9. Example of LZW Compressiom : Step 3
9
ENCODER OUTPUT STRING TABLE
Output code Representing Codeword String
66 B 256 BA
65 A 257 AB
256 BA 258 BAA
BABAABAAA P=A
C= B

10. Example of LZW Compressiom : Step 4
10
ENCODER OUTPUT STRING TABLE
Output code Representing Codeword String
66 B 256 BA
65 A 257 AB
256 BA 258 BAA
257 AB 259 ABA
BABAABAAA P=A
C= A

11. Example of LZW Compressiom : Step 5
11
ENCODER OUTPUT STRING TABLE
Output code Representing Codeword String
66 B 256 BA
65 A 257 AB
256 BA 258 BAA
257 AB 259 ABA
65 A 260 AA
BABAABAAA P=A
C= A

12. Example of LZW Compressiom : Step 6
12
ENCODER OUTPUT STRING TABLE
Output code Representing Codeword String
66 B 256 BA
65 A 257 AB
256 BA 258 BAA
257 AB 259 ABA
65 A 260 AA
260 AA
BABAABAAA P=AA
C= empty

13. Advantage Of LZW :
1- The LZW is simple, easy and requires no prior information about
the input data stream.
2- The LZW algorithm is faster compared to the other algorithms.
3- The LZW algorithm works more efficiently for files containing lots of
repetitive data.
4-The LZW algorithm compresses the data in a single pass.
5-This is a lossless compression technique, none of the contents in the file
are lost during or after compression.
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14. Disadvantage Of LZW :
1- LZW is a fairly old compression technique. All recent computer
systems have the powerful to use more efficient algorithms.
2- Creates entries in the dictionary that may never be used.
3- Useful only for large amount of text data when redundancy is high.
4- Royalties have to be paid to use LZW compression algorithms within
applications.
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THANKS FOR ATTENTION