Multimedia Specification Design and Production 2012 / Semester 1 / L3 Lecturer: Dr. Nikos Gazepidis...

Multimedia Specification Design and Production

2012 / Semester 1 / L3Lecturer: Dr. Nikos [email protected]

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Introduction

Compress methods are key enabling techniques for multimedia applications.

Raw media takes much storage and bandwidth

A raw video with 30 frame/sec, resolution of 640x480, 24bit color

One second of video 30 * 640 * 480 * 3 = 27.6480 Mbytes

fps width height 3*8bit (True Colour)

One hour video is about 100Gbytes

Lossless Compression

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Lossless vs. Lossy (Audio file example)


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Some Terms


Encoder(compression)

Storage ornetworks

Decoder(decompression)

Data Input(a sequence of symbols from an alphabet)

Recovered data sequenceCode (a sequence of codewords)

Informationsource

Lossless compression: The recovered data is exactly the same as the input

Lossy compression: The recovered data approximates the input data

Compression ratio = bits used to represent the input data

bits of the code

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Properties of Huffman Coding


…in Computer Science and information theory, Huffman coding is an entropy encoding algorithm used for lossless data compression.

Huffman coding uses longer codewords for symbols with smaller probabilities and shorter codewords for symbols that often occur.

The two longest codewords differ only in the last bit.

Huffman coding is not effective for cases when there are small number of symbols

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Properties of Huffman Coding


“this is an example of a huffman tree”

Char Freq

Code

x 1 10010

u 1 00111

r 1 11000

p 1 10011

o 1 00110

l 1 11001

s 2 1011

t 2 0110

m 2 0111

n 2 0010

i 2 1000

h 2 1010

f 3 1101

e 4 000

a 4 010

space 7 111

parent node

leaves

‘1’‘0’

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Dictionary Based


Dictionary based method is another way to capture the correlation of symbols.

Static dictionary Good when the data to be compressed is specific in some application. For instance, to compress a student database, the world “Name”, “Student ID” will often appear.

Static dictionary method does not work well if the source characteristics change.

Adaptive Dictionary

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Compress Multimedia Data Losslessly


Model the media (audio, image, graphics data, or video as) as a source that generates symbols.

The naive way of choosing symbols: Pixels for images and videos Samples for audios Characters for text messages

Apply the lossless compression methods to the string of symbols.

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PNG (Portable Network Graphics)


PNG is a lossless image compressing method based on LZ77

PNG supports three main image types: true color, grayscale and palette-based ("8-bit") PNG supports alpha channel

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PNG (cont.)


PNG also supports interlaced coding and decoding

1 6 4 6 2 6 4 67 7 7 7 7 7 7 75 6 5 6 5 6 5 67 7 7 7 7 7 7 73 6 4 6 3 6 4 67 7 7 7 7 7 7 75 6 5 6 5 6 5 67 7 7 7 7 7 7 7

The scanning pattern in a 8x8 block. The whole image is partitioned into 8x8 blocks and scanned based on the pattern in each block.

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File Compression - LZ Algorithm (Lempel & Ziv)


ZIP compression. This compression system is a very handy invention, especially for Web users, because it let users reduce the overall number of bits and bytes in a file so it can be transmitted faster over slower Internet connections, or take up less space on a disk.

Compression up to certain %Original files size

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File Compression - LZ Algorithm


“Ask not what your country can do for you - ask what you can do for your country.” Quote by J.F. Kennedy, 1961

The above quote has 17 words made up of:

1. 61 letters2. 16 spaces3. 1 dash (-)4. 1 period (.)

Total: 79 units

Words Frequency

ask 2

what 2

your 2

country 2

can 2

do 2

for 2

you 2ask not wha

tyou

rcountr

ycan do for you

Unique Words

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LZ Algorithm - Dictionary


“Ask not what your country can do for you - ask what you can do for your country.”

Dictionary(words+numbers)

1. ask 2. what 3. your 4. country 5. can 6. do 7. for 8. you

According to our dictionary the above quote can be:"1 not 2 3 4 5 6 7 8 - 1 2 8 5 6 7 3 4"

79units 3

7units

Quote ‘before’ compression

Quote ‘after’ compression

Dictionary

37

units 74units

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LZ Algorithm - Searching for Patterns



Possible patterns in a dictionary "t" followed by a space “ou” which appears in both "your" and "country“ “can do for you” which appears 2 times - 14 units

Dictionary

1. ask_2. what_ 3. you4. r_country 5. _can_do_for_you

"1not_2345_-_12354"

79units 4

1units

Quote ‘before’ compression

Quote ‘after’ compression

Dictionary

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units 59units

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LZ Algorithm - Searching for Patterns



79units

59units

79units

74units

≈ -7.5% ≈ -25%

Pattern A Pattern B

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GIF (Graphics Interchange Format)


GIF was devised by UNISYS and Compuserve.

GIF is based on LZW lossless compression.

GIF supports 8bit (256) color images only. Each image can have its own color table.

It supports transparency layer and simple animation functions.

It also supports interlaced coding and decoding.

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Audio Compression


Audio can also be compressed in a similar way to image data.

For lossless audio compression, prediction is usually applied first. Simple prediction. Adaptive prediction.

Stereo decorrelation.

Entropy coding.

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Audio Examples


FLAC (free lossless audio codec) Polynomial fitting prediction or linear prediction. Rice coding.

MPEG4-ALS Lossless audio coding standard in MPEG4. Adaptive Linear Prediction. Supports up to 65535 channels. Fast random access. Rice coding.

Apple lossless audio coding (ALAC)

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MPEG-4 ALS System Diagram


Multimedia Specification Design and Production 2012 / Semester 1 / L3 Lecturer: Dr. Nikos Gazepidis...

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