1. Università degli studi Roma Tre
Overview of the H.264AVC video coding standard
Maiorana Emanuele

2. Agenda Introduction Project overview & timeline
Standardization concepts
Codec technical design
New Fewtures
Prediction
De-Blocking
Entropy Coding
Profiles & levels
Comparisons

3. Introduction
H.264/AVC is newest video coding standard developed by the ITU-T/ISO/IEC Joint Video Team (JVT), consisting of experts from:
ITU-T Video Coding Experts Group (VCEG)
ISO/IEC Moving Picture Experts Group (MPEG)
Its design represents a delicate balance between:
Coding Gain (improved efficiency by a factor of two over MPEG-2)
Implementation complexity
Costs based on state of VLSI (ASICs and Microprocessors) design technology

4. Terminology The following terms are used interchangeably: H.26L
The Work of the JVT or “JVT CODEC”
JM2.x, JM3.x, JM4.x
The Thing Beyond H.26L
The “AVC” or Advanced Video CODEC
Proper Terminology going forward:
MPEG-4 Part 10 (Official MPEG Term)
ISO/IEC AVC
H.264 (Official ITU Term)

5. History
The digital representation of the TeleVision signals created many services for the content delivery:
Satellite
Cable TV
Terrestrial Broadcasting
ADSL and Fiber on IP
To optimize this services, there is the need of:
High Quality of Service (QoS)
Low Bit-Rate
Low Power Consumption
The source coding is responsible for the reduction of the bit-rate.
Example: the complete transmission of the TV signal, as in Recommendation ITU-R BT.601, would require:
720 × (360 × 576) × 25 × 8= 166 Mbit/s
Conflicting

6. Video Coding History
Efforts in maximizing coding efficiency while dealing with:
diversification of network types
characteristic formatting and loss/error robustness requirements.
ITU Standard for VideoTelephony
H.261, H.263, H.263+
ISO-MPEG Standard
MPEG-1:medium quality, physical support
MPEG-2:medium/high quality, physical and transmission support
MPEG-4:audio video objects

7. Evolution (1/2)
MPEG-2 Introduction
MPEG-4 in Comparison

8. Evolution (2/2)
H.26L Provides Focus
MPEG-4 “Adopts” H.26L

9. Codec Defects
Blocking Low Original High

10. Codec Defects
Packet Loss

11. Video Coding History
Early 1998: Started as ITU-T Q.6/SG16 (VCEG - Video Coding Experts Group) “H.26L” standardization activity
August 1999: first draft design
July 2001: MPEG open call for “AVC” technology: H.26L wins
December 2001: Formation of the Joint Video Team (JVT) between VCEG and MPEG to finalize H.26L as a joint project similar to MPEG-2/H.262
July 2002: Final Committee Draft status in MPEG
March 2003: formal approval submission
October 2004: final ITU-T and ISO approvation

12. Versions

13. JVT Project Technical Objectives
Primary technical objectives:
Significant improvement in coding efficiency: Average bit rate reduction of 50% compared to any other video standard
Network-friendly video representation for “conversational” (video telephony) and “non-conversational” (storage, broadcast or streaming) applications
Error resilient coding
Simple syntax specification and targeting simple and clean solutions
The scope of the standardization is only the central decoder, by:
imposing restrictions on the bitstream and syntax
defining the decoding process such that every conforming decoder will produce similar output with an encoded bitstream input
Pre-Processing
Encoding
Post-Processing
& Error Recovery
Decoding
Source
Destination
Scope of the Standard

14. Applications
The new standard is designed for technical solutions including at least the following application areas
Broadcast over cable, satellite, Cable Modem, DSL, terrestrial, etc.
Interactive or serial storage on optical and magnetic devices, DVD, etc.
Conversational services over ISDN, Ethernet, LAN, DSL, wireless and mobile networks, modems, etc. or mixtures of these.
Video-on-demand or multimedia streaming services over ISDN, Cable Modem, DSL, LAN, wireless networks, etc.
Multimedia Messaging Services (MMS) over ISDN, DSL, Ethernet, LAN, wireless and mobile networks, etc.
How to handle this variety of applications and networks?

