1. A Performance Analysis of the ITU-T Draft H.26L Video Coding Standard Anthony Joch, Faouzi Kossentini, Panos Nasiopoulos Packetvideo Workshop 2002 Department of Electrical and Computer Engineering University of British Columbia

2. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 2 Outline Overview of H.26L Comparison of H.26L to H.263 and MPEG-4 in conversational video applications Analysis of H.26L coding features –Variable block size motion compensation –Multiple reference frame prediction –Quarter-pixel and eighth-pixel accurate motion vectors –Entropy coding modes Conclusions

3. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 3 Introduction and Motivation Existing digital video coding standards (MPEG-2, H.263) have fuelled an industry ITU-T VCEG (and now ISO MPEG) completing development of H.26L Promise of significantly improved coding efficiency through added features, flexibility New features make fully optimized “brute-force” encoding far too complex for real-time Need for reduced-complexity encoding methods

4. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 4 H.26L Key Objectives –50% bit rate reduction compared to H.263 version 2 –Non-backward compatible, “back-to-basics” design –Network friendly, error resilient, low delay option Essentially same block-based hybrid coding model as earlier video standards, but with additional features and flexibility Draft standard continues to evolve. Our work is based on version “TML-8” Project has become joint with MPEG (JVT) Expected approval towards end of 2002

5. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 5 H.26L 7 motion compensation block sizes 16

6. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 6 H.26L Multiple reference frame prediction (similar to Annex U of H.263) Frame N Frame N-1 Frame N-2...

7. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 7 H.26L Quarter-pixel motion accuracy is the default Eighth-pixel accuracy is supported as an option that can sometime improve performance, at the cost of complexity B-frames supported, as in other standards Powerful deblocking filter in MC loop Two entropy coding modes –Universal VLC: simple single table VLC –Content-adaptive binary arithmetic coding (CABAC) Higher compression but more complex

8. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 8 H.26L Residual coding uses 4x4 integer transform with DCT-like properties Integer transform eliminates problem of rounding mismatch in the inverse transform Intra-coded blocks predicted in the spatial domain a b c d e f g h i j k l m n o p I A B C D EFGHEFGH

9. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 9 H.26L vs. H.263 and MPEG-4 For conversational video applications (low-delay, conversational content) Required implementation of the same Lagrangian RD-optimization algorithm in all video encoders All configured for best RD-performance with no consideration of complexity (full search, etc.)

10. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 10 Conversational Video Tested several CIF resolution conversational video sequences Low delay necessary (no B-pictures) H.263 Baseline H.263 Conversational High Compression MPEG-4 Simple Profile MPEG-4 Advanced Simple (without B pictures) H.26L with CABAC, 5 reference frames, no B- pictures

11. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 11 Conversational Video H.26L clearly outperforms other standards Average bit savings provided by H.26L –24% vs. H.263 CHC –28% vs. MPEG-4 ASP –33% vs. MPEG-4 SP –42% vs. H.263 Baseline RD Curves…

12. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 12 Conversational Video

13. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 13 Conversational Video

14. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 14 H.26L Analysis Goal is to characterize RD-performance of key H.26L coding features First step in finding best complexity-performance tradeoffs needed for efficient fast encoding RD-optimized, full search Several features tested on a large set of content (14 sequences)

15. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 15 H.26L Analysis: Block Size Tested various logical combinations of the 7 available block sizes Using all 7 block sizes can provide 16% bit savings compared to using only 16x16 > 80% of this gain can be achieved using only the block sizes of 8x8 and larger 4x4 mode provides no PSNR improvement

16. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 16 Motion Compensation Block Size

17. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 17 Motion Compensation Block Size

18. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 18 % Bit savings from variable block size

19. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 19 H.26L Analysis: Reference Frames Tested 1 vs. 5 reference frames Average bit savings near 5%, but for 2 of 14 sequences, savings are 20% Illustrates that savings from multiple reference frames are highly content dependent Does not provide large savings for most conversational video content Use of 2 consecutive B-pictures provides 10-12% bit savings

20. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 20 Multiple Reference Frames

21. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 21 Multiple Reference Frames

22. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 22 % Bit savings from 5 reference frames

23. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 23 H.26L Analysis: Eighth-pel Eighth-pel MV accuracy actually provides worse RD performance than quarter-pel on average Bit rate for motion vectors doubled Content dependent: works well for sequences with high spatial detail Bit rate dependent: improved gains at high bit rates

24. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 24 Fractional-pel MC Accuracy

25. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 25 Fractional-pel MC Accuracy

26. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 26 % Bit savings from eighth-pel

27. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 27 H.26L Analysis: CABAC CABAC entropy coding provides consistent coding improvement compared to UVLC Average bit savings 8% Largest improvements at very low and very high bit rates

28. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 28 Entropy Coding Modes

29. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 29 Entropy Coding Modes

30. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 30 % Bit savings from CABAC

31. H.26L Analysis & Encoding Algorithms © Anthony Joch, 2002 31 THE END