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Context-based Adaptive Coding and the Emerging H.26L Video Compression Standard

Context-based Adaptive Coding and the Emerging H.26L Video Compression Standard. Thomas Wiegand Heinrich Hertz Institute Berlin, Germany wiegand@hhi.de. ITU-T Project H.26L.

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Context-based Adaptive Coding and the Emerging H.26L Video Compression Standard

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  1. Context-based Adaptive Codingand the Emerging H.26L Video Compression Standard Thomas Wiegand Heinrich Hertz Institute Berlin, Germany wiegand@hhi.de

  2. ITU-T Project H.26L • New ITU-T Q.6/SG16 (VCEG) standardization activity for video coding especially aimed at 3G mobile networks (e.g. UMTS) and broadcast • Possible formation of a joint video team with MPEG • Goal for H.26L: 50% bit-rate reduction for same fidelity against every existing standard • 1999:3 proposalsfor definition of a first test model • HHI:Warping/OBMC motion model, wavelet- and context-based adaptive coding (CABAC) • Nokia:Affine motion model, multiple block transforms • Telenor:Block-matching with variable block-sizes, 4x4-DCT • Current Status:Definition of 8th test model (TML-8) based on Telenor proposal • Schedule:Final approval in November 2002

  3. Coder Control Control Data Transform/Quantizer Quant.Transf. coeffs - Decoder Deq./Inv. Transform Entropy Coding 0 Motion- Compensated Predictor Intra/Inter Motion Data Motion Estimator H.26L Structure

  4. Still using a hybrid of DPCM and transform codingas in prior standards. Common elements with other standards include: 16x16 macroblocks Conventional sampling of chrominance and association of luminance and chrominance data Block motion displacement Motion vectors over picture boundaries Variable block-size motion Block transforms (not wavelets or fractals) Scalar quantization The H.26L TML-8 Design, Part 1 of 4

  5. Coder Control Control Data Transform/Quantizer Quant.Transf. coeffs - Decoder Deq./Inv. Transform Mode 1 Mode 2 Mode 3 Mode 4 Entropy Coding 0 0 1 0 0 1 1 2 3 0 Motion- Compensated Predictor Mode 5 Mode 6 Mode 7 0 1 2 3 0 1 0 1 2 3 2 3 4 5 6 7 Intra/Inter 4 5 6 7 4 5 8 9 10 11 6 7 12 13 14 15 Motion Data 1/4 (QCIF) or 1/8 (CIF) pel Motion Estimator H.26L: Motion Compensation Accuracy

  6. Entropy Coding Multiple Reference Frames for Motion Compensation H.26L: Multiple Reference Frames Coder Control Control Data Transform/Quantizer Quant.Transf. coeffs - Decoder Deq./Inv. Transform 0 Motion- Compensated Predictor Intra/Inter Motion Data Motion Estimator

  7. Motion Compensation: Various block sizes and shapes for motion compensation (7 segmentations of the macroblock: 16x16, 16x8, 8x16, 8x8, 8x4, 4x8, 4x4) Multiple reference pictures (per H.263++ Annex U) Temporally-reversed motion B picture prediction weighting New “SP” transition pictures for sequence switching 1/4 sample (sort of per MPEG-4) and 1/8 sample accuracy motion 6x6 tap filtering to 1/2 sample accuracy, bilinear filtering to 1/4 sample accuracy, special position with heavier filtering 8x8 tap filtering applied repeatedly for 1/8 pel motion The H.26L TML-8 Design, Part 2 of 4

  8. Coder Control Control Data Transform/Quantizer Quant.Transf. coeffs - Decoder Deq./Inv. Transform Entropy Coding 0 Motion- Compensated Predictor Intra/Inter Motion Data Motion Estimator H.26L: Residual Coding • Residual coding is based on 4x4 blocks • Integer Transform

  9. Transform Integer transform approximating a DCT Based primarily on 4x4 transform size (all prior standards used 8x8) Expanded to 8x8 for chroma by 2x2 DC transform Intra Coding Structure Directional spatial prediction (6 types luma, 1 chroma) Expanded to 16x16 for luma intra by 4x4 DC Xfm The H.26L TML-8 Design, Part 3 of 4

  10. Quantization Two inverse scan patterns Logarithmic step size control Smaller step size for chroma (per H.263 Annex T) Deblocking Filter (in loop) Distinct Network Adaptation Layer (NAL) design for network transport Slice-structure coding Data partitioning The H.26L TML-8 Design, Part 4 of 4

  11. Coder Control Control Data Transform/Quantizer Quant.Transf. coeffs - Decoder Deq./Inv. Transform Entropy Coding 0 Motion- Compensated Predictor Intra/Inter Motion Data Motion Estimator H.26L: Entropy Coding