15. Network Abstraction Layer
H.264 Design
To address this need for flexibility and customizability, the H.264/AVC design covers a:
Video Coding Layer (VCL): representation of the video content (performing all the classic signal processing tasks)
Network Abstraction Layer (NAL): adaptation of VCL representations in a manner appropriate for conveyance by a variety of transport layers or storage media
Control Data
Video Coding Layer
Coded Macroblock
Data Partitioning
Coded Slice/Partition
Network Abstraction Layer
H.320
MP4FF
H323/IP
MPEG-2
etc.

16. Features enhancing coding efficiency (1)
Enhancements on picture encoding is enabled through value prediction methods
Variable block-size motion compensation with small block sizes
Quarter-sample-accurate motion compensation
Multiple reference picture motion compensation
Weighted prediction
Directional spatial prediction for intra coding
In-the-loop deblocking filtering

17. Features enhancing coding efficiency (2)
Enhancements on picture encoding is enabled through high performance tools
Small block-size transform
Hierarchical block transform
Short word-length transform
Exact-match inverse transform
Arithmetic entropy coding
Context-adaptive entropy coding

18. Features enhancing Robustness
Enhancements on Robustness to data errors/losses and flexibility for operation over a variety of network environments is enabled by new design aspects new
Parameter set structure
NAL unit syntax structure
Flexible slice size
Flexible macroblock ordering (FMO)
Redundant pictures
SP/SI synchronization/switching pictures

19. Network Abstraction Layer
The Network Abstraction Layer (NAL) is designed in order to provide "network friendliness“, facilitating the ability to map H.264/AVC VCL data to transport layers such as
RTP/IP for any kind of real-time wire-line and wireless Internet services (conversational and streaming)
File formats, e.g. ISO MP4 for storage and MMS
H.32X for wireline and wireless conversational services
MPEG-2 systems for broadcasting services, etc.
Some key concepts of the Network Abstraction Layer are:
NAL Units
Use of NAL Units in:
Byte stream format systems
Packet-Transport systems
Parameter Sets
Access Units

20. NAL Units
The coded video data is organized into NAL units, each of which is effectively a packet that contains an integer number of bytes
NAL units are classified into
VCL NAL units: contain the data associated to the video pictures
non-VCL NAL units: contain any associated additional information
Header byte: first byte of each NAL unit; contains an indication of the type of data in the NAL unit, and the remaining bytes contain payload data of the type indicated by the header
Emulation Prevention Bytes: bytes inserted in the payload data to prevent the accidentally generation of a particular pattern of data called a start code prefix
NAL unit stream: series of NAL units generated by an encoder

21. Use of NAL Units
Bitstream-oriented transport systems (H.320, MPEG-2 systems)
Delivery of the entire or partial NAL unit stream as an ordered stream of bytes or bits  the locations of NAL unit boundaries need to be identifiable
In the byte stream format, each NAL unit is prefixed by a specific pattern of three bytes called a start code prefix
Packet-oriented transport systems (IP, RTP systems)
The coded data is carried in packets that are framed by the system transport protocol  the boundaries of NAL units within the packets can be established without use of start code prefix patterns
The NAL units can be carried in data packets without start code prefixes.

22. Parameters Set
A Parameter Set contains information that is expected to rarely change.
Types of parameter sets:
sequence parameter sets: relative to a series of consecutive coded video pictures (coded video sequence)
picture parameter sets: relative to one or more individual pictures.
Parameter sets can be sent
One time (ahead the VCL NAL Units)
Many time (to provide robustness)
In-band (same VCL NAL Unit Channel)
Out-of-Band (different Channel)
Out-of-band Transmission

23. Access Units
A set of NAL units in a specified form is referred to as an Access Unit.
The decoding of each access unit results in one decoded picture.
It can be composed by:
access unit delimiter: to aid in locating the start
of the access unit.
supplemental enhancement information (SEI):
containing data such as picture timing information
primary coded picture: set of VCL NAL units that
represent the samples of the video picture.
redundant coded pictures: for use by a decoder
in recovering from loss or corruption
end of sequence: if the coded picture is the last
picture of a coded video sequence
end of stream: if the coded picture is the last
coded picture in the entire NAL unit stream

