2. Compression
Definition
Reduce size of data
(number of bits needed to represent data)
Benefits
Reduce storage needed
Reduce transmission cost / latency / bandwidth
3. Sources of Compressibility
Redundancy
Recognize repeating patterns
Exploit using
Dictionary
Variable length encoding
Human perception
Less sensitive to some information
Can discard less important data
4. Types of Compression
Lossless
Preserves all information
Exploits redundancy in data
Applied to general data
Lossy
May lose some information
Exploits redundancy & human perception
Applied to audio, image, video
5. Effectiveness of Compression
Metrics
Bits per byte (8 bits)
2 bits / byte ¼ original size
8 bits / byte no compression
Percentage
75% compression ¼ original size
6. Effectiveness of Compression
Depends on data
Random data hard
Example: 1001110100 ?
Organized data easy
Example: 1111111111 110
Corollary
No universally best compression algorithm
7. Effectiveness of Compression
Lossless Compression is not always possible
If compression is always possible (alternative view)
Compress file (reduce size by 1 bit)
Recompress output
Repeat (until we can store data with 0 bits)
8. Lossless Compression Techniques
LZW (Lempel-Ziv-Welch) compression
Build pattern dictionary
Replace patterns with index into dictionary
Run length encoding
Find & compress repetitive sequences
Huffman codes
Use variable length codes based on frequency
9. Huffman Code
Approach
Variable length encoding of symbols
Exploit statistical frequency of symbols
Efficient when symbol probabilities vary widely
Principle
Use fewer bits to represent frequent symbols
Use more bits to represent infrequent symbols
A A B A
A A
A B
10. Huffman Code Example
Expected size
Original 1/82 + 1/42 + 1/22 + 1/82 = 2 bits / symbol
Huffman 1/83 + 1/42 + 1/21 + 1/83 = 1.75 bits / symbol
Symbol A B C D
Frequency 13% 25% 50% 12%
Original
Encoding
00 01 10 11
2 bits 2 bits 2 bits 2 bits
Huffman
Encoding
110 10 0 111
3 bits 2 bits 1 bit 3 bits
11. Huffman Code Data Structures
Binary (Huffman) tree
Represents Huffman code
Edge code (0 or 1)
Leaf symbol
Path to leaf encoding
Example
A = “110”, B = “10”, C = “0”
Priority queue
To efficiently build binary tree 1
1 0
0
D
C
B
A
0
1
12. Huffman Code Algorithm Overview
Encoding
Calculate frequency of symbols in file
Create binary tree representing “best” encoding
Use binary tree to encode compressed file
For each symbol, output path from root to leaf
Size of encoding = length of path
Save binary tree
13. Huffman Code – Creating Tree
Algorithm
Place each symbol in leaf
Weight of leaf = symbol frequency
Select two trees L and R (initially leafs)
Such that L, R have lowest frequencies in tree
Create new (internal) node
Left child L
Right child R
New frequency frequency( L ) + frequency( R )
Repeat until all nodes merged into one tree
28. Huffman Code Properties
Prefix code
No code is a prefix of another code
Example
Huffman(“I”) 00
Huffman(“X”) 001 // not legal prefix code
Can stop as soon as complete code found
No need for end-of-code marker
Nondeterministic
Multiple Huffman coding possible for same input
If more than two trees with same minimal weight
29. Huffman Code Properties
Greedy algorithm
Chooses best local solution at each step
Combines 2 trees with lowest frequency
Still yields overall best solution
Optimal prefix code
Based on statistical frequency
Better compression possible (depends on data)
Using other approaches (e.g., pattern dictionary)
