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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

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

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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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.)

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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…

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

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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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)

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

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

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

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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

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

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

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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

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

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

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

H.26L Analysis & Encoding Algorithms © Anthony Joch, 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

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

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

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

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