1. Introduction to H.264/AVC Video Coding
Thomas Wiegand, Gary J. Sullivan, Gisle Bjøntegaard, and Ajay Luthra,
“Overview of the H.264/AVC Video Coding Standard,”
IEEE Transactions on Circuits and Systems for Video Technology, Vol. 13, No. 7, JULY 2003
Jörn Ostermann, Jan Bormans, Peter List, Detlev Marpe, Matthias Narroschke, Fernando Pereira, Thomas Stockhammer, and Thomas Wedi,
“Video coding with H.264/AVC: Tools, Performance, and Complexity,”
Circuits and Systems Magazine, IEEE , Vol. 4 , Issue: 1 , First Quarter 2004

2. Outline Goals of the H.264/AVC Structure of H.264/AVC video encoder
Design feature highlights
prediction methods
Transform details and VLC
Robustness on transmission
Video coding layer
Hypothetical reference decoder
Profiles and Levels
Network adaptation layer
Comparisons
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3. Goals of the H.264/AVC
Video Coding Experts Group (VCEG), ITU-T SG16 Q.6
H.26L project (early 1998)
Target – double the coding efficiency in comparison to any other existing video coding standards for a broad variety applications.
H.261, H.262 (MPEG-2),
H.263 (H.263+, H.263++)
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4. Structure of H.264/AVC video encoder
H.264/AVC Conceptual Layers
Video Coding Layer
Encoder
Video Coding Layer
Decoder
VCL-NAL Interface
Network Abstraction
Layer Encoder
Network Abstraction
Layer Decoder
NAL Encoder Interface
NAL Decoder Interface
Transport Layer
H.264 to
File Format
TCP/IP
H.264 to
H.320
H.264 to
MPEG-2
H.264 to
H.324/M …… …… ……
Wired Networks
Wireless Networks
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5. Design feature highlights (1) — improved on prediction methods
Variable block-size motion compensation with small block sizes
A minimum luma motion compensation block size as small as 4×4.
Quarter-sample-accurate motion compensation
First found in an advanced profile of the MPEG-4 Visual (part 2) standard, but further reduces the complexity of the interpolation processing compared to the prior design.
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6. Design feature highlights (2) — improved on prediction methods
Motion vectors over picture boundaries
First found as an optional feature in H.263 is included in H.264/AVC.
Multiple reference picture motion compensation
Decoupling of referencing order from display order
(X)IBBPBBPBBP… => IPBBPBBPBB…
Bounded by a total memory capacity imposed to ensure decoding ability.
Enables removing the extra delay previously associated with bi-predictive coding.
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7. Design feature highlights (3) — improved on prediction methods
Decoupling of picture representation methods from picture referencing capability
Ｂ－frame　could not be used as references for prediction
Referencing to closest pictures
Weighted prediction
A new innovation in H.264/AVC allows the motion-compensated prediction signal to be weighted and offset by amounts specified by the encoder.
For scene fading, etc
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8. Design feature highlights (4) — improved on prediction methods
Improved “skipped” and “direct” motion inference
Inferring motion in “skipped” areas => for global motion
Enhanced motion inference method for “direct”
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9. Design feature highlights (5) — improved on prediction methods
Directional spatial prediction for intra coding
Allowing prediction from neighboring areas that were not coded using intra coding
Something not enabled when using the transform-domain prediction method found in H.263+ and MPEG-4 Visual
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10. Design feature highlights (6) — improved on prediction methods
In-the-loop deblocking filtering
Building further on a concept from an optional feature of H.263+
The deblocking filter in the H.264/AVC design is brought within the motion-compensated prediction loop
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11. Design feature highlights (7) — other parts
Small block-size transform
The new H.264/AVC design is based primarily on a 4×4 transform.
Allowing the encoder to represent signals in a more locally-adaptive fashion, which reduces artifacts known colloquially as “ringing”.
Quantization: DPCM for DC terms
Spurious frequencies: truncation mismatch periods
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12. Design feature highlights (8) — other parts
Hierarchical block transform
Using a hierarchical transform to extend the effective block size use for low-frequency chroma information to an 8×8 array
Allowing the encoder to select a special coding type for intra coding, enabling extension of the length of the luma transform for low-frequency information to a 16×16 block size
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13. Design feature highlights (9) — other parts
Short word-length transform
While previous designs have generally required 32-bit processing, the H.264/AVC design requires only 16-bit arithmetic.
Exact-match inverse transform
Building on a path laid out as an optional feature in the H effort, H.264/AVC is the first standard to achieve exact equality of decoded video content from all decoders.