24. Video Coding Layer
The VCL design follows the so-called block-based hybrid video coding approach
There is no single coding element in the VCL that provides the majority of the significant improvement in compression efficiency in relation to prior video coding standards.
Entropy
Coding
Scaling & Inv. Transform
Motion-
Compensation
Control Data
Quant. Transf. coeffs
Motion Data
Intra/Inter
Decoder
Motion
Estimation
Transform/ Scal./Quant. -
Input Video Signal
Split into
Macroblocks
16x16 pixels
Intra-frame
Prediction
De-blocking
Filter
Output Video
Signal
Coder
Control

25. Video Coding Layer The picture is split into blocks.
Intra coded of the first picture or a
random access point
Inter coding for all remaining pictures
or between random access points
Transmission of the motion data
as side information
Transform of the residual
of the prediction (Intra or Inter)
Quantization of the transform coefficients
Entropy coding and transmission of the
quantized transform coefficients, together with the side information

26. Pictures, frames, and fields
A coded pictures can represent either an entire frame or a single field
A frame of video can be considered to contain two interleaved fields
interlaced frame: the two fields of a frame were captured at different time instants
progressive frame
The coding representation in H.264/AVC is primarily agnostic with respect to this video characteristic

27. Adaptive frame/field coding operation
In interlaced frames with regions of moving objects, two adjacent rows tend to show a reduced degree of statistical dependency
H.264/AVC design allows any of the following decisions for coding a frame:
Frame mode: combine the two fields together
Field mode: not combine the two fields together
The choice can be made adaptively for each frame and is referred to as Picture Adaptive Frame/Field (PAFF) coding
Field mode:
Motion compensation utilizes reference fields
De-blocking filter is not used for horizontal edges of macroblocks
Moving region  field mode
Non-moving region  frame mode

28. Sampling YCbCr color space
H.264/AVC uses a sampling structure called 4:2:0 sampling with 8 bits of precision per sample
The chroma component has one fourth of the number of samples than the luma component (in both the horizontal and vertical dimensions)
Y is called luma, and represents brightness.
Cb and Cr are called chroma, and represent
the deviation from gray toward blue and red

29. Macroblocks and Slices
Fixed-size macroblocks partition with 16x16 samples of the luma component and 8x8 samples of each of the two chroma components.
Slices are a sequence of macroblocks which are processed in the order of a raster scan when not using Flexible Macroblock Ordering (FMO).
A picture is a collection of one or more slices in H.264/AVC.
Indipendency  Each slice can be correctly decoded without use of data from other slices.

30. Flexible Macroblock Ordering
FMO uses the concept of slice groups
A set of macroblocks defined by a macroblock to slice group map, specified in the picture parameter set
A slice is a sequence of macroblocks within the same slice group
Useful for concealment in video conferencing applications

31. Slice Types
I slice: a slice in which all macroblocks of the slice are coded using intra prediction  is coded exploiting only the spatial correlation
P slice: In addition to the coding types of the I slice, some macroblocks of the P slice can also be coded using inter prediction with backward references (I or P slices)
B slice: In addition to the coding types available in a P, some macroblocks of the B slice can also be coded using inter prediction with forward references (I, P or B slices)
The following two coding types for slices are new:
SP slice: a slice that is coded such that efficient switching between different pre-coded pictures becomes possible
SI slice: a slice that allows an exact match of a macroblock in an SP slice for random access and error recovery purposes

32. Motivation for SP and SI slices
The best-effort nature of today’s networks causes variations of the effective bandwidth available to a user
For Video Streaming, the server should adjusting, on the fly, source encoding parameters
Representation of each sequence using multiple and independent streams
Prior video encoding standards
Switching is possible only at I-frames.
H.264
Identical SP-frames can be obtained
even when they are predicted using
different reference frames.