  12. Entropy Coding in H.26L: Type 1 of 2 • Universal Variable Length Code (UVLC) Simple design with some disadvantages: • Probability distribution is static • Correlations between symbols are ignored, i.e. no conditional probabilities are used • Codewords must have integer number of bits(Low coding efficiency for highly peaked pdfs)

  13. Entropy Coding in H.26L: Type 2 of 2 Context-based adaptive binary arithmetic codes(CABAC) • Usage of adaptive probability models • Exploiting symbol correlations by using contexts • Non-integernumber of bits per symbol by using arithmetic codes • Restriction to binary arithmetic coding • Simple and fast adaptation mechanism • But: Binarization is needed for non-binary symbols • Binarization enables partitioning of state space

  14. CABAC: Technical Overview update probability estimation Context modeling Probability estimation Coding engine Binarization Adaptive binary arithmetic coder Uses the provided model for the actual encodingand updates the model Maps non-binary symbols to a binary sequence Chooses a model conditioned on past observations

  15. Symbol Binarization 0 1 1 0 1 2 0 0 1 3 0 0 0 1 4 0 0 0 0 1 5 0 0 0 0 0 1 6 0 0 0 0 0 0 1 . ... Bin_no 1 2 3 4 5 6 7 ... Binarization • Mapping to a binary sequence using the unary code tree: • Applies to all non-binary syntax elements except for macroblock type • Ease of implementation • Optimal codes for a geometric pdfp(x) = 2–(x+1) • Discriminate between binary decisions (bins) by their position in the binary sequence •  Usage of different models for different bin_numbers in the arithmetic coder

  16. 0, for A+B < 2, 1, else ctx_no (C) = B A C Example: Context Modeling and Binarization Neighboring symbols A and B used for conditioning of current symbol C Context determination rule Current symbol C=4 A=2, B=3  ctx_no(C)=1 Context selection 0 0 0 0 1 Binarization Choice of model depends on bin_no (bit, model_no.): (0,1) (0,2) (0,3) (0,3) (1,3) Feed into the arithmetic coder

  17. Probability Estimation and Adaptation • Each model only consists of two counters: counts(„0“), counts(„1“) • Coding with multiple contexts (models) is easy to obtain because of a clean interface between model and coder • Model (probability estimate) is updated after each symbol is encoded  adaptive model

  18. Binary Arithmetic Coding • Standard implementations use integer arithmetic • Fast,multiplication-free variants of binary arithmetic coder exists: e.g. MQ-coder used in JBIG-2, JPEG-LS, JPEG-2000 • Estimation: Increasein computational complexity lower than 10% (MQ) and 20% (Standard-AC) of the total decoder execution time at medium bitrate

  19. Results: Bit-Rate Reduction

  20. Results: Bit-Rate Reduction

  21. MPEG-4: Advanced Simple Profile (ASP) Motion Compensation: 1/4 pel Global Motion Compensation H.26L: Motion Compensation: 1/4 pel (QCIF), 1/8 pel (CIF) Using CABAC entropy coding 5 reference frames in 7 of 8 cases (News: 17 / 25) Both: Sequence structure IBBPBBP... QPB=QPP+2 (step size: +25%) Search range: 32x32 around 16x16 predictor Well-known D+lR optimization techniques Comparison of H.26L to MPEG-4

  22. RD Curves: Foreman (QCIF, 10Hz) 39 38 37 36 35 34 33 Average PSNR(Y) [dB] 32 31 30 29 28 MPEG-4 27 H.26L 26 0 16 32 48 64 80 96 112 128 Bit-rate [kbit/s]

  23. RD Curves: Flowergarden (CIF, 30Hz) 38 37 36 35 34 33 32 31 Average PSNR(Y) [dB] 30 29 28 27 26 25 MPEG-4 24 23 H.26L 22 0 256 512 768 1024 1280 1536 1792 2048 2304 Bit-rate [kbit/s]

  24. PSNR Results: H.26L TML8vs. MPEG-4 ASP Anchor

  25. PSNR Results: H.26L TML8vs. MPEG-4 ASP Anchor

  26. Subjective Comparison

  27. Conclusions • Draft H.26L design is based on hybrid video coding • Similar in spirit to other standards but with important differences • Entropy coding can be conducted using • One VLC • Context-based adaptive arithmetic coding • Context-based adaptive arithmetic coding provides improvements between 5-15 % • H.26L shows a significant performance gain over existing standards including MPEG-4 • Bit-rate savings up to 50 % against MPEG-4 ASP

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