Integer transform
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14. Design feature highlights (10) — other parts
Arithmetic entropy coding
While arithmetic coding was previously found as an optional feature of H.263, a more effective use of this technique is found in H.264/AVC to create a very powerful entropy coding method known as CABAC (context-adaptive binary arithmetic coding)
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15. Design feature highlights (11) — other parts
Context-adaptive entropy coding
CAVLC (context-adaptive variable-length coding)
CABAC (context-adaptive binary arithmetic coding)
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16. Design feature highlights (12) — Robustness to data errors/losses and flexibility for operation over a variety of network environments
Parameter set structure
The parameter set design provides for robust and efficient conveyance header information
NAL unit syntax structure
Each syntax structure in H.264/AVC is placed into a logical data packet called a NAL unit
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17. Design feature highlights (13) — Robustness to data errors/losses and flexibility for operation over a variety of network environments
Flexible slice size
Unlike the rigid slice structure found in MPEG-2 (which reduces coding efficiency by increasing the quantity of header data and decreasing the effectiveness of prediction),
slice sizes in H.264/AVC are highly flexible, as was the case earlier in MPEG-1.
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18. Design feature highlights (14) — Robustness to data errors/losses and flexibility for operation over a variety of network environments
Flexible macroblock ordering (FMO)
Significantly enhance robustness to data losses by managing the spatial relationship between the regions that are coded in each slice
Arbitrary slice ordering (ASO)
sending and receiving the slices of the picture in any order relative to each other
first found in an optional part of H.263+
can improve end-to-end delay in real-time applications, particularly when used on networks having out-of-order delivery behavior
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19. Design feature highlights (15) — Robustness to data errors/losses and flexibility for operation over a variety of network environments
Redundant pictures
Enhance robustness to data loss
A new ability to allow an encoder to send redundant representations of regions of pictures
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20. Design feature highlights (15) — Robustness to data errors/losses and flexibility for operation over a variety of network environments
Data Partitioning
Allows the syntax of each slice to be separated into up to three different partitions for transmission, depending on a categorization of syntax elements
This part of the design builds further on a path taken in MPEG-4 Visual and in an optional part of H
The design is simplified by having a single syntax with partitioning of that same syntax controlled by a specified categorization of syntax elements.
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21. Design feature highlights (16) — Robustness to data errors/losses and flexibility for operation over a variety of network environments
SP/SI synchronization/switching pictures
A new feature consisting of picture types that allow exact synchronization of the decoding process of some decoders with an ongoing video stream produced by other decoders without penalizing all decoders with the loss of efficiency resulting from sending an I picture
Enable switching a decoder between different data rates, recovery from data losses or errors, as well as enabling trick modes such as fast-forward, fast-reverse, etc.
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22. Coded Video Sequences
A coded video sequence consists of a series of access units that are sequential in the NAL unit stream and use only one sequence parameter set.
Can be decoded independently
Start with an instantaneous decoding refresh (IDR) access unit – must be Intra.
A NAL unit stream may contain one or more coded video sequences.
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23. VCL (Video Coding Layer)
input
video
DCT Q
VLC -
output
bitstream
16×16
macroblocks IQ
Intra-
Prediction
IDCT
Intra / inter
Motion
Compensation
De-blocking
Filter
Motion
Estimation
Frame
Memory
output video
Clipping
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Decoder
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YCbCr Color Space and 4:2:0 Sampling

24. Pictures, Frames, and Fields
Progressive
Frame
Top
Field
Bottom
Field ∆t
Interlaced Frame (Top Field First)
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25. Slices and Slice Groups (1)
Subdivision of a picture into slices when not using FMO.
(Flexible Macroblock Ordering)
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26. Slices and Slice Groups (2)
Subdivision of a QCIF frame into slices utilizing FMO.
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27. Slice coding types I Slice P Slice B Slice SP Slice SI Slice
Switching between P slices
efficient switching between different pre-coded pictures becomes possible.
SI Slice
Switching between I slices
Allowing an exact match of a macroblock in an SP slice for random access and error recovery purposes.
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28. Adaptive Frame/Field Coding Operation
Three modes can be chosen adaptively for each frame in a sequence.
Frame mode
Field mode
Frame mode / Field coded
For a frames consists of mixed moving regions
The frame/field encoding decision can be made for each vertical pair of macroblocks (a 16×32 luma region) in a frame.
to code the nonmoving regions in frame mode and the moving regions in the field mode.
Macroblock-adaptive frame/field (MBAFF)
Picture-adaptive frame/field (PAFF)
16% ~ 20% save over frame-only
for ITU-R 601 “Canoa”, “Rugby”, etc.
