CT20A6100 MACHINE VISION AND DIGITAL IMAGE ANALYSIS

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