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Overview of the H.264/AVC Video Coding Standard T. Wiegand, G. Sullivan, G. Bjontegaard, and A. Luthra Overview of the H.264/AVC Video Coding Standard,

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Presentation on theme: "Overview of the H.264/AVC Video Coding Standard T. Wiegand, G. Sullivan, G. Bjontegaard, and A. Luthra Overview of the H.264/AVC Video Coding Standard,"— Presentation transcript:

1 Overview of the H.264/AVC Video Coding Standard T. Wiegand, G. Sullivan, G. Bjontegaard, and A. Luthra Overview of the H.264/AVC Video Coding Standard, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 13, No. 7, pp , July CMPT 820: Multimedia Systems

2 Outline Overview Network Abstraction Layer (NAL) Video Coding Layer (VCL) Profiles and Applications Feature Highlights Conclusions

3 Evolution of Video Compression Standards H.261 Video Telephony H.262/MPEG-2 Digital TV/DVD MPEG-4 Visual Object-based Coding H.263 Video Conferencing H.264 MPEG-4 AVC MPEG-1 Video-CD ITU-T MPEG

4 H.264/AVC Coding Standard Various Applications  Broadcast: cable, satellites, terrestrial, and DSL  Storage: DVDs (HD DVD and Blu-ray)  Video Conferencing: over different networks  Multimedia Streaming: live and on-demand  Multimedia Messaging Services (MMS) Challenge:  How to handle all these applications and networks  Flexibility and customizability

5 Structure of H.264/AVC Codec Layered design Network Abstraction Layer (NAL)  formats video and meta data for variety of networks Video Coding Layer (VCL)  represents video in an efficient way Scope of H.264 standard

6 Outline Overview Network Abstraction Layer (NAL) Video Coding Layer (VCL) Profiles and Applications Feature Highlights Conclusions

7 Network Abstraction Layer Provide network friendliness for sending video data over various network transports, such as:  RTP/IP for Internet applications  MPEG-2 streams for broadcast services  ISO File formats for storage applications We present a few NAL concepts

8 NAL Units Packets consist of video data  short packet header: one byte Support two types of transports  stream-oriented: no free unit boundaries  use a 3-byte start code prefix  packet-oriented: start code prefix is a waste Can be classified into:  VCL units: data for video pictures  Non-VCL units: meta data and additional info

9 Non-VCL NAL Units Two types of non-VCL NAL units  Parameter sets: headers shared by a large number of VCL NAL units a VCL NAL unit has a pointer to its picture parameter set a picture parameter set points to its sequence parameter set  Supplemental enhancement info (SEI): optional info for higher-quality reconstruction and/or better application usability Sent over in-band or out-of-band channels

10 Access Units A set of NAL units Decoding an access unit results in one picture Structure:  Delimiter: for seeking in a stream  SEI: timing and other info  primary coded picture: VCL  redundant coded picture: for error recovery

11 Video Sequences and IDR Frames Sequence: an independently decodable NAL unit stream  don’t need NALs from other sequences  with one sequence parameter set  starts with an instantaneous decoding refresh (IDR) access unit IDR frames: random access points  Intra-coded frames  no subsequent picture of an IDR frame will require reference to pictures prior to the IDR frame  decoders mark buffered reference pictures unusable once seeing an IDR frame

12 Outline Overview Network Abstraction Layer (NAL) Video Coding Layer (VCL) Profiles and Applications Feature Highlights Conclusions

13 Video Coding Layer (VCL) (Like other) hybrid video coding: H.264/AVC represents pictures in macroblocks  Motion compensation: temporal redundancy  Transform: spatial redundancy Small improvements add up to huge gain  Combining many coding tools together Pictures/Frames/Fields  Fields: top/bottom field contains even/odd rows  Interlaced: two fields were captured at diff time  Progressive: otherwise

14 Macroblocks and Slices Fixed size MBs: 16x16 for luma, and 8x8 for chroma Slice: a set of MBs that can be decoded without use of other slices  I slice: intra-prediction (I-MB)  P slice: possibly one inter-prediction signal (I- and P-MBs)  B slice: up to two inter-prediction signals (I- and B- MBs)  SP slice: efficient switch among streams  SI slice: used in conjunction with SP slices

15 Flexible Macroblock Ordering (FMO) MBs in a slice: in raster-order Slice group: more flexible Each slice group contains one or several slices Possible usages:  Region-of-interest (ROI)  Checker-board for video conferencing

16 Adaptive Field Coding Two fields of a frame can be coded as:  A single frame (frame mode)  Two separate fields (field mode)  A single frame with adaptive mode (mixed mode) Picture-adaptive frame/field (PAFF)  frame/field decision is made at frame level  16% - 20% bit rate reduction over frame only Macroblock-adaptive frame/field (MBAFF)  frame/field decision is made at MB level  14% - 16% bit rate reduction over PAFF [Ref] Chan Tsang - Project Presentation - Fast Macroblock Adaptive Coding in H264.ppthttp://scien.stanford.edu/2005projects/ee398/projects/presentations/Guerrero Chan Tsang - Project Presentation - Fast Macroblock Adaptive Coding in H264.ppt  suitable for Interlaced and high motion

17 Intra-frame Prediction In spatial domain, using samples to the left and/or on above to predict samples in a MB Types of intra-frame prediction:  Intra_4x4: detailed luma block  Intra_16x16: smooth luma blocks  Chroma_8x8: similar to Intra_16x16 as chroma components are smooth  I_PCM: bypass prediction/transform, send samples anomalous pictures, loseless, and predictable bit rate

18 Intra_4x4 Prediction Samples in 4x4 block are predicted using 13 neighboring sample 8 prediction mode: 1 DC and 8 directional Sample D is used if E-H is not available