33. Intra-frame Prediction
In all slice-coding types, the following types of intra coding are supported
Intra_4x4 with chroma prediction: areas of a picture with significant detail
Intra_16x16 with chroma prediction : very smooth areas of a picture
I_PCM: values of anomalous picture content (accurately representation)
Intra prediction in H.264/AVC is always conducted in the spatial domain
IDR: picture composed of slice I only
can be decoded without any reference
no subsequent picture in the stream will require reference to pictures prior to IDR
Chroma samples: similar prediction technique as for the luma component in Intra_16x16 macroblocks

34. Intra_4x4 mode M A G H I J K L a b c d e f g h i j k l m n o p B C D E
Labelling of prediction samples(4x4) M A G H I J K L B C D E F
0 (vertical) M A G H I J K L B C D E F
1 (horizontal) M A G H I J K L B C D E F
2 (DC)
(A+B+C+D+
I+J+K+L)/8 M A G H I J K L B C D E F
3 (diagonal down-left)

35. Intra_4x4 mode M A G H I J K L a b c d e f g h i j k l m n o p B C D E
Labelling of prediction samples(4x4) M A G H I J K L B C D E F
4 (diagonal down-right) M A G H I J K L B C D E F
5 (vertical-right) M A G H I J K L B C D E F
6 (horizontal-down) M A G H I J K L B C D E F
7 (vertical-left)

36. Intra_4x4 mode
When samples E-H are not available, they are replaced by D M A G H I J K L a b c d e f g h i j k l m n o p B C D E F
Labelling of prediction samples( 4x4) M A G H I J K L B C D E F
8 (horizontal-up)

37. Intra_16x16 and I_PCM mode Intra_16x16 mode 0: vertical
Mode 1: horizontal
mode 2: DC
Mode 3: plane (a linear “plane” function is fitted to the upper and left-hand samples  in areas of smoothly-varying luminance)
I_PCM  sends directly the values of the encoded samples, to to precisely represent them

38. Inter-frame Prediction in P Slices
Partitions with luma block sizes of 16x16, 16x8, 8x16, and 8x8 samples are supported by the syntax.
In case partitions with 8x8 samples are chosen, one additional syntax element for each 8x8 partition is transmitted.
The prediction signal is specified by
a translational motion vector
a picture reference index  a maximum of sixteen motion vectors may be transmitted for a single P macroblock.

39. Algorithm
The encoder selects the “best” partition size for each part of the frame, to minimize the coded residual and motion vectors.
The macroblock partitions chosen for each area are shown superimposed on the residual frame.
little change between the frames (residual appears grey)  a 16x16 partition is chosen
detailed motion (residual appears black or white)  smaller partitions are more efficient.
Residual (no motion compensation)

40. Effects (1/2) Frame Fn Frame Fn-1 Residual (no motion compensation)
Residual (16x16 bock size)

41. Effects (2/2) Residual (8x8 bock size) Residual (4x4 bock size)
Residual (4x4 bock size; half pixel)
Residual (4x4 bock size; quarter pixel)

42. Example (1/2) Frame Fn Reconstructed reference Frame F’n-1
Residual Fn – F’n-1 (no motion compensation)
16x16 Motion Vector Field

43. Example (2/2) Motion compensation reference frame
Motion compensation residual frame

44. Motion Estimation Accuracy
The accuracy of motion compensation is in units of one quarter of the distance between luma samples.
Integer-sample position  the prediction signal consists of the corresponding samples of the reference picture
Non integer-sample position  the corresponding sample is obtained using interpolation to generate non-integer positions.
The prediction values at half-sample positions are obtained by applying a one-dimensional 6-tap FIR Wiener filter horizontally and vertically

45. Motion Estimation Accuracy
Half sample positions (aa, bb, b, s, gg, hh and cc, dd, h, m, ee, ff) are derived by first calculating intermediate values
Ex.: b1 = ( E – 5 F + 20 G + 20 H – 5 I + J )
h1 = ( A – 5 C + 20 G + 20 M – 5 R + T )
b = (b1 + 16) >>5
h = (h1 + 16) >> 5
Position j
j1 = cc1 – 5 dd h m1 – 5 ee1 + ff1
j = ( j ) >> 10
Quarter sample positions (a, c, d, n, f, i, k, q) are derived by averaging with upward rounding of the two nearest samples at integer and half sample positions
Ex.: a = ( G + b + 1 ) >> 1
Quarter sample positions (e, g, p, r) are derived by averaging with upward rounding of the two nearest samples at half sample positions in the diagonal direction as, for example, by
Ex.: e = ( b + h + 1 ) >> 1

