Video Transcoding in H.264

Prof. Maurizio Bonuccelli

Francesca Martelli

Francesca Lonetti

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Outline

H.264 coding and transcoding

Motion Vector Composition algorithms

Overview of our temporal transcoding results (MPEG4, H.263)

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H.264 coding

H.264 mean features:

Variable and small block sizes (4×4)

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Average bit rate reduction!

High complexity!

Quarter-pixel resolutionNAL unitsNew entropic coding algorithms (CABAC)Deblocking filterMoto compensation with multiple reference pictures

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H.264 temporal transcoding

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TranscoderInput bit-rate 64 Kb Output bit-rate 32 Kb

123 13123

Skipping frames to reduce the output bit-rate

Three main issues:– Motion Vector Computation(MVC)– Prediction Error Computation– Frame Skipping Policy

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Motion Vector CompositionThe motion vectors are computed by

– Motion Vector Composition Algorithms (BI, TVC,

FDVS, ADVS)

– Restricted Motion Estimation (RME)

MVC

skipped

F(n-1) F(n)F(n-2)

mvn-1mvn

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Telescopic Vector Composition

F(n-1)

(skipped)

MBn

MB4n-1

MB1n-1

MB2n-1

MVn

MB3n-1

MV1n-1

MV2n-1

MV4n-1

MV3n-1

F(n)

MVn=MVn+MVTVC

New reference area

F(n-2)

MBnMVTVC=MV1

n-1

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Bilinear InterpolationMVBI=(1-α)(1-β)MV1

n-1+(α)(1-β)MV2n-1+(1-α)(β)MV3

n-1+(α)(β)MV4n-1

F(n-2)

F(n-1)

(skipped)

MBn

MB4n-1

MB1n-1

MB2n-1

MVn

MB3n-1

MBn

MV1n-1

MV2n-1

MV4n-1

MV3n-1

F(n)

MVn=MVn+MVBI

New reference area

MVBI

αβ

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H.264 features New in H.264: variable macroblock partition (16 motion vectors for each macroblock)

Transcoder keeps the same partitions of the remote encoder (most efficient solution)

How to apply MVC in H.264?

– BI and TVC adaptation

– New MVC algorithm

?

H.264

?

Previous standards

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MVC in H.264MB1 MB2

MB3 MB4

LIV.0

Mv_MVC=

f(mv_MB1,mv_B2, mv_B3, mv_B4)

MB1

B1 B2

MB3

B1 B2

B3 B4

MB4

B1

B2

LIV.1

MB2

B1

B3 B4

B2

Mv_B4

LIV.3

Mv_B2 Mv_B2 Mv_B2Mv_B1 Mv_B1=

f(sb1,sb2)Mv_B2

Mv_MB4=

f(mv_B1,mv_B2)

Mv_MB3=

f(mv_B2,mv_B4)Mv_MB2=

f(mv_B1,mv_B2)

Mv_MB1=

mv_B2

B1

sb1

B2B2

B2 B4 B2

LIV.2

B1

sb2

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A new MVC algorithmBasic Idea: looking for a reference area, the most similar to the current macroblock

Two steps:– For each motion vector of a macroblock (or block) of

the reference area in the skipped frame, compute the difference between the macroblock and an area pointed by the motion vector with the same size of the current macroblock

– Choose the motion vector of the reference area that minimizes this difference

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New MVC algorithm: example

F(n)

A

MB

N

M

Vf=argmin i Є S MSE(A, Ai) = argmin i Є S (1/NxM |A-Ai|2)

F(n-2)

mv

F(n-1)

MB4MB3

MB1 MB2b1 b2

b3 b4

b2b2

b1

b1

(skipped)

mv2

A2

A4

mv4

A3

mv3

A6

mv6

mv8

A8

mv7

A7

mv5

A5

mv1

A1

new_mv

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MVC performance (time)

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TVC BI New ME

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MVC performance (quality)P

SN

R(d

B)

Frames

Akiyo

Motion Estimation

New MVC Algorithm

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MVC performance (quality)Akiyo

PS

NR

(dB

)

New MVC Algorithm

BI Algorithm TVC Algorithm

Frames

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MVC performance (quality)

Motion Estimation

New MVC Algorithm

PS

NR

(dB

)

Frames

Coastguard

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MVC performance (quality)P

SN

R(d

B)

Frames

Coastguard

New MVC Algorithm

BI Algorithm TVC Algorithm

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Our activities in video transcodingWe started in studying video features in MPEG4

