Guidelines for SelectingPractical MPEG
Group of Pictures
The IASTED International Conference onInternet and Multimedia Systems and Applications (EuroIMSA 2006)
February 13-15, 2006 Innsbruck, Austria
Huahui Wu, Mark Claypool and Robert KinickiWorcester Polytechnic InstituteComputer Science Department
Worcester, MA, 01609 USA
Presenter - Bob Kinicki
Euro IMSA February 14, 20062
Outline
•Introduction•GOP Choices for Static MPEG
•GOP Choices for Streaming MPEG
•Conclusions
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I 0 B 00 B 01 P 1 B 10 P 2 I 0
MPEG• Group Of Pictures (GOP)
– IBBPBBPBBPBBIBBPBBPBBPBBI…
• Important Definitions– NP: number of P frames in one GOP– NBP : number of B frames between two
reference frames (I or P frame)
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GOP Choice• GOP choice determines MPEG properties– Frame size, picture quality– Streaming data rate, perceived quality
Our exhaustive search approach needs practical limits for the GOP length.• Current GOP choice is mostly intuitive!– 300 frames long by default in VirtualDub– 10 seconds long in NOSSDAV 2005
online video.• Mayer-Patel [6] searches a large GOP range
(i.e, 35 P frames).
[MLC02]
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• Playable Frame Rate– Counting frames– Temporal domain– Appropriate when every frame does not change
its content.• Peak Signal to Noise Ratio (PSNR)– Counting pixel differences for each frame– Quality domain– Appropriate when there is no frame loss.
• VQM– Temporal + Quality domain– Handles temporal loss and quality distortion.
Video Quality Metrics
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Outline
• Introduction
•GOP Choices for Static MPEG
•GOP Choices for Streaming MPEG
•Conclusions
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Static GOP Tradeoffs
• More B frames– Advantage: smallest frame size reduces the average
frame size– Disadvantage: decreases inter-frame similarity, increase
B frame size
• More P frames– Advantages: smaller than I frame, reduces the average
frame size
• However, supporting VCR functions is better with short GOPs.
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Static MPEG Methodology
• Use the Berkeley MPEG encoder and decoder.
• Choose varying combinations of NBP and NP.
• Encode raw images into video clips.
• Study the impact of NBP and NP on MPEG file size (MB) and frame quality measured by PSNR.
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9 Video Clips
• Each video is 300 raw images with CIF picture size (352x288) encoded at 30 fps.• Quantization level is set at 3.
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NBP Choice
PSNR of B frames
30
31
32
33
34
35
36
37
0 1 2 3 4 5 6 7 8 9 10 11 12NBP
PSNR
NP=1
NP=4
MPEG File Size
3
3.2
3.4
3.6
3.8
4
0 1 2 3 4 5 6 7 8 9 10 11 12NBP
MB NP=1
NP=4
Impact of NBP on Foreman Video
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High Motion
Low Motion
Medium Motion
NBP Impact on Eight Videos
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NP Choice
PSNR of P frames
38
39
40
41
42
43
0 5 10 15 20 25 30 35NP
PSNR
NBP=1
NBP=2
Impact of NP on Foreman Video
MPEG File Size
3
3.2
3.4
3.6
3.8
4
0 5 10 15 20 25 30 35NP
MB
NBP=1
NBP=4
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NP Impact on Eight Videos
NBP = 2
High Motion
Low Motion
Medium Motion
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Outline
• Introduction
•GOP Choices for Static MPEG
•GOP Choices for Streaming MPEG•Conclusions
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Streaming GOP Tradeoffs• More B frames– Advantages
• Low data rate• Easy to get through a lossy network
– Disadvantages:• Low inter-frame similarity• Add decoding latency
• More P frames– Advantage
• low data rate– Disadvantages
• Increases propagation errors– Repair techniques need to be used
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Streaming MPEG Methodology
• Develop a streaming MPEG model with packet loss and a capacity constraint– Forward Error Correction (FEC)– Pre-Encoding Temporal Scaling (PETS)– Estimate the video quality (measured by
playable frame rate). [WCK03
• Choose varying combinations of NBP and NP.
• Study the impact of NBP and NP on streaming performance.
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Forward Error Correction (FEC)
1 2 K Or i gi nal Vi deo Fr ame
1 2 K K+1 N Af t er Addi ng FEC Packet s
1 K K+1 N
1 2 K
Af t er Net wor k Tr ansmi ssi on
Af t er Reconst r uct i on( wi t h any K cor r ect packet s)
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Pre-Encoding Temporal Scaling (PETS)
• GOP rate
• Frame Size
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Streaming MPEG Optimization Model
where:R is playable frame rateSI, SP and SB are the frame sizesSIF, SPF and SBF are the FEC frame sizesNP and NB are the number of P and B framesG is the GOP rateT is the capacity constraint
[WCK03]
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Streaming MPEG Performance Analysis
• Three FEC choices– Non-FEC– 5% Fixed FEC– Adjusted FEC via the optimization model
•Use PETS to respond to the capacity constraint.
• Study varying combinations of NBP and NP.
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Streaming Foreman Video
2 percent packet loss 1.5 Mbps capacity
constraint
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Eight Streaming MPEG Videos
2 percent packet loss1.5 Mbps capacity constraint
Low Motion
High Motion
Medium Motion
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Outline
• Introduction
•GOP Choices for Static MPEG
•GOP Choices for Streaming MPEG
•Conclusions
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Conclusions
• For Static MPEG Videos– The number of B frames between two
references frames should not exceed 2.– The number of P frames in a GOP need not be
more than 5.• For Streaming MPEG Videos– The number of B frames between two
reference frames should be kept at 2 except when dealing with a delay constraint.
– The number of P frames in a GOP need not be more than 5.
Guidelines for SelectingPractical MPEG
Group of PicturesHuahui Wu, Mark Claypool and Robert Kinicki
Worcester Polytechnic InstituteComputer Science Department
Worcester, MA, 01609 USA
Thank You!Questions?