46. Motion Estimation Accuracy
The prediction values for the chroma component are always obtained by bi-linear interpolation.
For chroma the resolution is halved (4:2:0) therefore the motion compensation precision is down to one-eighth pixel  ¼ pixels accuracy
a = round{[(8-dx)·(8-dx)·A]+dx·(8-dy)·B+(8-dx)·dy · C+dx · dy · D ]/64}
Ex.: a = round[(30A+10B+18C+6D)/64]

47. Multi-picture Prediction
Multi-picture motion compensation using previously-encoded pictures as references allows up to 32 reference pictures to be used in some cases
Very significant bit rate reduction for scenes with
rapid repetitive flashing
back-and-forth scene cuts
uncovered background areas

48. Inter-frame Prediction in B Slices
The concept of B slices is generalized in H.264/AVC
Other pictures can reference pictures containing B slices for motion-compensated prediction
Some macroblocks or blocks may use a weighted average of two distinct motion-compensated prediction values for building the prediction signal
In B slices, four different types of inter-picture prediction are supported:
list 0 (backward)
list 1 (forward)
bi-predictive: weighted average of motion-compensated list 0 and list 1 prediction signals
direct prediction: inferred from previously transmitted syntax elements
It is also possible to have both motion predictions from past, or both motion predictions from future.

49. Transform: types
Each residual macroblock is transformed, quantized and coded
H.264 uses a smaller size transform
H.264 uses three transforms depending on the type of residual data that has to be coded
a 4x4 transform for the luma DC coefficients in Intra_16x16 macroblocks
a 2x2 transform for the chroma DC coefficients
a 4x4 transform for all other blocks
Adaptive block size transform mode
Further transforms are (eventually) chosen depending on the motion compensation block size (4x8, 8x4, 8x8, 16x8, etc)

50. Transform: Order For a 16x16 Intra mode coded Macroblock
“-1” Block DC coefficient of each 4x4 luma block
“0-15” Blocks Luma residual blocks
“16-17” Blocks DC coefficients from the Cb and Cr components
“18-25” Bloks Chroma residual blocks
Coding of smooth areas

51. 4x4 residual transform
This transform operates on 4x4 blocks of residual data (labelled 0-15 and 18-25)
Fundamental differences from DCT transform:
integer transform
fully specified inverse transform  mismatch between encoders and decoders should not occur
requires only additions and shifts
scaling multiplication integrated into the quantizer
transform and quantization can be carried out using 16-bit integer arithmetic
Reasons
better prediction method  less spatial correlation in the residual
visual benefits: less noise around edges
less computations and a smaller processing wordlength

52. Transform: Order The 4x4 DCT of an input array X is given by: d=c/b
CXCT  “core” 2-D transform
E  matrix of scaling factors

53. Transform: Order
To simplify the implementation of the transform, while keeping it orthogonal, the parameters must be modified
The inverse transform is given by:

54. Quantization Requirements
avoid division and/or floating point arithmetic
incorporate the post- and pre-scaling matrices Ef and Ei described above
A total of 52 values of Qstep are supported by the standard and these are indexed by a Quantization Parameter, QP.
Qstep doubles in size for every increment of 6 in QP
Qstep increases by 12.5% for each increment of 1 in QP
QP may be different for luma and chroma (an offset can be signalled in a Picture Parameter Set)

55. DC Components Transform
Luma DC coefficient (Intra_4x4 block)
4x4 Hadamard transform:
Chroma DC coefficient (any block)
2x2 Hadamard transform:

56. Scanning
The quantized transform coefficients of a block generally are scanned in a zig-zag fashion and transmitted using entropy coding methods
The 2x2 DC coefficients of the chroma component are scanned in raster-scan order and transmitted using entropy coding methods

57. De-Blocking Filter
Benefits of the application to every decoded macroblock:
reduced blocking distortion block  improved image appearance
better motion-compensated prediction of further frames
Filtering is applied to vertical or horizontal edges of 4x4 blocks:
Filter 4 vertical boundaries of the luma component (a,b,c,d)
Filter 4 horizontal boundaries of the luma component (e,f,g,h)
Filter 2 vertical boundaries of each chroma component (i,j)
Filter 2 horizontal boundaries of each chroma component (k,l)