MBAFF
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29. Macroblock-adaptive frame/field (MBAFF)
A Pair of Macroblocks
in Frame Mode
Top/Bottom Macroblocks
in Field Mode
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30. PAFF vs. MBAFF
The main idea of MBAFF is to preserve as much spatial consistency as possible.
In MBAFF, one field cannot use the macroblocks in the other field of the same frame as a reference for motion prediction.
PAFF coding can be more efficient than MBAFF coding in the case of rapid global motion, scene change, or intra picture refresh.
MBAFF was reported to reduce bit rates 14 ~ 16% over PAFF for ITU-R 601 (Mobile and Calendar, MPEG-4 World News)
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31. Intra-Frame Prediction (1)
Well suited for coding of parts of a picture with significant detail.
Intra_16×16 together with chroma prediction
More suited for coding very smooth areas of a picture.
4 prediction modes
I_PCM
Bypass prediction and transform coding and, send the values of the encoded samples directly
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32. Intra-Frame Prediction(2)B
A +B
Intra_16  16
Vertical prediction
Horizontal prediction
DC-prediction
Plane-prediction
Works very well in areas of a gently changing luminance.
Chrominance signals
8  8 blocks
Very smooth in most cases.
Use the same modes as in Intra_16  16.
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33. Intra-Frame Prediction (3)
In H.263+ and MPEG-4 Visual
Intra prediction is conduced in the transform domain
In H.264/AVC
Intra prediction is always conducted in the spatial domain
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34. Intra-Frame Prediction (3)
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35. Intra-Frame Prediction (4)
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Across slice boundaries is not allowed.

36. Inter-Frame Prediction in P slices (1) Segmentations of the macroblock
MB Types 16 8 8 16 8 8 8 8 16 16 8 8
8x8 Types 8 4 8 4 4 4 4 8 8 4
*P_Skip
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H.264 / MPEG-4 Part 10 : Inter Prediction

37. Inter-Frame Prediction in P slices (2)
Inter-Frame Prediction in P slices (2) The accuracy of motion compensation A aa B
b1=(E-5F+20G+20H-5I+J)
h1=(A-5C+20G+20M-5R+T)
b=(b1+16) >> 5
h=(h1+16) >> 5
j1=cc-5dd+20h1+20m1-5ee+ff
j = (j1+512) >>10
a=(G+b+1) >>1
e=(b+h+1) >> 1 C bb D
clipped to
0~255 E F G a b c H I J d e f g cc dd h i j k m ee ff
clipped to
0~255 n p q r K L M s N O P R gg S T hh U
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38. 4 Prior Decoded Pictures
Inter-Frame Prediction in P slices (3) Multiframe motion-compensated prediction
∆=1
∆=4
∆=2
4 Prior Decoded Pictures
As Reference
Current
Picture
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39. Inter-Frame Prediction in B slices
Other pictures can refer pictures containing B slices
Weighted averaging of two distinct motion-compensated prediction
Utilizing two distinct lists of reference pictures (list0, list1)
4 prediction types
list0, list1, bi-predictive, direct prediction, B_Skip
For each partition, the prediction type can be chosen separately.
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40. Transform, Scaling, and Quantization(1)
4  4 and 2  2 DCT
Integer transform matrix -1 16 17
INTRA_16 16 H2 H3 H3 H1 1 4 5 18 19 22 23 2 3 6 7 20 21 24 25
DCT Cb Cr 8 9 12 13 Cb Cr 10 11 14 15 Y
Transmission order:
-1,0,1, …, 24,25
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41. Transform, Scaling, and Quantization(2) Repeated Transforms
Intra_16×16, chroma intra modes are intend coding for smooth areas
The DC coefficients undergo a second transform with the results that we have transform coefficients covering the whole macroblock 00 01 1
indices correspond to the indices of
2×2 inverse Hadamard transform 10 2 3 11
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Repeat transform for chroma blocks

42. Transform, Scaling, and Quantization(3)
Quantized by scalar quantizer; the quantization step size is chosen by a so-called quantization parameter (QP) that has 52 values.
An increment of QP by 1 results in an increase of the required data rate of approximately 12%. (The step size doubles with each increment of 6 of QP.)
A change of step size by approximately 12% also means roughly a reduction of bit rate by approximately 12%
QSTEP = 2(QP-4)/6
21/6  1.12
R  (1/1.12)R if
QSTEP  1.12QSTEP
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43. Transform, Scaling, and Quantization(4)
Scanning order
Zig-zag scan
For 2×2 DC coefficients of the chroma component
raster-scan order
All inverse transform operations in H.264/AVC can be implemented using only additions and bit-shifting operations of 16-bit integer values. No drift problem between encoders and decoders.
