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