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Transcoding of an MPEG-2 bit stream to an H.264 bit stream

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[22] D-Y. Chan, S-J. Lin and C-Y. Chang, A rate control scheme using Kalman filtering ... MPEG-2 bit stream Adaptive directional prediction of 4x4 ... – PowerPoint PPT presentation

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Title: Transcoding of an MPEG-2 bit stream to an H.264 bit stream


1
Transcoding of an MPEG-2 bit stream to an H.264
bit stream

2
What is Transcoding ?
  • The operation of converting video in one format
    to another format.
  • Need Compatibility between MPEG-2 and H.264
    devices
  • Applications To adapt the bit rate of a
    compressed stream to the channel bandwidth, to
    change the spatial or temporal resolution of a
    compressed stream etc.

3
Criteria considered in Heterogeneous Transcoding
  • Quality of the transcoded stream should be
    comparable to that obtained by complete decoding
    and re-encoding with full motion search and to
    that of the initial input stream.
  • The information in the input bit stream should be
    re-used as much as possible to reduce
    multigenerational degradation.
  • The computational cost and complexity should be
    kept minimal.

4
MPEG-2 Decoder
MPEG-2 bit stream
5
H.264 encoder
6
Transcoding Algorithm

7
Intra frame coding

MPEG-2 H.264
Macroblock modes supported 8x8 16x16 with 4 directional modes, 4x4 with 9 directional modes
Type of intra prediction Fixed prediction of D.C. coefficient Adaptive directional prediction of 4x4 or 16x6 pixel blocks
Transform 8x8 DCT 4x4 Integer transform
8
Intra Frame Transcoding
9
Complexity of applying mode decisions in
transform domain
  • Example Vertical prediction

Predicted block
10
Intra modes in H.264/AVC
Directional modes for an intra 4x4 macroblock
Directional prediction
Directional modes for an intra 16x16 macroblock
11
Mode decision algorithm
12
Why use standard deviation?
  • Simple metric
  • Can be easily computed as the transform domain
    coefficients are already available.

13
Post mode decision
14
Intra Frame transcoding results
15
Subjective Quality of Intra framesMPEG-2 Input
Stream H.264 bit stream obtained
H.264 bit stream Test clip Akiyo
by the proposed method obtained from
complete Bit rate 1
Mbps Bit rate 768 Kbps decoding and
re-encoding Spatial resolution 352x240
of the input MPEG-2 bit stream

Bit
rate 670 Kbps
16
Subjective Quality of Intra framesMPEG-2 Input
H.264 bit stream obtained
H.264 bit stream bit stream by the
proposed method obtained from complete
Test clip Foreman Bit rate 1Mbps
decoding and re-encoding Bit rate 1 Mbps
of the input MPEG-2 bit Spatial resolution
352x240 stream. Bit rate 1Mbps

17
Inter frame coding
MPEG-2 H.264
MC prediction with ¼ pel accuracy No, only ½ pel accuracy yes
MC modes 16x16 16x16,16x8,8x16, 8x8,8x4,4x8, 4x4
Multiple reference prediction no yes
Direct modes in B frames no yes
Use of B frames as reference frames no Allowed, can be selected by the user
18
Inter frame transcoding
MPEG-2 bit stream
Inverse quantise
Sum residuals
VLD
IDCT
Hierarchical mode decisions
MV Refinement
Pass parameters
Inter prediction
Rate control
H.264 bit stream
4x4 Integer transform
Quantise
VLC/CABAC
Inverse VLC/CABAC
Inverse Quantise
Motion compensate
Store as reference frame
19
Inter frame transcoding
  • Features
  • Motion vector extraction
  • Motion vector refinement
  • Motion vector reuse
  • Hierarchical mode decision

20
Motion vector extraction
  • Motion vectors can be extracted from the MPEG-2
    bit stream after variable length decoding.

