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Basics of MPEG

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					              Basics of MPEG
• Picture sizes: up to 4095 x 4095
• Most algorithms are for the CCIR 601 format for
  video frames
   • Y-Cb-Cr color space
   • NTSC: 525 lines per frame at 60 fps, 720 x 480 pixel
     luminance frame, 360 x 480 pixel chrominance frame
   • PAL: 625 lines per frame at 50 fps, 720 x 576 pixel
     luminance frame, 360 x 576 pixel chrominance frame
• SIF (source input format) for digital TV
   • Luminance resolution: 360 x 240 pixels at 30 fps or 360
     x 288 pixels at 25 fps
   • Chrominance resolution: half the luminance resolution
     in both dimensions
               Detour: Motion Vectors with
                   Subpixel Accuracy
    • Find motion vector (u,v) with integer pixel accuracy
            • Let the MAE be m0
    • Compute the MAE at its 4-neighbor pixels (m1 .. m4)
    • Horizontal pixels
    1
            • Model with the function p(i)=a|i-b|+c
            • If 2(m3 – m0) < (m4 – m0), the i coordinate is to the left of
        0      the center
3            4
            • If (m3 – m0) > 2(m4 – m0), the i coordinate is to the right
               of the center
    2       • Otherwise it is along the center line
    • Similarly for the vertical direction
                 Basics of MPEG
• Types of pictures
   • I (intra) frame
      • compressed using only intraframe coding
      • Moderate compression but faster random access
   • P (predicted) frame
      • Coded with motion compression using past I frames or P frames
      • Can be used as reference pictures for additional motion
        compensation
   • B (bidirectional) frame
      • Coded by motion compensation by either past or future I or P frames
   • D (DC) frame
      • Limited use: encodes only DC components of intraframe coding
       MPEG: Video Encoding
• The MPEG standards
  • do not define an encoding process
  • define syntax of the coded stream
  • define a decoding process
             MPEG: Video Encoding
                                                     Regulator

Frame              +                                 Quantizer                     VLC
                                             DCT
Memory                            -                    (Q)                        Encoder

                                                        Q-1
                                                                                  Buffer
               Predictive frame



   Pre




                                                                 Motion vectors
processing                                            IDCT

                                                        +                         Output
  Input
                                        Motion       Frame
                                      Compensation   Memory

                                         Motion
                                        Estimation
      MPEG: Video Encoding
• Some highlights
  • Interframe predictive coding (P-pictures)
     • For each macroblock the motion estimator produces the best
       matching macroblock
     • The two macroblocks are subtracted and the difference is DCT
       coded
  • Interframe interpolative coding (B-pictures)
     • The motion vector estimation is performed twice
     • The encoder forms a prediction error macroblock from either
       or from their average
     • The prediction error is encoded using a block-based DCT
  • The encoder needs to reorder pictures because B-frames
    always arrive late
        MPEG: Structure of the Coded
               Bit-Stream
• Sequence layer: picture dimensions,
    pixel aspect ratio, picture rate,
    minimum buffer size, DCT                   GOP-1 GOP-2               GOP-n
    quantization matrices
•   GOP layer: will have one I picture,                          Sequence layer
    start with I or B picture, end with I or P
    picture, has closed GOP flag, timing
                                                I B B B P B B.. GOP layer
    info, user data
•   Picture layer: temporal ref number,
    picture type, synchronization info,
    resolution, range of motion vectors                               Slice layer
•   Slices: position of slice in picture,            Slice-1
                                                                   mb-1 mb-2      mb-n
    quantization scale factor                        Slice-2
•   Macroblock: position, H and V motion               …        Macroblock layer
    vectors, which blocks are coded and
    transmitted                                     Slice-N        01
                                                                   23 4 5
                                                  Picture layer                8x8 block
              MPEG: Macroblock Coding
                                                      Picture Type


      I picture                                P picture                                      B picture


 change       no change to      motion comp.                 motion vector    Fwd motion     Bwd motion     interpolated
MQUANT         MQUANT                                          set to 0      compensation   compensation   compensation


                   interframe                  intraframe            A            A              A              A


           coded           not coded      change     no change to
                                         MQUANT       MQUANT
                                                                         MQUANT= scale factor q
  change
 MQUANT
              no change to
               MQUANT
                                         A


                                                                             Quantization
                                                                               matrix
                         MPEG-2
• Why another standard?
  • Support higher bit rates e.g., 80-100 Mbits/s for HDTV
    instead of the 1.15 Mvits/s for SIF
  • Support a larger number of applications
  • The encoding standard should be a toolkit rather than a
    flat procedure
     •   Interlaced and non-interlaced frame
     •   Different color subsampling modes e.g., 4:2:2, 4:2:0, 4:4:4
     •   Flexible quantization schemes – can be changed at picture level
     •   Scalable bit-streams
     •   Profiles and levels
 MPEG-2: Effects of Interlacing
• Fields or frame pictures can be encoded
• Prediction Modes and Motion Compensation
   • Frame prediction: current frame predicted from previous frame
   • Field prediction:
       • Top and bottom fields of reference frame predicts first field
       • Bottom field of previous frame and top field of current frame predicts
          the bottom field of current frame
   • 16 X 8 motion compensation mode
      • A macroblock may have two of them
      • A B picture macroblock may have four!
   • Dual prime motion compensation
      • Top field of current frame is predicted from two motion vectors
         coming from the top and bottom field of reference frame
       • Works for P vectors
            MPEG-2: Profiles and Levels
                                                             Profiles
            Levels                SNR             Spatial                  High         Multiview
                                  4:2:0            4:2:0                4:2:0;4:2:2      4:2:0
                 Enhancement                                      1920 X 1151/60      1920 X 1151/60
                     Lower                                          960 X 576/30      1920 X 1151/60
  High               Bitrate                                            100, 80,25      130, 50, 80
                 Enhancement                  1440 X 1152/60      1440 X 1152/60      1920 X 1152/60

                     Lower                     720 X 576/30         720 X 576/30      1920 X 1152/60
High-1440

                     Bitrate                    60, 40, 15              80, 60, 20      100, 40, 60
                 Enhancement   720 X 576/30                         720 X 576/30       720 X 576/30
  Main               Lower                                          352 X 288/30       720 X 576/30
                     Bitrate      15, 10                                 20, 15, 4      25, 10, 15
                 Enhancement   352 X 288/30                                            352 X 288/30
  Low                Lower                                                             352 X 288/30
                     Bitrate       4, 3                                                   8, 4, 4
         MPEG-2 Applications
• Digital Betacam: 90 Mbits/s video
• MPEG-2
   • Main Profile, Main Level, 4:2:0: 15 Mbits/s
   • High Profile, High Level, 4:2:0: adequate, expensive
   • Image quality preserved across generations of
     processing
   • Multiview Profile
      • Stereoscopic view – disparity prediction
      • Virtual walk-throughs composed from multiple viewpoints

				
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