CT20A6100 MACHINE VISION AND DIGITAL IMAGE ANALYSIS MPEG Pauli Jutila Cristina Petre

Contents 1. Introduction to MPEG 2. MPEG Family 3. Intra Frame Coding Techniques 4. Non-Intra Frame Coding Techniques 5.Summary

Why do we need video compression? Example video compression x1080 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 ● : 720x480 at 30 frames/sec ● : 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 FilterDCTQuantizer Run-Lenght VLC Bit-Stream Buffer 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, ● Joan L. Mitchell, William B. Pennebaker, Chad E. Fogg, Didier J. LeGall, MPEG Video Compression Standard, Chapman and Hall, ● 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