With this codec, we developed two temporal transcoders:– One based on an architecture known as FSC (Frame

Skipping Control)– One based on an architecture known as DFS

(Dynamic Frame Skipping)

By simulation, we realized that the second architecture results better in terms of video quality, while the first one results better in terms of processing times

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MPEG4 transcoder architecturesAfter a skipped frame:– In the FSC architecture, motion vectors and

prediction errors are computed without executing motion estimation worse quality, re-encoding errors amassment

– In the DFS architecture, motion vectors and prediction errors are computed with a restricted motion estimation better quality, but worse time performance

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Motion Vector CompositionIn literature we found 4 MVC algorithms:– Bilinear interpolation– Telescopic Vector Composition– Forward Dominant Vector Selection– Activity Dominant Vector Selection

By using the DFS architecture (that performs motion estimation) we realized by simulation that these algorithms are equivalent

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Architectures and MVC algorithms

M. A. Bonuccelli, F. Lonetti, F. Martelli. Video Transcoding Architectures for Multimedia Real Time Services, ERCIM News No. 62, pp. 39-40, July 2005.

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Skipping policiesWe developed 4 skipping policies, all of them based on the transcoder buffer occupancy:– Buffer based– Motion activity– Consecutive skipping– Random

M. A. Bonuccelli, F. Lonetti, F. Martelli. Temporal Transcoding for Mobile Video Communication. In Proceedings of 2nd Annual International Conference on Mobile and Ubiquitous Systems: Networking and Services (Mobiquitous 2005), pp.502-506, July 17-21, 2005, San Diego, CA.

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H.263 Video TranscoderThen, we studied the H.263 video codec, and we developed a temporal transcoder, based on DFS architecture.By observing the test results, we realize that video quality is influenced by the rate control of the front encoderThen, we tested and implemented several rate control algorithms for the front encoder:– TMN5– TMN8 domain– Perceptual rate control– Multiple zone (Activity)

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H.263 Video TranscoderIn our transcoder architecture, every frame is first transcoded, and then transmitted or dropped.To avoid the computation of frames that will be discarded, we developed a new frame skipping policy that predicts the frames to be transcodedM. A. Bonuccelli, F. Lonetti, F. Martelli. A Fast Skipping Policy for H.263 Video Transcoder. In Proceedings of 12th International Workshop on Systems, Signals and Image Processing (IWSSIP'05). September 22-24, 2005, Chalkida, Greece.

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Finally, H.264This codec is the product of the union between VCEG (H.263) and MPEG, forming the Joint VideoTeam (JVT)

It includes all benefits of previous standards in order to achieve good quality performance even at low bit-rate

We worked in two directions:– Optimizing the codec, to speed up it– Implementing a transcoder

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Optimizing H.264 encoderWe operated some modifications to the reference software in order to obtain acceptable encoding times:– instead of computing all half and quarter pixels in two

rounds, we compute them in only one round– fast way for choosing the optimal partitioning: instead of

using the SAD (Sum of Absolute Differences) measure as decision parameter, we use other metrics:

• the number of differences in terms of pixels • the maximum difference value• the average difference value• the most popular difference value

compared with proper self-adjusting thresholds.

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H.264 transcoderWe first implemented the cascade pixel domain transcoder by simply concatenating a decoder with an encoder, in order to develop the motion vector composition algorithms

Then we are implementing the frame skipping to obtain a transcoder that decides the frames to be dropped

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H.264 rate controlFinally, we are implementing the TMN8 rate control algorithm to be used in the front encoder

We think that, with a rate control algorithm able to skip frame in encoding phase, the transcoding process may be improved both in terms of quality and computation time

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Master ThesesLuigi D’Amaro. Algoritmi per la transcodifica video.Gianni Rosa. Transcodifica video per comunicazione mobile: studio di rate control.Luca Leonardi. Transcodifica video temporale: politiche di frame skipping.Marina Paletta. Realizzazione di un transcodificatore video temporale H.264 per video comunicazione mobile.Riccardo Vagli. Implementazione di un transcoder video basato sullo standard H.264/AVC.Alsona Dema. Rate Control in H.264.

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ConclusionsWe studied the video transcoding problem in real-time communications

We developed temporal transcoders with MPEG4, H.263 and H.264 codecs

We developed some skipping policies to be used in each transcoder

We developed three MVC algorithms to be used in the H.264 transcoder

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Acknowledgements

We thank all ERI people who introduced us in this research area, for the helpful discussions and advices

We thank PisaTel Lab people

We thank all students who worked with us in this project