MPEG-2. ISO/IEC (or ITU-T H.262)

Transcription

1 MPEG-2 1 MPEG-2 ISO/IEC (or ITU-T H.262) High quality encoding of interlaced video at 4-15 Mbps for digital video broadcast TV and digital storage media Applications Broadcast TV, Satellite TV, CATV, HDTV, video services on networks (e.g., ATM) Killer applications: DVD 2

2 MPEG-2 Parts of MPEG-2: ISO/IEC : Systems ISO/IEC : Video ISO/IEC : Audio ISO/IEC : Compliance Testing ISO/IEC : Software ISO/IEC : DSM-CC ISO/IEC : NBC Audio ISO/IEC : 10-Bit Video (dropped!) ISO/IEC : Real-Time Interface ISO/IEC : DSM-CC Conformance 3 MPEG-2 Requirements Coding of interlaced video with high quality at 4-15 Mbps Random access/channel switching in limited time Fast forward/reverse (FF/FR) using access points Scalable video coding for multi-quality video applications System supporting audio-visual synchronized play/access for multiple streams A practical/implementable decoder 4

3 MPEG-2 Main New Feature Frame/ picture structure Frame//dual prime adaptive motion compensation Frame/ adaptive DCT Alternate scan for DCT coefficients Chrominance formats: 4:2:0, 4:2:2, 4:4:4 Nonlinear quantization table increased accuracy for small values 5 Positions of Samples 4:2:0 4:2:2 : Y samples : Cr, Cb samples 6

4 Positions of Samples top pixels bottom pixels 7 Positions of Samples Interlaced 4:2:0 top first=1 top bottom Interlaced 4:2:0 top first=0 bottom top Interlaced 4:2:2/4:4:4 top first=1 top bottom Progressive time time time time 8

5 Group of Pictures Encoder input: I B B P B B P B B I B B P Encoder output: I P B B P B B I B B P B B Decoder output: I 1 B 2 B 3 P 4 B 5 B 6 P 7 B 8 B 9 I 10 B 11 B 12 P 13 9 MPEG-2 Slice Slice a series of an arbitrary number of consecutive macroblocks The first and last macroblocks of a slice shall not be skipped macroblocks Every slice shall contain at least one macroblock Slices shall not overlap The position of slices may change from picture to picture The first and last macroblock of a slice shall be in the same horizontal row of macroblocks 10

6 MPEG-2 Slice Two slice structure General slice structure the slices does not cover the entire picture Restricted slice structure every macroblock shall be enclosed in a slice 11 MPEG-2 Slice E A B C G F H I general slice structure D A B C D E F G H I J K L M N O P Q restricted slice structure 12

7 MPEG-2 Macroblocks Three different chrominance format for a macroblock 0 1 4:2:0 2 3 Y 4 5 Cb Cr 4:2: Y Cb Cr 4:4: Y Cb Cr 13 MPEG-2 Streams Program streams for error-free environments (such as a disk) use long and variable-length packets for softwarebased processing Transport streams offer robustness necessary for noisy channels use fixed-length packets of 188 bytes well suited for delivering compressed video and audio over error-prone channels such as CATV and satellite transponders 14

8 MPEG-2 Scalability Scalability allows decoder of various complexities to be able to decode video of resolution/quality commensurate with their complexity from the same bit stream Data Partitioning SNR scalability Spatial scalability Temporal scalability 15 MPEG-2 Scalability Data Partitioning All header, MVs, first few DCT coefficients in the base layer Can be implemented at the bit stream level simple 16

9 MPEG-2 Scalability MPEG-2 non-scalable video codec Video in Inter frame/ DCT encoder Frame/ motion estimator and compensator Variable length encoder System MUX // System DeMUX Variable length decoder Inter frame/ DCT dncoder Frame/ motion compensator Video out 17 MPEG-2 Scalability MPEG-2 scalable video codec MPEG-2 enhancement video encoder MPEG-2 enhancement video decoder Video in System MUX // System DeMUX Preprocessor Midprocessor Midprocessor Postprocessor Enhanced quality MPEG-1/MPEG-2 non-scalable video encoder MPEG-1/MPEG-2 non-scalable video decoder base quality 18

10 MPEG-2 Scalability SNR Scalability Base layer includes coarsely quantized DCT coefficients Enhancement layer further quantizes the base layer quantization error 19 MPEG-2 Scalability 20

11 MPEG-2 Scalability Spatial Scalability 21 MPEG-2 Scalability Temporal Scalability option 1 22

12 MPEG-2 Scalability Temporal Scalability option 2 23 MPEG-2 Levels and Profiles Levels define the resolution of the picture Low level SIF ( ) Main level standard 4:2:0 resolution ( ) High-1440 level HDTV ( ) High level wide-screen HDTV ( ) 24

13 MPEG-2 Levels and Profiles Profiles determine the set of compression tools, compromise between compression rate and the cost of the decoder Simple profile higher bit-rate, no bidirectional prediction (B pictures) Main profile the best compromise between rate and cost, use all three image types (I, P and B) SNR scalable profile enhance quantization accuracy Spatially scalable profile enhance spatial resolution High profile for HDTV broadcast applications 25 MPEG-2 Levels and Profiles Profile Level Simple Main SNR Scalable Spatial Scalable High High MP@HL HP@HL High MP@H1440 SSP@H1440 HP@H1440 Main SP@ML MP@ML SNP@ML HP@ML Low MP@LL SNP@LL 26

