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  • 1. 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
  • 2. 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 • Model with the function p(i)=a|i-b|+c • If 2(m3 – m0) < (m4 – m0), the i coordinate is to the left of the center • If (m3 – m0) > 2(m4 – m0), the i coordinate is to the right of the center • Otherwise it is along the center line • Similarly for the vertical direction 0 43 2 1
  • 3. 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
  • 4. MPEG: Video Encoding • The MPEG standards • do not define an encoding process • define syntax of the coded stream • define a decoding process
  • 5. MPEG: Video Encoding Pre processing Frame Memory + - DCT Motion Compensation Motion Estimation Frame Memory + IDCT Quantizer (Q) Regulator VLC Encoder Buffer Q-1 Output Input Predictiveframe Motionvectors
  • 6. 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
  • 7. MPEG: Structure of the Coded Bit-Stream • Sequence layer: picture dimensions, pixel aspect ratio, picture rate, minimum buffer size, DCT quantization matrices • GOP layer: will have one I picture, start with I or B picture, end with I or P picture, has closed GOP flag, timing info, user data • Picture layer: temporal ref number, picture type, synchronization info, resolution, range of motion vectors • Slices: position of slice in picture, quantization scale factor • Macroblock: position, H and V motion vectors, which blocks are coded and transmitted GOP-1 GOP-2 GOP-n I B B B P B B.. Slice-1 Slice-2 … Slice-N mb-1 mb-2 mb-n 0 1 2 3 4 5 Sequence layer GOP layer Picture layer Slice layer Macroblock layer 8x8 block
  • 8. MPEG: Macroblock Coding c h a n g e M Q U A N T n o c h a n g e t o M Q U A N T I p ic t u r e c h a n g e M Q U A N T n o c h a n g e t o M Q U A N T c o d e d n o t c o d e d in t e r fr a m e c h a n g e M Q U A N T n o c h a n g e t o M Q U A N T in t r a fr a m e m o t io n c o m p . A m o t io n v e c t o r s e t t o 0 P p ic t u r e A F w d m o t io n c o m p e n s a t io n A B w d m o t io n c o m p e n s a t io n A in t e r p o la t e d c o m p e n s a t io n B p ic t u r e P ic t u r e T y p e A MQUANT= scale factor q ],[ ],[8 ],[ jiqQ jiDCT jiQDCT = Quantization matrix
  • 9. 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
  • 10. 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
  • 11. MPEG-2: Profiles and Levels Levels Profiles SNR 4:2:0 Spatial 4:2:0 High 4:2:0;4:2:2 Multiview 4:2:0 High Enhancement 1920 X 1151/60 1920 X 1151/60 Lower 960 X 576/30 1920 X 1151/60 Bitrate 100, 80,25 130, 50, 80 High-1440 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 Bitrate 60, 40, 15 80, 60, 20 100, 40, 60 Main Enhancement 720 X 576/30 720 X 576/30 720 X 576/30 Lower 352 X 288/30 720 X 576/30 Bitrate 15, 10 20, 15, 4 25, 10, 15 Low Enhancement 352 X 288/30 352 X 288/30 Lower 352 X 288/30 Bitrate 4, 3 8, 4, 4
  • 12. 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