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Technion - IIT Dept. of Electrical Engineering

Research Proposal to. Technion - IIT Dept. of Electrical Engineering. Signal and Image Processing lab. Transrating and Transcoding of Coded Video Signals. David Malah Ran Bar-Sella Michael Lavrentiev. STRIMM Meeting June 25, 2003. Transcoding.

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Technion - IIT Dept. of Electrical Engineering

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  1. Research Proposal to Technion - IITDept. of Electrical Engineering Signal and Image Processing lab Transrating and Transcoding of Coded Video Signals David Malah Ran Bar-Sella Michael Lavrentiev STRIMM Meeting June 25, 2003

  2. Transcoding • Converting a coded bit stream to another bitstream to • match a destination profile in terms of: • Media Formats{e.g., MPEG-2 H.264} • Resolutions (Spatial, Temporal) • Bit Rates (Transrating) • Color depths / formats {e.g., 4:4:4  4:2:0} Coded Bit Stream Transcoded Bit Stream Transcoder

  3. Cell-phone

  4. Generic Hybrid Video Encoder 0101 ... Q VLC DCT + + .. Bit stream - Q - 1 Intra / Inter Image sequence switch DCT - 1 + + + MPEG Frame Structure MEM MC ME MC - Motion Compensation DCT - Discrete Cosine Transform ME - Motion Estimation Q - Quantization VLC - Variable Length Code MEM - Frame store H.264 Intra Spatial Prediction

  5. Transrating Methods (Bit-Rate Reduction - BRR) • Frame dropping (B, P) • Color suppression • Discarding high-frequency DCT coefficients • Reducing spatial resolution (size reduction) • DCT coefficients Re-quantization

  6. Transcoder Architectures Transcoded Bit Stream Full Decoder Full Encoder Coded Bit Stream Cascading Decoder and Encoder Coded Bit Stream Partial Decoder Partial Encoder Processing Transcoded Bit Stream Motion vectors / coding modes Complexity-Reduced Transcoding (Compressed Domain) • Related Issues • Open Loop • Closed Loop • - Pixel-Domain • - DCT-Domain

  7. Q1 Re-quantization Error Q2 Original pixel value Quantized values with Q1 Quantized values with Q2

  8. Re-quantization Step-size Selection (Wang and Woods, 1998)

  9. Preferred Step-size Ratios (Wang and Woods, 1998)

  10. MSE Re-quantization p(x) Centroid x c d d 1 2 Finer quantizer Q1 c R R Coarserquantizer 1 2 D Q2

  11. MAP Re-quantization p(x) p p x 1 2 d d 2 1 Finer quantizer p < p p > p Q1 2 1 2 1 R R 2 1 Coarserquantizer Q2 D

  12. Re-quantization by Lagrangian Optimization Constrained Minimization problem: Lagrangian cost function becomes sum of independent MB level calculated parts: Lagrangian parameter, λ≥0, is iteratively updated to achieve desired bit-rate RT (Assuncao and Ghanbari, 1997) • Recent result(Laventer and Porat – ICIP-03):Optimal quantizer steps • are obtained at values close to specific multiples of the input quant. step.

  13. Variable Length Coding

  14. Trellis Re-quantization q*kmax q*k2 q*k1 block_1 block_2 block_N Macro-Block

  15. HVS-based Bit Allocation • Video frames can be partitioned (segmented) into regions • having different characteristics: • Textured regions • Smooth regions • Edges • Moving objects • Different regions have different perceptual importance and • may be encoded at different rates for similar subjective quality. • For efficiency, segmentation should be based on compressed- • domain data: Block DCT Coeff., MB type, MB q-step, MB rate, • and Motion Vectors (MV). • Possible approach: Block classification, Region-growing, and • Segment tracking. • Bit allocation to each region should reflect perceptual importance.

  16. MPEG-2  H.264 Transcoding • Quantization and Coeff. Scanning • 32 logarithmic steps • no ‘dead-zone’ • 2 scan modes • De-blocking Filter (4x4) • Entropy Coding • - Universal VLC (UVLC) • - Arithmetic Coding (CABAC) • Network Friendly Representation • - Video Coding Layer • - Network Adaptation Layer • (packetization, priority control) H.264 – Main Differences vis-à-vis MPEG-2 • MEC • - 7 modes (block sizes 16x16 – 4x4) • ¼ Pel resolution • Improved MV coding • Transform • Integer transform (4x4) • Intra Prediction • spatial prediction (9 modes) • 16x16 or 4x4 blocks • Inter Prediction • Multiple reference frames (1 – 5) • SP frame for stream transition

  17. Proposed Activity I.Transrating(MPEG2  MPEG2) • Re-quantization • Restricted Lagrangian optimization with MSE / MAP • Trellis re-quantization (MSE optional) • Bit-rate Control • Integrating other rate-reduction techniques • (temporal and spatial resolution reduction – BGU) • HVS-based bit-rate reduction • Frame segmentation and tracking • HVS-based bit allocation to segments • Restricted Lagrangian optimization in each segment • Trellis re-quantization in each segment

  18. Proposed Activity (Cont’d) • Full Transcoder • Similar rate • Intra prediction • All motion modes • Multiple ref. Frames • Coding control decisions II.Transcoding(MPEG-2  H.264) • Base-line Transcoder • Maximum matching to MPEG-2 • MEC – MB based; ½ pixel res. • B&P frames, Single ref. frame • No Intra spatial prediction • Same MB types decisions • Adapted H.264 features: • Syntax • UVLC • Transform • Quantization • Reduced rate • Adapting developed • re-quantization techniques • from MPEG-2 • Integrating other rate- • reduction techniques • (temporal and spatial • resolution reduction – BGU)

  19. END

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