19 Sample Intra_4x4 Prediction Interpolation is used in some modes [Ref] Foreman sequence,

20 Intra_16x16 Prediction 4 modes  Vertical  Horizontal  DC  Planer (Diagonal)

21 Inter-Prediction in P Slices Two-level segmentation of MBs  Luma MBs are divided into at most 4 partitions (as small as 8x8) 8x8 partitions are divided into at most 4 partitions  Chroma – half size horizontally and vertically Maximum of 16 motion vectors for each MB

22 Examples of Segmentation [Ref]

23 Inter-Prediction Accuracy ¼-pixel for luma, 1/8-pixel for Chroma Half-pixel samples: 6-tap FIR filter Quarter-pixel samples: average of neighbors Chroma predictions: bilinear interpolation CD AB E KLMNOP FGHIJ TU RS ccddeeff aa bb gg hh ba c efg ijk pqr d h n m s

24 Multiframe Inter-Prediction in P Slices More than one prior reference pictures (by diff. MBs) Encoders/decoders buffer the same reference pictures for inter-prediction  Reference index is used when coding MVs  MVs for regions smaller than 8x8 uses the same index for all MVs in the 8x8 region P_skip mode:  Don’t send residual signals nor MVs nor reference index  Use buffered frame 0 as the reference picture  Use neighbor’s MVs  Large areas with no change or constant motion like slow panning can be represented with very few bits.

25 Multiframe Inter-Prediction in B Slices Weighted average of 2 predictions B-slices can be used as reference Two reference picture lists are used One out of four pred. methods for each partition:  list 0  list 1  bi-predictive  direct prediction: inferred from prior MBs The MB can be coded in B_skip mode (similar to P_skip)

26 4x4 Integer Transform Why smaller transform:  Only use add and shift, an exact inverse transform is possible  no decoding mismatch  Not too much residue to code  Less noise around edge (ringing or mosquito noise)  Less computations and shorter data type (16-bit) An approximation to 4x4 DCT:, where

27 2 nd Transform and Quantization Parameter 2 nd Transform: Intra_16x16 and Chroma modes are for smooth area  DC coefficients are transformed again to cover the whole MB Quantization step is adjusted by an exponential function of quantization parameter  to cover a wider range of QS  QP increases by 6 => QS doubles  QP increases by 1 => QS increases by 12% => bit rate decreases by 12%

28 Entropy Coding Non-transform coefficients: an infinite-extent codeword table Transform coefficients:  Context-Adaptive Variable Length Coding (CAVLC) several VLC tables are switched dep. on prior transmitted data  better than a single VLC table  Context-Adaptive Binary Arithmetic Coding (CABAC) flexible symbol probability than CAVLC  5 – 15% rate reduction efficient: multiplication free

29 In-loop Deblocking Filter Operate within coding loop Use filtered frames as ref. frames  improve coding efficiency Adaptive deblocking, need to determine  Blocking effects or object edges  Strong or weak deblocking Intuitions  Large difference near a block edge -> likely a block artifact  If the difference is too large to be explained by the QP difference -> likely a real edge E.g., Filter p0 and q0 if

30 Hypothetical Reference Decoder (HRD) Standard receiver buffer models  encoders must produce bit streams that are decodable to HRD Two buffers  Coded picture buffer (CPB) models the bit arrival and removal time  Decoded picture buffer (DPB) models the frame decoded and output time in reference frame lists

31 Outline Overview Network Abstraction Layer (NAL) Video Coding Layer (VCL) Profiles and Applications Feature Highlights Conclusions

32 Profiles and Applications Defines a set of coding tools and algorithms Conformance points for interoperability 3 Profiles for different applications  Baseline – video conferencing  Main – broadcast, media storage, digital cinema  Extended – streaming over IP (wire/wireless) 15 Levels  pic size  decoding rate (MB/s)  bit rate  buffer size [Ref] J. Ostermann, J. Bormans, P. List, D. Marpe, M. Narroschke, F. Pereira, T. Stockhammer, and T. Wedi Video coding with H.264/AVC: tools, performance and complexity IEEE Circuits and Systems Magazine 4(1) pp May 2004

33 Outline Overview Network Abstraction Layer (NAL) Video Coding Layer (VCL) Profiles and Applications Feature Highlights Conclusions

34 Feature Highlights -- Prediction Variable blocksize MCs Quarter-sample accurate MCs MVs over pic. Boundaries Multiple reference pictures Weighted Bi-directional prediction Decoupling of referencing from display orders Decoupling prediction mode from reference capability (uses B frames as reference) Improved Skip/Direct modes Intra prediction in Spatial domain In-loop deblocking filter

35 Feature Highlights -- Transform Small block-size transform 2-level block transform (repeated DC transform) Short data-type transform (16-bit) Exact inverse transform Context-adaptive entropy coding Arithmetic entropy coding

36 Feature Highlights -- Network Parameter set structure (efficient) NAL unit syntax structure (flexibility) Flexible slice size Flexible macroblock ordering (FMO) Arbitrary slice ordering (ASO) Redundant pictures Data partitioning (unequal error protection) SP/SI switching pictures

37 Outline Overview Network Abstraction Layer (NAL) Video Coding Layer (VCL) Profiles and Applications Feature Highlights Conclusions

38 Key improvements  Enhanced prediction (intra- and inter-)  Small block size exact match transform  Adaptive in-loop deblocking filter  Enhanced entropy coding method [Ref] G Sullivan and T. Wiegand, Video Compression—From Concepts to the H.264/AVC Standard,Video Compression—From Concepts to the H.264/AVC Standard Proc. of IEEE, 93(1), Jan 2005


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