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FAST MODE DECISION IN H264/AVC VIDEO CODEC NIranjan Mulay (0393251) Chen Gao (0401840) (El6123: Project Presentation). 05/06/2010. Outline:. Introduction to H.264/AVC coding standard Mode decisions in H.264/AVC - Intra Block - Inter Block RDO algorithm and the need for FMD

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  1. FAST MODE DECISION IN H264/AVC VIDEO CODECNIranjanMulay (0393251)Chen Gao(0401840)(El6123: Project Presentation) 05/06/2010

  2. Outline: • Introduction to H.264/AVC coding standard Mode decisions in H.264/AVC - Intra Block - Inter Block • RDO algorithm and the need for FMD • FMD (for Intra and Inter) Literature survey: edge-map based FMD • Study of x264 code and encoding options • Implementation: -Generation of MB mode statisticsfile from X264 -Visualize the modes in Matlab -Intra FMD; Inter FMD • Summary and future work

  3. Introduction to H.264/AVC Coding Standard The key features of H.264: • Improved Intra prediction: Directional spatial prediction • Enhanced Temporal Prediction: -Motion compensation with variable block sizesfrom 4x4 to 16x16: reduces ‘prediction error’ -Quarter-pelaccurate motion estimation -Multiple reference for motion estimation -Weighted prediction (for B and P frames) • DCT-like integer transform: No mismatch between encoder and decoder

  4. Introductionto H.264/AVC Coding Standard(Cntd) • Efficient entropy coding: -Uses arithmetic entropy coding, has option for VLC coding -Context adaptive entropy coding: 2 options – CAVLC and CABAC • Variable size (primarily 4x4 along with 8x8,16x16) transform: - Smaller size helps to represent a signal in locally adaptive manner which reduces ringing artifacts. - Generally high frequency=> 4x4 and low frequency=> 16x16 • In-loop deblocking filter: Reduces blocking artifacts, improves quality. • Special Error Resilient Tools

  5. H.264 Intra Modes: • Intra 4x4 : useful for a MB with significant detail • Intra 16x16 : good for coding very smooth areas (Intra 8x8 chroma: similar to intra 16x16) • I_PCM: no prediction or transform

  6. ‘Intra 16x16’: • Mode 0 (vertical): extrapolation from upper samples. • Mode 1 (horizontal): extrapolation from left samples. • Mode 2 (DC): mean of upper and left-hand samples. • Mode 3 (Plane): plane prediction based on a linear spatial interpolation by using the upper and left-hand samples of the MB.

  7. ‘Intra 4x4’: Figure:4x4 luma prediction mode

  8. Intra 4x4(Cntd): • Mode 0: Vertical • Mode 1: Horizontal • Mode 2: DC prediction • Mode 3: Diagonal down-left • Mode 4: Diagonal down-right • Mode 5: Vertical-right • Mode 6: Horizontal-down • Mode 7: Vertical-left • Mode 8: Horizontal-up

  9. H.264 Inter Modes: • Hierarchical Decision • Level-1 (Partition): Compute RD-cost for: 16x16, 16x8, 8x16, 8x8. • Level-2 (Sub-Partition): If level-1 => 8x8, Then, compute RD cost of 8x4,4x8 and 4x4 Select the most optimal block! • P_Skip Mode

  10. RDO Algorithm • Formula:RD_cost(s,c,MODE|Qp) = D +  . R • ------------------------------------------------------------------------------ • Computational Complexity of brute-force RDO: • INTRA block: Total Modes = 4(16x16) + 9(4x4) + 1 (I_PCM) + 4(chroma_8x8) = 18 Total # of RDO calculations = M8 * ( M4*16 + M16) Theoretical Bound for a MB: 4 x (9x16+4)=592! • INTERblock: Total Modes = [ 7+1(P_SKIP) ] + Intra counterparts HUGE Computations!! Problem for real time application => So, Need of FMD!

  11. FMD-Intra : Edge-Histogram approach • Main Idea: Use Prediction in Edge Direction Generate edge map using Sobel operator Build edge direction histogram Fastintra mode decision

  12. Generate Edge Map • Sobel Operator (Compute Gradients):

  13. Edge Direction Histogram for Intra_4x4

  14. FMD for Intra_4x4 Contd… As per observations in Reference[5]: - The ideal 4x4 mode is either the primary mode or one of the two neighboring modes - DC mode (Mode 2) is always evaluated - Total Modes = 1(Prime) + 2 (neighbors) + DC = 4

  15. Edge Direction Histogram for Intra_16x16 Total Modes = 1(Prime) + DC = 2

  16. Fast Mode Decision-Inter • Main idea: If we can reasonably decide that MB is temporally stationary orspatially homogeneous, we can encode MB using larger block-size and safely skip all other modes!