58. Filtering Choice The choice of filtering outcome depends on
boundary strength
gradient across the boundary BS
Rule 4
p or q is intra coded and boundary is a macroblock boundary 3
p or q is intra coded and boundary is not a macroblock boundary 2
neither p or q is intra coded; p or q contain coded coefficients 1
neither p or q is intra coded; neither p or q contain coded coefficients;
p and q have different reference frames or a different number of reference frames or different motion vector values
p and q have same reference frame and identical motion vectors  no filtering
The filter is “stronger” where there is likely to be significant blocking distortion (high values of BS)

59. Filter decision
A group of samples is filtered only if BS > 0 and the following conditions is
satisfied:
|p0-q0| < (QP)
|p1-p0| < (QP)
|q1-q0| < (QP) with (QP)< (QP)
Small QP
Anything other than a very small gradient across the boundary is likely to be due to image features  low (QP) and (QP) value
Large QP
blocking distortion is likely to be more significant  high (QP) and (QP) value, so that more filtering takes place.

60. Filter implementation
BS=1,2,3
A 4-tap linear filter is applied with inputs p1, p0, q0 and q1, producing filtered outputs P0 and Q0
For luma only
If |p2-p0| < (QP), a 4-tap linear filter is applied with inputs p2, p1, p0 and q0, producing filtered output P1
If |q2-q0| < (QP), a 4-tap linear filter is applied with inputs q2, q1, q0 and p0, producing filtered output Q1
II. BS=4
If |p2-p0|< (QP) and |p0-q0|< (QP) /4
P0 is produced by 5-tap filtering of p2, p1, p0, q0 and q1
P1 is produced by 4-tap filtering of p2, p1, p0 and q0
Luma  P2 is produced by 5-tap filtering of p3, p2, p1, p0 and q0
else:
P0 is produced by 3-tap filtering of p1, p0 and q1
(the same for qi pixels)

61. De-blocking: Example Reconstructed Frame; QP=32; No Filter
Reconstructed Frame; QP=32; With Filter
Original Frame
Reconstructed Frame; QP=36; No Filter
Reconstructed Frame; QP=36; With Filter

62. Entropy Coding Parameters that require to be encoded and transmitted
Macroblock Type (Prediction method) QP
Motion data: Reference frame and Motion Vector
Coded block pattern (blocks containing coded coefficients)
Residual Data
H.264/MPEG-4 AVC uses a number of techniques for entropy coding:
Exp-Golomb codes
All syntax elements except the quantized transform coefficients.
Context Adaptive Variable Length Coding (CAVLC)
Quantized transform coefficients
Context Adaptive Binary Arithmetic Coding (CABAC).

63. Exp-Golomb Codes
Exp-Golomb codes (Exponential Golomb codes) are variable length codes with a regular construction.
uses a single infinite-extent codeword table
only the mapping to the single codeword table is customized according to the data statistics
Each codeword is constructed as follows:
[M zeros][1][INFO] codeword length  (2M+1) bits
Encoding
M = round[log2(code_num+1)]
INFO = code_num+1-2M
Decoding
Code_num = 2M +INFO-1

64. Exp-Golomb Codes: Value mapping
Each element v is assigned a type reflecting how data is to be mapped
Unsigned Exponential  ue(v)
Maping: code_num = v
Used for macroblock type, reference frame index
Signed Exponential  se(v)
Mapping
code_num = 2|v| (v < 0)
code_num = 2|v| - 1 (v > 0)
Used for motion vector difference, QP.
Mapped Exponential  me(v)
Mapping specified in the standard
Used for the Coded block patterns.
Truncated Exponential  te(v)
Each mapping is designed to produce short codewords for frequently occurring values and longer codewords for less common parameter values.
Table for Inter predicted macroblocks: coded_block_pattern indicates which 8x8 blocks in a macroblock contain non-zero coefficients

65. CAVLC Method for coding residual, zig-zag ordered transformed blocks
VLC tables for various syntax elements are switched depending on already transmitted syntax elements
CAVLC takes advantage of several characteristics of quantized 4x4 blocks:
Sparse blocks (containing mostly zeros)
Highest non-zero coefficients after the zig-zag scan are often sequences of +/-1 (“Trailing 1s” or “T1s”)
The number of non-zero coefficients in neighbouring blocks is correlated
The level (magnitude) of non-zero coefficients tends to be higher at the start of the reordered array (near the DC coefficient) and lower towards the higher frequencies