Only 16-bit memory accesses are needed for a good implementation of the forward transform and quantization process in the encoder
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44. Entropy Coding Two methods of entropy coding are supported
An exp-Golomb code - a single infinite-extent codeword table for all syntax elements.
For quantized transform coefficients
Context-Adaptive Variable Length Coding (CAVLC)
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45. CAVLC (1)
# of nonzero quantized coefficients (N) and the actual size, and position of the coefficients are coded separately
7, 6, -2, 0, -1, 0, 0, 1, 0, 0, 0, 0, 0, 0 ,0 ,0.
# of nonzero coefficients (N) and “Trailing T1s
T1s = 2, N = 5,
These two values are coded as a combined event. One out of 4 VLC tables is used based on the number of coefficients in neighboring blocks.
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46. CAVLC (2) For T1s, only sign need to be coded.
7, 6, -2, 0, -1, 0, 0, 1, 0, 0, 0, 0, 0, 0 ,0 ,0.
2) Encoding the value of Coefficients
For T1s, only sign need to be coded.
Coefficient values are coded in reverse order:
-2, 6, …
A starting VLC is used for -2, and a new VLC may be used based on the just coded coefficient. In this way adaptation is obtained in the use of VLC tables, Six exp-Golomb code tables are available for this adaptation.
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47. CAVLC (3) For T1s, this is sent as single bit.
7, 6, -2, 0, -1, 0, 0, 1, 0, 0, 0, 0, 0, 0 ,0 ,0.
3) Sign Information
For T1s, this is sent as single bit.
For the other coefficients, the sign bit is included in
the exp-Golomb codes
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48. CAVLC (4)
7, 6, -2, 0, -1, 0, 0, 1, 0, 0, 0, 0, 0, 0 ,0 ,0.
4) TotalZeroes
The number of zeros between the last nonzero coefficient
of the scan and its start.
TotalZeroes = 3
N=5, => the number must in the range 0-11, 15 tables are available for N
in the range (If N=16 there is no zero coefficient.)
5) RunBefore
In this example it must be specified how the 3 zeros are
distributed.
The number of 0s before the last coefficient is coded.
2, => range:0-3 => a suitable VLC is used.
1, => range:0-1
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49. CAVLC vs CABAC
The efficiency of entropy coding can be improved further if the Context-Adaptive Binary Arithmetic Coding (CABAC) is used.
Compared to CAVLC, CABAC typically provides a reduction in bit rate between 5%~15%.
The highest gains are typically obtained when coding interlaced TV signals.
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50. CABAC
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51. In-Loop Deblocking filter
Apply deblocking filter on p0 and q0 if each of conditions satisfied
|p0-q0|<α(QP)
|p1-p0|<β(QP)
|q1-q0|<β(QP) q0 q2 q1 p0 p2 p1
β < α
. p1 and q1: if
|p2-p0|<β(QP) or
|q2q0|< β(QP)
4×4 block edge
*The filter reduces the bit rate by 5%~10% typically.
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52. Hypothetical Reference Decoder
In H.264/AVC HRD specifies operation of two buffers:
The coded picture buffer (CPB)
Modeling the arrival and removal time of the coded bits.
The decoded picture buffer (DPB)
Similar in spirit to what MPEG-2 had, but is more flexible in support at a variety of bit rates without excessive delay.
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53. Hypothetical Reference Decoder
H.264 hypothetical reference decoder (HRD): guarantee that the buffers never overflow or underflow
rate allocation: allocate proper bits to each coding unit according to the buffer status
quantization parameter adjustment: how to adjust the encoder parameters to properly encode each unit with the allocated bits
Find the relation between the rate and the quantization parameter
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54. The Relation between QSTEP and QP
In H.264/AVC, the relation between QSTEP and QP is
QSTEP = 2 (QP-4)/6.
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55. The Relation between PSNR and the quantization parameter QP
The Relation between PSNR and the quantization parameter QP is
where l and b are the constants.
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56. Profiles and Levels Baseline, Main, and Extended
Baseline supports all features in H.264/AVC except
Set 1: B slices, weighted prediction, CABAC, field coding, and picture or macroblock adaptive switching between frame and field coding.
Set 2: SP/SI slices, and slice data partitioning.
