mpeg compression 3

7/31/2019 Mpeg Compression 3

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Overview of MultimediaCompression Technologies

Vinay Kumar


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Presentation Overview

Introduction

Data Compression Image Compression

Video Compression

Audio Compression Summary

Q&A


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What is Compression and Why?

Compression ratio:

Number of bytes in the original imageNumber of bytes in the compressed image

e.g. input image = 10 MB

output image = 5 MB

compression ratio = 2:1

Whytransmission and storage

Compression

Algorithm

x bytes y bytes

y


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Common Terminology

Lossless Compression perfect reconstruction

Lossy Compression data is lost Spatial Compression 2D or single image

Temporal Compression 3D or video

Codec Compression / Decompression Colour / intensity same thing


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1. Data Compression

Oldest form of compression 1940s

Still important used to encode output frommodern codecs

Lossless


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Huffman Coding

1. Show symbols as leaf nodes

Symbol a b c d e

Frequency 19 10 8 8 5

a b c

2. Combine nodes with lowest frequencies

d e8 5

13

d e

Decreasing frequency


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Huffman Coding II

dea bc

13 18

31

50

0 1

0

0

1

11 0

Symbol HuffmanCode

a 0

b 111

c 110

d 101

e 100

=

Fewer bits per symbol compression achieved


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LZ77 - PKZIP

Replaces a repeated stream with a symbol

Search is finitewindowed

Need to store and transmit codebook inefficient

LZW needs no codebook generated by Tx / Rx

anbfcatkdfjs lcatjfl

look ahead windowtext window

Symbols code

cat 01

dictionary


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2. Image Compression

Can break free from stream compression

associated with data compression

Intra-frame coding


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Run-length Encoding

Data compression method

Replaces repetitive stream data with tuple in format

(symbol, count) aaaaazz encoded as (a,5) (z,2)

Good at horizontal runs

Still 1D


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Vector Quantization

2D based

Loads of enhancements

n[i]

n

- =

=

=

=

+

source block codebook

matched entry

average intensity


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Quadtree Encoding

Look for blocks of similar colour - recursive

Leaf node

Pixel intensity 1

Pixel intensity 0

(a) (b)

-1 0 1 0 1 0 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1

Good for few colours


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GIF - PNG

Uses LZW compression patent problems

A little knowledge useful for web designers

PNG replaces GIF no patent problems

Uses older (but free) LZ77 algorithm


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Pyramidal Techniques

Based on averaging pixels

Image reconstructed by summing beta pyramid

pel[i]

i=3

i=0

4

original image reconstructed image

pyramid pyramid


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Surface Approximation

View single image as 3D

Uses parametric equations

Recursive for complex images

- 2

- 1

0

1- 2

- 1

0

1

2

0

0 . 2 5

0 . 5

0 . 7 5

1

- 2

- 1

0

1

- 2

- 1

0

1- 2

- 1

0

1

2

0

0 . 2 5

0 . 5

0 . 7 5

1

- 2

- 1

0

1


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Directional Filtering

2nd generation technique HVS

Reconstruct image by adding all components

(a) (b)

(c) (d)


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JPEG - the theory

Uses Discrete Cosine Transform (DCT)

More waves can be added for more accuracy

(a)

(t)


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JPEG II in practice

DC Component AC Components

DC component represents average Usually 0

DCTSource

Image

IDCT


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Fractals

Can a block be found similar to Domain block?

If so, its called a Range block

Only store Range blocks plus affine transformations

Expensive!

Range

Blocks

Original

Domain Blocks


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3. Image Sequence Compression

Uses Inter-frame encoding

Also known an image sequence or temporalcoding


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MPEG Process

Hows does it work?

1. Subsample

Reduces data by around 50%

R G B Y U V

luminance


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MPEG Process II

2. Motion Detection on luminance block only

3 types of frame I Frames intra-coded

P Frames prediction from previous frame

B frames use bi-directional prediction

1

I

2

B

3

B

4

P

5

B

6

P

7

B

8

I


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MPEG Process III - Schematic

Image

Sequences

RGB

ToYUV

Motion

Estimation DCT

QuantizeRLEVLCOutputBuffer

No compression

achieved


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MPEG-1 & MPEG-2

MPEG-1

Designed for video playback at 150KB/s - singlespeed CD-ROM

Used in VCD technology

MPEG-2 Much higher bandwidth 3MB/s

DVD technology


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MPEG-4

Very different from previous generations

Aimed at low-bandwidth applications at upper end,good enough for digital TV

Digital Camcorders

How does it work then


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MPEG-4 II - Meshes

2-D animated meshes

Textures mapped onto meshes

Store vertices of mesh and movement parameters


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MPEG-4 III - Sprites

MPEG-4 is object based state of the art

Panoramic images massive compression ratios

1000:1


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4. Audio Compression

Techniques from image compression can be used

Huffman encodes output

DCT

MP3 huge!!!!

How does it work


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MP3 MPEG-1 Layer 3

1. Minimal Audition Threshold

Dont store anything under 5Khz

2. Masking Effect

Uses psychoacoustic model of the ear

Dont store quiet and loud noises simultaneously


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MP3 II

3. Joint Stereo (JS) coding

1. Intensity Stereo (IS) Ear unable to locate some frequencies bass

Store signal in mono + minimum for spatialization

2. Mid/Side (MS) Stereo

Used if left and right speakers are similar

Store middle (L+R) plus a side speaker (L or R) e.g.

L R10 5

7 5Store:Raw:

Fewer bits

Decompress: 10 5


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MP3 III - schematic


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Summary

Technique CompressionRatio

When?

Huffman 1.5-2:1 1952

RLE 4-10:1 1966

LZW 2-10:1 1977&84

Quadtree 2:1 1980

VQ 10:1 1984

Directional

Filtering

10-40:1 1985

Fractals 10-1000:1 1988

MPEG-1 10-100:1 1993

Surface Methods 10-50:1 1995

MPEG-2 10-200:1 1995

MPEG-4 10-500:1 1999

time


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Q&A

Anyone still awake?

Did that make sense?

Questions


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The End!

Contact Me!

Vinay Kumar -NIC, DIT, MOCIT, GOI

[email protected]

mpeg compression 3

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