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Multimedia Systems

Entropy Coding

Mahdi Amiri

October 2015

Sharif University of Technology

Course Presentation

Assumptions:

Single source and user

Unlimited complexity and delay

Page 1 Multimedia Systems, Entropy Coding

Source and Channel CodingShannon's Separation Principle

Information

Source

Generates information

we want to transmit or

store.

Source

Coding

Channel

Coding

Reduces number of bits

to store or transmit

relevant information.

Increases number of bits

or changes them to protect

against channel errors.

What about joint source and channel coding?

Coding related elements in a communication system.

Claude E.

Shannon,

1916-2001

Ref.: en.wikipedia.org/wiki/Information_theory

information source: en.wikipedia.org/wiki/Information_source

source coding : en.wikipedia.org/wiki/Data_compression

Channel coding: en.wikipedia.org/wiki/Forward_error_correction


Source CodingMotivation

Data storage and transmission cost money.

Use fewest number of bits to represent information source.

Pro:

Less memory, less transmission time.

Cons:

Extra processing required.

Distortion (if using lossy compression ).

Data has to be decompressed to be represented, this

may cause delay.


Source CodingPrinciples

Example

The source coder shall represent the video signal by the minimum number of

(binary) symbols without exceeding an acceptable level of distortion.

Two principles are utilized:

1. Properties of the information source that are known a priori result in

redundant information that need not be transmitted (“redundancy

reduction“).

2. The human observer does not perceive certain deviations of the

received signal from the original (“irrelevancy reduction“).

Approaches:

Lossless coding: completely reversible, exploit 1. principle only.

Lossy coding: not reversible, exploit 1. and 2. principle.


Data CompressionLossless and Lossy

Lossless

Exact reconstruction is possible.

Applied to general data.

Lower compression rates.

Examples: Run-length, Huffman, Lempel-Ziv.

Lossy

Higher compression rates.

Applied to audio, image and video.

Examples: CELP, JPEG, MPEG-2.


Data CompressionCodec (Encoder and Decoder)

T

Transform,

prediction

Reconstructed

signal

Original

signal

T-1

Q

Q-1

E

E-1

Compressed

bit-stream

Inverse

Transform

Quantization

Dequantization

Entropy

encoder

Entropy

decoder

General structure of a Codec.

In information theory an entropy encoding is a

lossless data compression scheme that is independent

of the specific characteristics of the medium.Ref.: en.wikipedia.org/wiki/Entropy_(information_theory)

en.wikipedia.org/wiki/Entropy_encoding

Run-length encoding

Fixed Length Coding (FLC)

Variable Length Coding (VLC)

Huffman Coding Algorithm

Entropy, Definition

Lempel-Ziv (LZ77)

Lempel-Ziv-Welch (LZW)

Arithmetic Coding


Entropy CodingSelected Topics and Algorithms


Lossless CompressionRun-Length Encoding (RLE)

BBBBHHDDXXXXKKKKWWZZZZ 4B2H2D4X4K2W4Z

Image of a rectangle

0, 40

0, 40

0,10 1,20 0,10

0,10 1,1 0,18 1,1 0,10

0,10 1,1 0,18 1,1 0,10

0,10 1,1 0,18 1,1 0,10

0,10 1,20 0,10

0,40

RLE used in

Fax machines.


Lossless CompressionFixed Length Coding (FLC)

A simple example

►♣♣♠☻►♣☼►☻The message to code:

5 different symbols � at least 3 bits

Message length: 10 symbols

Total bits required to code: 10*3 = 30 bits

Codeword table


Lossless CompressionVariable Length Coding (VLC)

Intuition: Those symbols that are more frequent should have smaller codes, yet since

their length is not the same, there must be a way of distinguishing each code

►♣♣♠☻►♣☼►☻The message to code:

Total bits required to code: 3*2 +3*2+2*2+3+3 = 24 bits

Codeword tableTo identify end of a codeword as

soon as it arrives, no codeword can

be a prefix of another codeword

How to find the optimal codeword table?


Lossless CompressionVLC, Example Application

Morse code

nonprefix code

Needs separator symbol

for unique decodability


Lossless CompressionHuffman Coding Algorithm

Step 1: Take the two least probable symbols in the alphabet

(longest codewords, equal length, differing in last digit)

Step 2: Combine these two symbols into a single symbol, and repeat.

P(n): Probability of

symbol number n

Here there is 9 symbols.

e.g. symbols can be

alphabet letters ‘a’, ‘b’, ‘c’,

‘d’, ‘e’, ‘f’, ‘g’, ‘h’, ‘i’


Lossless CompressionHuffman Coding Algorithm

David A. Huffman

1925-1999

Paper: "A Method for the Construction of

Minimum-Redundancy Codes“, 1952

Results in "prefix-free codes“

Most efficient

No other mapping will produce a smaller average output size,

If the actual symbol frequencies agree with those used to create the code.

Cons:

Have to run through the entire data in advance to find frequencies.

‘Minimum-Redundancy’ is not favorable for error correction techniques (bits

are not predictable if e.g. one is missing).

Does not support block of symbols: Huffman is designed to code single

characters only. Therefore at least one bit is required per character, e.g. a word of

8 characters requires at least an 8 bit code.


Entropy CodingEntropy, Definition

The entropy, H, of a discrete random variable X is a measure of the

amount of uncertainty associated with the value of X.