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57. H.264/AVC Profiles
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58. Structure of H.264/AVC video encoder
H.264/AVC Conceptual Layers
Video Coding Layer
Encoder
Video Coding Layer
Decoder
VCL-NAL Interface
Network Abstraction
Layer Encoder
Network Abstraction
Layer Decoder
NAL Encoder Interface
NAL Decoder Interface
Transport Layer
H.264 to
File Format
TCP/IP
H.264 to
H.320
H.264 to
MPEG-2
H.264 to
H.324/M …… …… ……
Wired Networks
Wireless Networks
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59. NAL (Network Abstraction Layer)
Designed in order to provide “network friendliness”
facilitates 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.
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60. Key concepts of NAL NAL Units
Byte stream and Packet format uses of NAL units
Parameter sets
Access units
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61. NAL units 1 byte header payload Integer number of bytes
Interleaved as necessary with emulation prevention bytes, which are bytes inserted with a specific value to prevent a particular pattern of data called a start code prefix from being accidentally generated inside the payload.
The NAL unit structure definition specifies a generic format for use in both packet-oriented and bitstream-oriented transport systems, and a series of NAL units generated by an encoder is referred to as a NAL unit stream.
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62. NAL units in byte-stream format use
H.320 and MPEG-2/H systems
require delivery of the entire or partial NAL unit stream as an ordered stream of bytes or bits.
Each NAL unit is prefixed by a specific pattern of three bytes called a start code prefix.
payload
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63. NAL units in packet-transport system use
Internet protocol/RTP systems
The inclusion of start code prefixes in the data would be a waste of data carrying capacity, so instead the NAL units can be carried in data packets without start code prefixes.
payload
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64. VCL and no-VCL NAL units
The data that represents the values of the samples in the video pictures
Non-VCL NAL
Any associated additional information such as parameter sets (important header data that can apply to a large number of VCL NAL units) and supplemental enhancement information (timing information and other supplemental data that may enhance usability of the decoded video signal but are not necessary for decoding the values of the samples in the video pictures).
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65. Parameter Sets (1)
A parameter set is supposed to contain information that is expected to rarely change and offers the decoding of a large number of VCL NAL units.
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66. Parameter Sets (2) Two types of parameter sets:
Sequence parameter sets
Apply to a series of consecutive coded video pictures called a coded video sequence;
Picture parameter sets
Apply to the decoding of one or more individual pictures within a coded video sequence.
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67. Parameter Sets (3) The Structure
VCL NAL unit
Identifier to Picture parameter set
Picture parameter set
Identifier to Sequence parameter set
Sequence parameter set
Non VCL NAL unit
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68. Parameter Sets (4) Transmission
VCL NAL unit
Non VCL NAL unit
In-band
VCL NAL unit
Out of band
Non VCL NAL unit
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69. Parameter set use with reliable “out-of-band” parameter set exchange
NAL unit with VCL Data encoded
with PS#3 (address in Slice Header )
H.264/AVC Encoder
H.264/AVC Decoder
Reliable Parameter Set Exchange 1 2 3 3 2 1
Parameter Set #3
Video format PAL
Entr. Code CABAC …
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70. redundant coded picture
Access Units
A set of NAL units in a specified form is referred to as an access unit.
start
redundant coded picture
access unit delimiter
Supplemental
Enhancement
Information
end of sequence
SEI
end of stream
VCL NAL units
slices
or slice data partitions
primary coded picture
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end

71. Comparisons (1)
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72. Comparisons (2)
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73. Comparisons (3)
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74. Comparisons (4)
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75. New Features in H.264 Multi-mode, multi-reference MC
Motion vector can point out of image border
1/4-, 1/8-pixel motion vector precision
B-frame prediction weighting
44 integer transform
Multi-mode intra-prediction
In-loop de-blocking filter
UVLC (Uniform Variable Length Coding)
NAL (Network Abstraction Layer)
SP-slices
MV pointing out of image: pan, zoom in
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76. MPEG-4: H.263 + Additions + Variable Shape Coding
Goal: Support for interactive multimedia
Visual Object (AO), Audio Object (AO) and AVO
18 video coding profiles
Roughly follows H.263 design and adds all prior features and (most important) shape coding
Includes zero-tree wavelet coding of still textured pictures, segmented coding of shapes, coding of synthetic content
2D & 3D mesh coding, face animation modeling
10-bit and 12-bit video
Contains 9 parts. Part 10 is H.264/AVC
Simple visual profile supports very low bit rate coding (under 64 kbps), suitable for applications on mobile network. •
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77. SP-Slices Efficiently switching between two bitstreams
Provides VCR-like functions
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78. B-frame Prediction Weighting
I B B B P B B6
Time
A block is predicted by weighting several blocks from multiple references.
Playback order: I0 B1 B2 B3 P4 B5 B6 ……...
Bitstream order: I0 P4 B1 B3 B2 P B5 ……...
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