14 MPEG-2 Levels and Profiles Digital TV DVD Digital CATV and VCR 1/2 buffer needed HDTV 27 Motion Estimation/Compensation Performed on luminance macroblock (16 16) Supporting half-pixel motion compensation Chrominance motion vectors are half of luminance MB s to for half-pixel motion vector Depending on motion types: Frame motion vector Field motion vector Motion vector in forward direction Motion vector in backward direction 28

15 Motion Estimation/Compensation MPEG-2 provides two types of picture structures Field picture Frame picture Five motion compensation modes Frame prediction for frame pictures Field prediction for pictures Field prediction for frame pictures Dual-prime prediction for p-pictures 16 8 MC for pictures 29 Motion Estimation/Compensation Mode 1-1 frame prediction for frame pictures Reference frame Possible interleaving B-picture (Not yet decoded) Frame-prediction for P-pictures 30

16 Motion Estimation/Compensation Mode 1-1 frame prediction for frame pictures Reference frame Reference frame Possible interleaving B-picture (Already decoded) Possible interleaving B-picture (Not yet decoded) Frame-prediction for B-pictures 31 Motion Estimation/Compensation Mode 2: prediction for pictures Top reference Bottom reference Possible interleaving B-picture (Not yet decoded) Field-prediction for the first of P- pictures 32

17 Motion Estimation/Compensation Mode 2: prediction for pictures Top reference Top reference Bottom reference Possible interleaving B-picture (Not yet decoded) Field-prediction for the 2nd of P- pictures when it is bottom 33 Motion Estimation/Compensation Mode 2: prediction for pictures Top reference Bottom reference Bottom reference Possible interleaving B-picture (Not yet decoded) Field-prediction for the second of P- pictures when it is top 34

18 Motion Estimation/Compensation Mode 3: prediction for frame pictures The target MB in a frame picture is split into top pixels and bottom pixels Field prediction is carried out independently for each 16 8 For P-frames, two motion vectors are assigned to each target MB, and two or four motion vectors are assigned to each target MB for B-frames 35 Motion Estimation/Compensation Mode 3: prediction for frame pictures Frame Macroblock 16 Top pixels blocks 8 16 Bottom pixels 36

19 Motion Estimation/Compensation Mode 3: prediction for frame pictures Top reference Top reference Bottom reference Possible interleaving Possible interleaving B-picture B-picture (Already decoded) (Not yet decoded) Field-prediction for B-frame pictures Bottom reference 37 Motion Estimation/Compensation Mode 4: dual-prime for P-pictures Only one motion vector is transmitted per MB together with a small differential motion vector For pictures two motion vectors are used to form predictions from two reference s (one top, one bottom) For frame pictures, a total of four predictions are made 38

20 Motion Estimation/Compensation Mode 4: dual-prime for P-pictures Top mv Top Bottom dmv Field prediction in picture 39 Motion Estimation/Compensation Mode 4: dual-prime for P-pictures Top Bottom mv1 dmv2 dmv1 mv2 Top Bottom Field prediction in frame picture 40

21 Motion Estimation/Compensation dmv Mode 4: dual-prime for P-pictures : Derived Vectors Top Bottom Top Bottom Reference Picture Picture Being Predicted Motion vector from prediction p to reference r : mv Differential motion vector: dmv Vertical shift correction: e Transmitted MV mv11 = (mvx11, mvy11) dmv = (dmvx, dmvy) Derived MV mvx22 = mvx11 mvy22 = mvy11 Field Vector For mvy12: e = -1 from bitstream mvx12 = mvx11/2 + dmvx mvy12 = mvy11/2 + e + dmvy For mvy21: e = +1 mvx21 = 3 mvx22/2 + dmvx mvy21 = 3 mvy22/2 + e + dmvy 41 Motion Estimation/Compensation Mode 5: MC for pictures The target MB in a picture is split into upper half region and lower half region Field prediction is carried out independently for each 16 8 half region For p-frames, two motion vectors are assigned to each target MB, and two or four motion vectors are assigned to each target MB for B-frames 42

22 Motion Estimation/Compensation Mode 5: MC for pictures Field Macroblock 16 Upper half region region blocks 8 16 Lower half region 43 Motion Estimation/Compensation Motion Compensation Mode Frame Prediction for Frame Pictures Field Prediction for Field Pictures Field Prediction for Frame Pictures Dual-Prime for P-Pictures 16 8 MC for Field Pictures Use in Field Pictures NO YES NO YES YES Use in Frame Pictures YES NO YES YES NO 44

23 Motion Mode Decision For P-Pictures Compute mean square error (MSE) between block and zero motion prediction Compute MSE between block and its MC frame prediction block Compute MSE between block and its MC prediction block Compute MSE between block and its MC dual-prime prediction block Choose the prediction mode with the least MSE A better strategy may be to weight MSE before mode selection 45 Motion Mode Decision For B-Pictures Compute MSE between block and its forward MC frame prediction block Compute MSE between block and its forward MC prediction block Compute MSE between block and its backward MC frame prediction block Compute MSE between block and its interpolated MC frame prediction block Compute MSE between block and its interpolated MC prediction block 46

24 DCT Coding Two types of luminance macroblock structure for DCT coding Frame DCT coding - each block shall be composed of lines from the two s alternately Field DCT coding - each block shall be composed of lines from only one of the two s 47 DCT Coding frame DCT coding DCT coding 48

25 DCT Coefficients Scan Scan order should depend on frequency energy distribution Zigzag scan for inter DCT Alternate scan for intra DCT 49