Measure of information content (in bits)

A quantitative measure of the disorder of a system

It is impossible to compress the data such that the average

number of bits per symbol is less than the Shannon entropy

of the source(in noiseless channel)

The Intuition Behind the Formula

( ) ( )( )2

1log

x X

H X P xP x∈

= ⋅∑

Claude E. Shannon

1916-2001( )( )1

amount of uncertatinty P x HP x

↑ ⇒ ↓ ⇒ ∼

( )( )2

1bringing it to the world of bits log , information content of H I x x

P x⇒ ∼ =

( )weighted average number of bits required to encode each possible value and P x⇒ × ∑

X � Information Source

P(x) � Probability that symbol x in X will occur

Information Theory

Point of View


Lossless CompressionLempel-Ziv (LZ77)

Algorithm for compression of character sequences

Assumption: Sequences of characters are repeated

Idea: Replace a character sequence by a reference to an earlier occurrence1. Define a: search buffer = (portion) of recently encoded data

look-ahead buffer = not yet encoded data

2. Find the longest match between

the first characters of the look ahead buffer

and an arbitrary character sequence in the search buffer

3. Produces output <offset, length, next_character>

offset + length = reference to earlier occurrence

next_character = the first character following the match in the look ahead buffer


Lossless CompressionLempel-Ziv-Welch (LZW)

Drops the search buffer and keeps an explicit dictionary

Produces only output <index>

Used by unix "compress", "GIF", "V24.bis", "TIFF”

Example: wabbapwabbapwabbapwabbapwoopwoopwoo

Progress clip at 12th entry

Encoder output sequence so far: 5 2 3 3 2 1


Lossless CompressionLempel-Ziv-Welch (LZW)

Example: wabbapwabbapwabbapwabbapwoopwoopwoo

Progress clip at the end of above example

Encoder output sequence: 5 2 3 3 2 1

6 8 10 12 9 11 7 16 5 4 4 11 21 23 4


Lossless CompressionArithmetic Coding

Encodes the block of symbols into a single number, a

fraction n where (0.0 ≤ n < 1.0).

Step 1: Divide interval [0,1) into subintervals based on probability of

the symbols in the current context �� Dividing Model.

Step 2: Divide interval corresponds to the current symbol into sub-

intervals based on dividing model of step 1.

Step 3: Repeat Step 2 for all symbols in the block of symbols.

Step 4: Encode the block of symbols with a single number in the

final resulting range. Use the corresponding binary number in this

range with the smallest number of bits.

See the encoding and decoding examples in the following slides


Lossless CompressionArithmetic Coding, Encoding

Example: SQUEEZE

Using FLC: 3 bits per symbol � 7*3 = 21 bits

P(‘E’) = 3/7 Prob. ‘S’ ‘Q’ ‘U’ ‘Z’: 1/7

We can encode the word SQUEEZE with a

single number in [0.64769-0.64772) range.

The binary number in this range with the

smallest number of bits is 0.101001011101,

which corresponds to 0.647705 decimal. The '0.'

prefix does not have to be transmitted because

every arithmetic coded message starts with this

prefix. So we only need to transmit the sequence

101001011101, which is only 12 bits.

Dividing Model


Lossless CompressionArithmetic Coding, Decoding

Input Probabilities: P(‘A’)=60%, P(‘B’)=20%, P(‘C’)=10%, P(‘<space>’)=10%

Decoding the input value of 0.538

Dividing model from input probabilities �

60% 20% 10% 10%

The fraction 0.538 (the circular point) falls into the

sub-interval [0, 0.6) � the first decoded symbol is 'A'

The subregion containing the point is successively

subdivided in the same way as diviging model.

Since .538 is within the interval [0.48, 0.54), the

second symbol of the message must have been 'C'.

Since .538 falls within the interval [0.534, 0.54), the

Third symbol of the message must have been '<space>'.

The internal protocol in this example indicates <space> as the

termination symbol, so we consider this is the end of decoding process


Lossless CompressionArithmetic Coding

Pros

Typically has a better compression ratio than

Huffman coding.

Cons

High computational complexity.

Patent situation had a crucial influence to decisions

about the implementation of an arithmetic coding

(Many now are expired).


Lossless CompressionCAVLC and CABAC

CAVLC: Context-based adaptive variable-length coding.

CABAC: Context-based adaptive binary arithmetic

coding.

Both, a form of entropy encoding and lossless

compression used in the H.264/MPEG-4 AVC and h.265.

Have multiple look-up tables (CAVLC) and multiple

probability modes (CABAC). The different tables or

probability modes are selected based on the context (input

bit stream).

Supplementary Materials

Ref.: en.wikipedia.org/wiki/Context-adaptive_variable-length_coding

en.wikipedia.org/wiki/Context-adaptive_binary_arithmetic_coding


Lossless CompressionCAVLC vs. CABAC

Though results are source-dependent, CABAC

is generally regarded as being between 5–15%

more efficient than CAVLC. This means that

CABAC should deliver equivalent quality at a 5–

15% lower data rate, or better quality at the same

data rate.

CAVLC requires considerably less processing to

decode than CABAC.

Supplementary Materials


Thank You

1. http://ce.sharif.edu/~m_amiri/

2. http://www.aictc.ir/

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