66. CAVLC encoding
CAVLC encoding of a block of transform coefficients proceeds as follows:
Encode the number of coefficients and trailing ones in a 4x4 block
Encode the sign of each T1
Encode the levels of the remaining non-zero coefficients
Encode the total number of zeros before the last coefficient
Encode each run of zeros

67. CAVLC: Steps (1/3)
Step 1: Encode the number of coefficients (TotalCoeffs) and T1s
TotalCoeffs range from 0 to 16 (in a 4x4 block)
T1s range from 0 to 3 (max value allowed)
Coding: 4 choices for look-up table (coeff_token)
Num_VCL0  biased towards small numbers of coefficients
Num_VCL1  biased towards medium numbers of coefficients
Num_VCL2  biased towards higher numbers of coefficients
Num_FLC  fixed 6-bit length code
The choice depends on the number of non-zero coefficients in upper and left-hand previously coded blocks NU and NL (context adaptivity)
U,L available  N=(NU+NL)/2
U available  N=NU
L available  N=NL
U,L unavailable  N=0

68. CAVLC: Steps (2/3) Step 2: Encode the sign of each T1.
For each T1 up to three, a single bit encodes the sign (0=+, 1=-). Encoding is in reverse order, starting with the highest-frequency T1
Step 3: Encode the levels of the remaining non-zero coefficients.
The level (sign and magnitude) of each remaining non-zero coefficient in the block is encoded in reverse order
Encoding VLC table depends on successive coded level (context adaptivity)
Initialise the table to Level_VLC0
Encode highest-frequency coefficient
If magnitude is larger than a threshold, move up to the next VLC table

69. CAVLC: Steps (2/3)
Step 4: Encode the total number of zeros before the last coefficient
Coding with a VLC of the number of all zeros preceding the highest non-zero coefficient.
Step 5:Encode each run of zeros
The number of zeros preceding each non-zero coefficient (run_before) is encoded in reverse order.
The VLC for each run of zeros is chosen depending on
the number of zeros that have not yet been encoded (ZerosLeft)
value of run_before parameter.

70. CAVLC: example (1/2) 4x4 Block
Reordered Block: 0,3,0,1,-1,-1,0,1,0,0,0,0,0,0,0,0
TotalCoeff = 5; TotalZeros=3; T1s =3 (max value)
Encoding 1 2 3 4 5
Transmitted bitstream  (24 bits)

71. CAVLC: example (2/2) Decoding Input string: 000010001110010111101101
Values added to the output array at each stage are underlined
The decoder has inserted two zeros; however, TotalZeros is equal to 3 and so another 1 zero is inserted before the lowest coefficient, making the final output array: 0,3,0,1,-1,-1,0,1,…

72. CABAC CABAC uses arithmetic coding
effective use of probability models of occurrence of symbols
Particularly beneficial for symbol probabilities greater than 0.5
The use of adaptive codes permits adaptation to non-stationary symbol statistics
easily adapt to changing statistical characteristics of the data to be coded
Context modelling: The statistics of already-coded syntax elements are used to estimate the conditional probabilities
CABAC provides a reduction in bit-rate between 5% to 15% over CAVLC, when coding TV signals at the same quality.

73. CABAC basis Encoding with CABAC consists of three stages Binarization
needed for syntax elements that are non-binary valued
Context modeling
a model is selected such that the choice may depend on previous encoded syntax elements or bins
Adaptive binary arithmetic coding
coding of bin sequence with updating of probabilities takes place

74. Binarization Requirements for a successful context coding method
fast and accurate estimation of conditional probabilities
the computational complexity must be kept at a minimum
“Pre-processing” step to reduce the alphabet size of the syntax elements
The design of binarization schemes in CABAC (mostly) relies on a few basic code trees
Unary or Truncated Unary code
x>0  x “1” bits plus a terminating “0” bit (unary)
0<x<S  for x=S the terminating “0” bit is neglected (truncated)
Exp-Golomb code
codes are constructed by a concatenation of a prefix and a suffix code word
Fixed-length code
a finite alphabet of values of the corresponding syntax element is assumed.

