CT20A6100MACHINE VISION AND DIGITAL

IMAGE ANALYSIS

MPEG

Pauli Jutila

Cristina Petre

Contents

1. Introduction to MPEG 2. MPEG Family3. Intra Frame Coding Techniques4. Non-Intra Frame Coding Techniques5.Summary

Why do we need video compression?

Example video compression- 1920x1080 pixels 30 frames/sec 8 bits/color => 1.5Gb/sec

- 6MHz channel bandwidth => 19.2Mb/sec

=> compression 83:1

The MPEG Family

MPEG-1

● Basic form using motion compensated block-based transform coding techniques

● Optimized to work for 352x240 at 30frames/sec

or 352x288 at 25frames/sec ,possible to go up

to 4095x4095

● Defined for progressive frames only

● No direct provision for interlaced video applications

MPEG-2

● Designed for digital television broadcasting

● Potentially high quality video (4-9Mb/sec)

● Profiles and levels :

profile - bitstream scalability, colorspace resolution

level - image resolution, maximum bit-rate/profile

● MP@ML : 720x480 at 30 frames/sec

● MP@HL : 1920x1080 at 30 frame/sec

MPEG-4

● Interactive content

● Object-oriented composite files

● Image blocks are arbitrary shaped

● Multiplexes and synchronizes data

● Divided into a number of parts-compatibility issue

Coding Techniques

Intra Frame Coding

Video Filter DCT Quantizer Run-Lenght VLC

Bit-StreamBuffer

Bit-Rate Control

Video Filter

• The human eye is less sensitive to chrominance than luminance.

• RGB YCbCr• Choice of sub-sampling ratios:

• 4:4:4 (Professional, Post-Production)• 4:2:2 (same as above)• 4:1:1 (DV Camcorder) • 4:2:0 (Web, DVD)

Discrete Cosine Transform

• Nearly optimal in energy consentration and decorrelating

• The human eye is less sensitive to changes in higher frequencies

• Calculated in blocks, usually 8x8

DCT Coefficient Quantization

• 12-bit coefficient is divided by a corresponding value from intra quantization matrix

• Each value in the resultant matrix is pre-scaled by multiplying by a single value, known as the quantizer scale code (1-112)

• The goal is to force as many of the DCT coefficients to zero or near zero

Variable Length Coding

• Zigzag-scanning long runs of consecutive zeros

• Alternate pattern in MPEG-2 (for interlaced video)• Huffman coding

Coding Techniques

I,P,B Frames

I-pictures: intra-coded no motion-compensated prediction used as reference picture

P-picture: inter-coded forward predicted (from I or P frame)

B-picture: inter-coded bi-directional prediction high coding efficiency

I,P,B Frames

Motion Estimation

Matching

Prediction Error Compensation

Non-Intra Decoding

Summary

● Video compression is needed!

● MPEG enables a wide range of applications

● Different standards but same principles

● Spatial and temporal processing

References

● Barry G. Haskell, Atul Puri, Arun N. Netravali, Digital Video: An Introduction to MPEG-2, Chapman and Hall, 1997.

● Joan L. Mitchell, William B. Pennebaker, Chad E. Fogg, Didier J. LeGall, MPEG Video Compression Standard, Chapman and Hall, 1997.

● John Wiseman, An Introduction to MPEG Video Compression, 1998

● Marius Preda, Les standards MPEG dans l'espace multimedia, 2005

● E. G. Richardson, Video Codec Design, Wiley, 2002● Wikipedia