compression and decompression 10/22/20151 a.aruna, assistant professor, faculty of information...

COMPRESSION AND DECOMPRESSION

04/20/23 1A.Aruna, Assistant Professor, Faculty of Information Technology

IntroductionVideo and audio have much higher

storage requirements than textData transmission rates (in terms of

bandwidth requirements) for sending continuous media are considerably higher than text

Efficient compression of audio and video data, including some compression standards

COMPRESSION

Compression is a reduction in the number of bits needed to represent data.

save storage capacityspeed file transferdecrease costs for storage hardware and network bandwidth.

04/20/23 A.Aruna, Assistant Professor, Faculty of Information

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Multimedia Compression

Audio, image and video require vast amounts of data320x240x8bits grayscale image: 77Kb1100x900x24bits color image: 3MB640x480x24x30frames/sec: 27.6 MB/sec

Low network’s bandwidth doesn't allow for real time video transmission

Slow storage or processing devices don't allow for fast playing back

Compression reduces storage requirements

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Classification of Techniques

Lossless: recover the original representation.Mechanisms:

Packbits encoding(Run Length Encoding)CCITT Group 3 1DCCITT Group 3 2DCCITT Group 4Lempel – Ziv and Welch Algorithm LZW

A.Aruna, Assistant Professor, Faculty of Information Technology

Classification of Techniques

Lossy: recover a representation similar to the original onegraphics, audio, video and imagesMechanisms:

Joint Photographic Experts Group Moving Picture Experts GroupIntel DVI CCITT H.26l Video Coding AlgorithmFractals


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BINARY IMAGE COMPRESSION

Used for Documents (Black & White) Continuous Tone Information

Office & Business Document Handwritten TextLine GraphicsEngineering DrawingScanning Documents


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BINARY IMAGE COMPRESSION

Scanning ProcessScanline – Top to Bottom, Left to rightComposed of Various ObjectsCCD Array Sensor – B/W Dots- Memory

Eg: Faxing – 1 Page – 20 seconds


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Packbits Encoding or Runlength Encoding

Simplest and earliest Data Compression Schemes

Binary Image Consecutive Repeated – Two Bytes

First Byte – No.of times Character is Repeated

Second Byte – Character itself


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Packbits Encoding or Runlength Encoding


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CCITT Group 3 1-D Compression

Based on Runlength Encoding Facsimile & Early document Imaging

System Large Size even after Compression Modified Runlength encoding is

Huffman Encoding


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Huffman Encoding Variable Length Encoding Shorter Code – Frequently Longer code – Less Frequently

Probability of Occurrence of white and black Pixel

It is based on a coding Tree, which is constructed based on the probability of occurrences of white pixels and black pixels in the run length or bit streams


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probability of occurrences of bit stream of length Rn = P(Rn)


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Large Pixel Sequences


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Example

16 White Pixel = 101010 - Frequently16 Black Pixel = 0000010111Quicker decoding Tree Structure to be constructed


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Makeup code : length in Multiples of 64 pixels

Terminating code: length less than 64 pixels

132 white pixels is 100101011Make up code for 128 = 10010Terminating code for 4 = 1011


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Coding Tree

16 white 101010 and black pixel 0000010111


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1

0

1

0

1

0

CCITT Group 3 2D Compression

2-dimensional codingImages are divided into several

groups of K linesthe first line of each group is encoded

using CCITT Group 3 1D methodThe rest of lines are encoded using

some "differencial schemes"Typically compression ratio 10 ~

20


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The "K-factor" allows more error-free transmission

World-wide fassimile standardThe 2D scheme uses a

combination of additional codes called vertical code, pass code, and horizontal code


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Only one pass code, i.e. 0001 and one horizontal code, i.e. 001If vertical code and horizontal code

are not applied, then the horizontal code is appied

Horizontal Code + Group 3 1D Code = 001 + markup code + terminating code


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COLOR, GRAY SCALE& STILL VIDEO IMAGE COMPRESSION


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http://cs.stanford.edu/people/eroberts/courses/soco/projects/data-compression/lossy/jpeg/coeff.htm

INTRODUCTION

Adds a another Dimension to image.Indicate the states

Red - ?Green?

Adds Depth to the image – Background & Dense in Nature

Presenting Information


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In Physics?

Visible Light – Electromagnetic Spectrum Radiation or Radiant Energy

Frequency Ranges ??????????Radiant Energy is measured in terms

Wavelength & FrequencyRelationship??????Velocity of light c = 3 x 108 Meters


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COLOR

Primary ColorComplementary ColorApproaches

AdditiveSubtrative


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COLOR CHARACTERISTICS

Luminance or Brightness – Emitted or reflected from object

Hue – Color AppearancesSaturation – Color Intensity


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COLOR MODELS

Chromacity ModelRGB ModelHSI ModelCMY ModelYUV or YUI Model

B/W TV or COLOR IMAGE COMPOSITION


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JOINT PHOTOGRAPHIC EXPERTS GROUP COMPRESSION

JPEG – Joint ISO & CCITT Working Committee - exclusively for Still Image

Joint Committee – MPEG – Full Motion Standards

Works with colour and greyscale images Up to 24 bit colour images Suitable for many applications e.g.,

satellite, medical, general photography...