75. Context Modeling
A model probability distribution is assigned to the given symbols, to drive the actual coding engine to generate a sequence of bits as a coded representation of the symbols
Define a modeling function F:T  C operating on a set T of past symbols
For each symbol x to be coded, a conditional probability p(x|F(z)) is estimated according to the already coded neighboring symbols z  T
After encoding x, the probability model is updated with the value of the encoded symbol x
Context template consisting of two neighboring syntax elements A and B to the left and on top of the current syntax element C

76. Profiles and Levels Profile
Defines a set of coding tools or algorithms that can be used in generating a conforming bit-stream
Level
Places constraints on certain key parameters of the bitstream
Decoders conforming to a profile must support all features in that profile
In H.264/AVC, three profiles are defined (plus extensions)
Baseline
Main
Extended Profile
Fidelity Range Extensions: four HighProfile versions (High, High 10, High 4:2:2, and High 4:4:4), for high quality uses
Fifteen levels are defined specifying upper limits for
picture size
decoder-processing rate
size of the multi-picture buffers
video bit rate
video buffer size

77. Profile features
Baseline profile supports all features in H.264/AVC except the following two feature sets:
Set 1
B slices
Weighted prediction
CABAC
Field coding
Picture or macroblock adaptive switching between frame and field coding
Set 2
SP/SI slices
Main profile supports Set 1, and does not support the FMO and redundant pictures features
Extended Profile supports all features except for CABAC.

78. Profile Map

79. Application Areas Conversational services
operate typically below 1 Mbps; low latency requirements
Baseline profile for following application
H.320 conversational services with circuit-switched ISDN-based video conferencing
3GPP conversational H.324/M services
H.323 conversational services over the Internet with best effort IP/RTP protocols.
3GPP conversational services using IP/RTP for transport and SIP for session set-up.
Entertainment video applications
operate between 1-8 Mbps; moderate latency (0.5 to 2 seconds)
Main profile for following application
Broadcast via satellite, cable, terrestrial, or DSL
DVD for standard and high-definition video
Video on demand via various channels.
Streaming services
operate at 50 kbps–1.5 Mbps; latency of 2 or more seconds
Baseline or Extended profile; differences based on the use for wired or wireless environments as follows:
3GPP streaming using IP/RTP for transport and RTSP Baseline profile
Streaming over the wired Internet using IP/RTP protocol and RTSP for session set-up  Extended profile

80. Differences

81. Complexity of Codec Design
Codec design includes relaxation of traditional bounds on complexity (memory & computation) – rough guess 2-3x decoding power increase relative to MPEG-2, 3-4x encoding
Problem areas:
Smaller block sizes for motion compensation (cache access issues)
Longer filters for motion compensation (more memory access)
Multi-frame motion compensation (more memory for reference frame storage)
More segmentations of macroblock to choose from (more searching in the encoder)
More methods of predicting intra data (more searching)
Arithmetic coding (adaptivity, computation on output bits)

82. Comparison (1/2) Foreman QCIF 10Hz 39 38 37 36 JVT/H.264/AVC 35 MPEG-434
Y-PSNR [dB]
Quality
MPEG-2 33
H.263 32 31 30 29 28 27 50
100
150
200
250
Bit-rate [kbit/s]

83. Comparison (2/2) Tempete CIF 30Hz 38 37 36 35 34 JVT/H.264/AVC 33
Y-PSNR [dB]
Quality
MPEG-4 32
MPEG-2 31 30
H.263 29 28 27 26 25
500
1000
1500
2000
2500
3000
3500
Bit-rate [kbit/s]

84. Test Set Results for Perceptual Quality
Informal perceptual tests
At the same PSNR, people generally prefer JVT
Small motion compensation block size (breaks up block structure)
Small transform block size (breaks up block structure, reduces ringing)
In-loop deblocking filter
By how much?
Needs further study
No rigorous testing reported
10-15% might be a good guess

85. References
IEEE Transactions on Circuits and Systems for Video Technology (2003):
Special Issue on the H.264/AVC Video Coding Standard
Signal Processing: Image Communication (2004):
Video coding using the H.264/MPEG-4 AVC compression standard