21
Need for motion vector refinement
  • Need
  • Differences in the quantization parameters of the
    incoming bit stream and those selected may
    differ. When these differences are large it
    results in quality degradation.
  • MPEG-2 supports certain modes in which no motion
    information is coded. However, since H.264
    supports more fine motion estimation block sizes,
    a small amount of motion may result upon
    refinement.
  • Re-evaluation of the decision to intra code
    macroblocks in a P frame.
  • Improves accuracy of the motion vectors and helps
    achieve compatibility between ½ pel MV accuracy
    in MPEG-2 and ¼ pel MV accuracy in H.264

22
Need for motion vector refinement
  • Compensates for field coding to frame coding
    changes and vice versa

23
Motion vector refinement
  • MPEG-2 motion vectors are refined over a one
    pixel window i.e. dx dy 3 pixels , in the
    most recent reference frame in List 0.
  • Half pixel and quarter pixel refinement is
    performed with the defined window.

24
Search window size (dx,dy) selection
  • Before window size selection, different
    increasing window sizes were tested to verify the
    effect of varying the search window size on the
    PSNR.
  • The graph for one such test clip Akiyo is as
    follows
  • It was observed that the PSNR obtained for a one
    pixel window closely approximated the steady
    state value and using a one pixel window provided
    a good tradeoff between complexity and the PSNR.

25
Motion vector reuse
26
Hierarchical mode decisions
  • Coding modes are compared and selected based on
    the sum of absolute difference (SAD) value. In
    the full mode decision method, every coding mode
    is evaluated ,the SAD value is computed and the
    mode with the minimum SAD value is selected as
    the best mode. However , although this method
    would give the best results ,it is very
    computationally intensive. For instance, each
    macroblock in the P frame would have to be
    evaluated for 16x16, 2 16x8, 2 8x16, 4 8x8, 8
    8x4,8 4x8, 16 4x4 intra and skip modes
  • Hierarchical mode decision process makes use of
    the fact that after evaluating a mode and the
    next level of sub partitioned modes , if sub
    partitioning does not reduce the SAD value then
    further sub partitioning need not be evaluated.

27
Hierarchical mode decisions
  • The top down splitting approach is shown below

28
P frame transcoding Results

29
P frame results

(see previous slide)
30
P frame results
31
Motion vectors in P framesMPEG-2 Input Stream
H.264 bit stream obtained H.264
bit stream Test clip Akiyo by
the proposed method obtained from
complete Bit rate 1 Mbps Bit rate 768
Kbps decoding and re-encoding Spatial
Resolution352x240 of the input MPEG-2 bit
stream

Bit rate 670 Kbps
H.264 motion vectors after transcoding
MPEG-2 motion vectors
H.264 motion vectors after full motion search
32
Mode decisions in P framesMPEG-2 Input Stream
H.264 bit stream obtained H.264
bit stream bit stream. by
the proposed method obtained from
complete Test clip Akiyo Bit rate 768
Kbps decoding and re- Bit rate 1 Mbps
encoding of the input Spatial
resolution352x240 MPEG-2 bit stream

Bit rate
670 Kbps
16x16 modes in the MPEG-2 bit stream
16x16 and sub-macroblock modes in the H.264
transcoded bit stream
16x16 and sub-macroblock modes in the re-encoded
H.264 bit stream
33
B frame transcoding results

34
B frame transcoding results
35
B frame transcoding results

36
Motion vectors in B framesMPEG-2 Input Stream
H.264 bit stream obtained H.264
bit stream. Test clip Akiyo
by the proposed method obtained
from complete Bit rate
1 Mbps Bit rate 768 Kbps decoding and
re-encoding Spatial resolution352x240
of the input MPEG-2 bit stream

Bit
rate 670 Kbps
Backward motion vectors in the H.264 transcoded
bit stream
Forward motion vectors in MPEG-2
Forward motion vectors in the H.264
transcoded bit stream
Forward motion vectors in the H.264 transcoded
bit stream
Backward motion vectors in the H.264 transcoded
bit stream
Backward motion vectors in MPEG-2
37
Mode decisions in B framesMPEG-2 Input Stream
H.264 bit stream obtained H.264
bit stream bit stream. by
the proposed method obtained from
complete Test clip Akiyo Bit rate 768
Kbps decoding and re-encoding Bit rate 1 Mbps
of the input MPEG-2 bit Spatial
resolution352x240 stream

