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

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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?