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JPEG Modes of Operation

Sequential DCT: the image is encoded in one left-to-right, top-to-bottom scan

Progressive DCT: the image is encoded in multiple scans (if the transmission time is long, a rough decoded image can be reproduced)

Hierarchical: encoding at multiple resolutions

Lossless : exact reproductionA.Aruna, Assistant Professor,

Faculty of Information Technology

JPEG Standards Level

Baseline – Maintain High Compression Ratio

Special Lossless Function – No Loss of Data

Extended System – Various Encoding Variable Length Encoding Progressive Length Encoding Hierarchical Encoding


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OVERVIEW OF JPEG STANDARDS

Components Baseline Sequential Codec – DCT

Coefficients, Quantization And Entropy Encoding

DCT Progressive Mode – Multiple Scans – Until Reached Picture Quality (Based on Quantization)

Predictive Lossless EncodingHierarchical Mode – Different Resolution


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JPEG IMPLEMENTATION

Discrete Cosine Transformation Reduce the level in Gray Scale and

Color Image ( 2D – Amplitude & Frequency)

Reduce Series of DataRemove The redundant data ( time to

frequency Domain)


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DCT


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INPUT


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140 144 147 140 140 155 179 175

144 152 140 147 140 148 167 179

152 155 136 167 163 162 152 172

168 145 156 160 152 155 136 160

162 148 156 148 140 136 147 162

147 167 140 155 155 140 136 162

136 156 123 167 162 144 140 147

148 155 136 155 152 147 147 136

OUTPUT


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186 -18 15 -9 23 -9 -14 19

21 -34 26 -9 -11 11 14 7

-10 -24 -2 6 -18 3 -20 -1

-8 -5 14 -15 -8 -3 -3 8

-3 10 8 1 -11 18 18 15

4 -2 -18 8 8 -4 1 -7

9 1 -3 4 -1 -7 -1 -2

0 -8 -2 2 1 4 -6 0

Quantization

Precision of Integer – Reduce No. of bits is used to store the values


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Quantization Step

Reduces the amplitude of coefficients which contribute little or nothing to 0

Discards information which is not visually significant

Quantization coefficients Q(u,v) are specified by quantization tables

A set of 4 tables are specified by JPEG


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Quantization Tables

for (i=0; i < 64; i++)

for (j=0; j < 64; j++) Q[i,j] = 1 + [ (1+i+j) quality]; quality = 1: best quality, lowest compressionquality = 25: poor quality, highest compression


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Entropy Encoding

Encodes sequences of quantized DCT coefficients into binary sequences

AC: (runlength, size) (amplitude)DC: (size, amplitude)runlength: number consecutive 0’s, up to 15

takes up to 4 bits for coding(39,4)(12) = (15,0)(15,0)(7,4)(12)

amplitude: first non-zero valuesize: number of bits to encode amplitude 0 0 0 0 0 0 476: (6,9)(476)


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Huffman coding

Converts each sequence into binaryFirst DC following with ACsHuffman tables are specified in JPEGEach (runlength, size) is encoded

using Huffman codingEach (amplitude) is encoded using a

variable length integer code(1,4)(12) => (11111101101100)


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Example of Huffman table


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VIDEO COMPRESSION


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INTRODUCTION

Distribute the information to larger places

ApplicationVideo teleconferencing Digital Telephony


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STANDARDS

P*64 (CCITT) – Video ConferencingJPEG (ISO)- Still Image MPEG (ISO) – Stored Video


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Requirements for Full Motion Video Compression

Random Access – IndexingVCR Paradigm –

play,fast,forward,rewind,stop,search forward, etc.,

Audio & Video SynchronizationMultiplexing multiple compressed Audio

and Video Bit StreamsEditabilityPlayback Device Flexibility


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CCITT H.261 Video Coding Algorithm(px64)

Developed in 1990’sVideophone and Video ConferencingCIF (Common Interchange File

Formats) & QCIF(Quarter CIF)Hierarchical Block Structure –

Encoding DataDCT & DPCM


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AUDIO COMPRESSION

ADAPTIVE DIFFERENCIAL PULSE CODE MODULATION


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compression and decompression 10/22/20151 a.aruna, assistant professor, faculty of information...

Documents

assistant professor

runlength encoding

d compression

huffman encoding

dccitt group

video data

higher storage requirements

storage hardware