Bit rate 670 Kbps
16x16 and sub 16x16 modes in the H.264
transcoded bit stream
16x16 and sub 16x16 modes in the H.264
re-encoded bit stream
16x16 modes in the MPEG-2 bit stream
38
B frame transcoding results
39
Comparison of the Input MPEG-2 bit stream vs. the
transcoded H.264 bit stream
  • The table below illustrates the comparison
    between the PSNR of the input MPEG-2 bit stream
    and the PSNR of the transcoded H.264 bit stream
    obtained by transcoding 35 frames at 1Mbps with
    the IBBPBBP GOP structure

40
Comparison of the proposed method with the DCT
domain transcoder proposed by Chang and
Messerschmitt 23
  • The graph shown compares the proposed method with
    DCT domain transcoding 23 and complete decoding
    and re-encoding of a 1 Mbps MPEG-2 bit stream
    (test clip Foreman) to an H.264 bit stream with
    an IBBPBBP.GOP structure at a constant bit rate.

41
Proposed method transcoded stream
  • Proposed method
  • Full re-encoding

42
References
  • 1 J. Youn and M-T. Sun , Motion Vector
    Refinement for high-performance transcoding, in
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  • 2 J. Xin, C-W. Lin and M-T. Sun, Digital Video
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  • 3 T. Wiegand et. al., Overview of the
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    CSVT, Vol. 13, pp. 560-576, July 2003.
  • 4 A. Vetros, C. Christopoulos and H. Sun,
    Video transcoding architectures and techniques
    an overview, IEEE Signal Processing magazine,
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  • 5 H. Kalva, Issues in H.264/MPEG-2 Video
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    p
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  • 13 Commercially available transcoders, PSP
    Video 9, http//www.pspvideo9.com
  • 14 K.R. Rao and J. J. Hwang, Techniques and
    Standards for Image, Video and Audio coding,
    Upper Saddle River, N.J. Prentice Hall, 1996.

43
References continued
  • 15 M. Ghanbari, Video Coding an introduction
    to standard codecs, London, U.K. Institution of
    Electrical Engineers, 1999.
  • 16 I. E. G. Richardson, H.264 and MPEG-4
    video compression video coding for next
    generation multimedia, Chichester Wiley, 2003.
  • 17Test streams obtained from ftp//ftp.tek.com/t
    v/test/streams/Element/MPEG-Video/525/ and
    http//www.cipr.rpi.edu/resource/sequences/sif.htm
    l
  • 18 Y-J. Chuang, Y-C. Huang and J-L Wu, An
    efficient block algorithm for splitting an 8x8
    DCT into four 4x4 modified DCT used in
    AVC/H.264, EURASIP 2005, pp. 311-316.
  • 19 P. Assunco and M. Ghanbari, Post Processing
    of MPEG-2 coded video for transmission at lower
    bit rates, Proc. IEEE ICASSP, pp. 1998-2001,
    Atlanta, GA, 1996.
  • 20 T. Shanableh and M. Ghanbari, Transcoding
    Architectures for DCT domain heterogeneous video
    transcoding, Proc. IEEE ICIP, Vol. 1, pp.
    433-436, Thessaloniki, Greece, Sept. 2001,.
  • 21 J. Xin, M.T. Sun and K. Chun, Motion
    re-estimation for MPEG-2 to MPEG-4 simple profile
    transcoding, Proc. Int. Workshop Packet Video,
    Pittsburgh, PA, Apr. 2002.
  • 22 D-Y. Chan, S-J. Lin and C-Y. Chang, A rate
    control scheme using Kalman filtering for H.263,
    Journal of Visual Communication and Image
    Representation, Vol. 16, pp. 734-748, Dec. 2005.
  • 23 S. Liu and A. Bovik, Foveated embedded DCT
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  • 25 G. Sullivan, T. Wiegand and A. Luthra,
    Draft of Version 4 of H.264/AVC (ITU-T
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  • 26 G. F-Escribano et.al., Computational
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    707-710, July 2005.

44
References continued
  • 27 I. Ahmad et. al., Video transcoding an
    overview of various techniques and research
    issues, IEEE Trans. on multimedia, vol. 7, pp.
    793-804, Oct. 2005.
  • 28 S. Benyaminovich, O. Hadar and E. Kaminsky,
    Optimal transrating via DCT coefficients
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    June 2005.
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  • 30 J. Wang et. al., An AVS to MPEG-2
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  • 31 J. McVeigh et. al., A software based
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