  17. Stationary Region Determination • Refers to the stillness between consecutive frames in the temporal dimension • Evaluate Zero-MV Diff : • If (Diff < Threshold Ts) => “Stationary” So, choose16x16 mode and skip other sizes ! • Threshold Ts = 200 (Reference[6])

  18. Homogeneous Region Determination • Refers to texture similarities inside a single video frame • Edge amplitude computation is already done in fast intra mode decision • Threshold values (Reference[6]): for 16x16 block : 20000 for 8x8 block : 5000

  19. Flow Chart of FMD_Inter

  20. Wait...Changing the mode:Theory to Practice! Implementation & Demo

  21. H.264/AVC Profiles • H264/AVC Profiles 

  22. Q. What is X264 ? • ‘x.264’ : • Open source H264/AVC encoder by VideoLAN • ‘C’ code library, Platform : Linux • Optimized as compared to reference JSVM software • Bunch of encoding options! • We finalized the options for “benchmarking” performance of Non-FMD vs FMD case E.g.: Command to encode ‘foreman_qcif.yuv’ sequence… ./x264 -o foreman_qcif.264 foreman_qcif.yuv 176x144 --profile baseline --frame 30 --verbose --keyint 15 --min-keyint 15 --no-scenecut --bframes 0 --ref 1 --slices 1 --fps 15 --qp 25 --partitions all --weightp 0 --me esa --subme 7 --no-chroma-me --no-8x8dct --trellis 0 --no-fast-pskip --visualize

  23. X264 Coding Options: • --keyint 15/--min-keyint 15: Sets GOP size to 15 • --bframes 0: Disables B-frame • --slices 1: Sets 1 slices per frame • --ref 1: Only 1 frame can be used as reference • --me esa: Select exhaustive motion estimation • --no-chroma-me: Ignore chroma in motion estimation • --qp 25: Fixed quantization step-size • --partitions all: Do all possible partitions • --no-scenecut: Disables adaptive I-frame decision

  24. Implementation I:‘Generation of Mode Statistics’ • Intra MB:3 Types :: I_4x4=0 (11 Modes), I_16x16=2 (4 Modes), I_PCM=3, • Inter MB: 3 Types :: P_L0=4, P_8x8=5, P_SKIP=6 • P_LO (Level-1): can have 3 Partitions: D_16x8=14, D_8x16=15, D_16x16=16 • P_8x8 (Level-2): has D_8x8 partition and can have 4 Sub-partitions: D_L0_8x8=3, D_L0_4x4=0, D_L0_8x4=1, D_L0_4x8=2

  25. Implementation II: ‘Visualization Utility’ I-Frame RED : Intra_4x4 CYAN: Intra_16x16 P-Frame GREEN: P_SKIP BLUE:P_8X8 (and below) MAGENTA: P_16x16,P_16x8, P_8x16 Motive:“Seeing is Believing  !” Let’s see a Demo…

  26. Key observations: • I- Frame: • 16x16 size chosen for spatially homogeneous region • 4x4 size chosen for a MB with manyspatial details/local edges ------------------------------------------------------------------------------------- • P-Frame:

  27. Contd… Though H.264 allows variable size MC up-to 4x4 size… • Real world video sequences: Certain percentage of ‘Skipped’ blocks • Spatially Homogeneous regions gets best compensated with 16x16 (such blocks have similar motion; very seldom split to smaller blocks) • Temporally Stationary blocks ( e.g. stationary background even with strong edges) gets best compensated with 16x16 or P_SKIP • Nonetheless, Blocks containing motion boundaries or motion in smaller objects benefit from 8x8 or 4x4 MC

  28. Implementation III: FMD Intra in x264 • ~1000 lines of C code: Edge Map computation, Prime mode computation based on histogram, Modification of mode decision logic in .x264 • Number of candidate modes in Intra-FMD:

  29. Results: Intra FMD (All I frames, Qp=25) Avg. Time Saving: 36.70% Avg. PSNR drop: 0.11 dB

  30. Results:Intra FMD (PSNR vs R) Sequence: Mobile, Coding: All I, Qp= 37,33,29,25 Avg PSNR drop: 0.044 dB, Avg. Increase in R: ~6%, Avg Time Saving: 37.51%

  31. Summary and future work: To Conclude: • Learnt x264 code-flow, different encoding options • Matlab ‘modevisualization script’ is ready • Intra-FMD ready, Inter-FMD (in progress) • Important:FMD framework is ready! Different FMD algorithms can be plugged in to evaluate prime mode selection… Future Work: • Inter FMD • FMD enhancement: Analysis of different modes with conditional probabilistic model

  32. Reference • [1] URL: http://www.videolan.org/developers/x264.html • [2] Thomas Wiegand, Gary J Sullivan, “Overview of the H264/AVC Video Coding Standard”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 13, No. 7,July 2003 • [3]URL: http://www.vcodex.com/files/H.264_overview.pdfWhite Paper: An Overview of H.264 Advanced Video Coding • [4] Iain E G Richardson, “H.264 and MPEG4 Video Compression”, WILEY Publications, 2003 • [5] Feng Pan et al, “Fast Mode Decision Algorithm for Intra-prediction in H264/AVC Video Coding”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 15, No. 7,July 2005 • [6] D. Wu et al, “Fast Intermode Decision in H264/AVC Video Coding”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 15, No. 6,July 2005 • [7] Rui Su, Guizhong Liu, TongyuZhang,”Fast Mode Decision Algorithm for Intra Prediction In H264/AVC”, ICASSP-2006

  33. Thank you!

  34. Questions? Questions? Questions?

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