Transcript Ch.6 H.264/AVC Part2 (pp.200~222)

Ch.6 H.264/AVC
Part2 (pp.200~222)
Chun-Wei Hsieh
Mikemage@CMLab, 2004 NTU
CSIE
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Outline
• 6.4.13.2 CAVLC
• Main profile
– B slices, weighted prediction, CABAC
• Extended profile
– SP & SI slices
• Transport of H.264
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CAVLC
• Context-Based Adaptive Variable
Length Coding
• Characteristics:
– Run-level coding to compact zero string
– Trailing ones (+1, -1 after 0)
– Number of nonzero coefficient in
neighboring blocks is correlated
– Choice VLC lookup table for level
parameter for level magnitude
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CAVLC Encoding process
• 1. Encode the number of coefficients
and trailing ones (coeff token)
– TotalCoeffs : 0 ~ 16
– TrailingOnes : 0 ~ 3
• if more than tree TrailingOnes, only last
three are treated as ‘special cases’
– Four look up table
• Three variable-length, one fixed-length
• Choice depend on neighboring blocks
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CAVLC Encoding
process(2)
• 2. Encode the sign of each
TrailingOne
– In reverse order
• 3. Encode the levels of the remaining
nonzero coefficients
– level_prefix, level_suffix
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CAVLC Encoding
process(3)
• 4.Encode the total number of zeros
before the last coefficient
– Zero-runs at start of the array need
not to be encoded
• 5. Encode each run of zeros
• If less then 3 TrailingOnes, the first
nonzero coefficient is adjusted
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Main profile
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Main profile(2)
• Suitable application:
– broadcast media
– applications such as digital television
and stored digital video
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B slices
• May be predicted from one or two
reference pictures, before or after the
current in temporal order.
• List 0
– The closest past picture, followed by any other
past pictures, followed by any future pictures
• List 1
– The closest future picture, followed by any
other future picture followed by any past
picture
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B slices(2)
• Prediction options
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B slices:
Bi-direction prediction
• Each sample of the prediction block
is calculated as an average of the list
0 and list 1 prediction sample
• pred(i,j) = (pred0(i,j) + pred1(i,j) + 1)
>> 1
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B slices:
Direction prediction(2)
• No motion vector is transmitted
• Spatial direct mode
– Using the process described in 6.4.5.4
• Temporal direct mode
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Weighted prediction
• 1. P slice macroblock, ‘explicit’ weighted
prediction;
• 2. B slice macroblock, ‘explicit’ weighted
prediction;
• 3. B slice macroblock, ‘implicit’ weighted
prediction.
• Explicit: weighting factor are transmitted
• Implicit: weighting factor are calculated
based on reference picture
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Interlaced video
• In macroblock-adaptive frame/field
(MB-AFF) coding mode, the choice of
field or frame coding may be
specified at the macroblock level
– 16 luminance samples wide and 32
luminance samples high
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Interlaced video(2)
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Interlaced video(3)
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CABAC
• Context-based Adaptive Binary
Arithmetic Coding
– Selecting probability models for each
syntax element according to the
element’s context
– Adapting probability estimates based on
local statistics
– Using arithmetic coding rather than
variable-length coding
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CABAC(2)
• Binarisation
• Context model selection
– Context model is a probability model for
one or more bins of binarised symbol
• Arithmetic encoding
• Probability update
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Extended profile
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SP & SI slices
• SP and SI slices are specially-coded slices that
enable efficient switching between video streams
and efficient random access for video decoders
• SP-slices are designed to support switching
between similar coded sequences without the
increased bitrate penalty of I-slices
• SI-slice may be used to switch from one sequence
to a completely different sequence
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SP slices
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SI slice
• Prediction is formed using the 4 × 4
Intra Prediction modes (see Section
6.4.6.1) from previously-decoded
samples of the reconstructed frame
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Data partitioned slices
• The coded data that makes up a slice is placed in
three separate Data Partitions (A, B and C)
• Partition A contains the slice header and header
data for each macroblock in the slice
• Partition B contains coded residual data for Intra
and SI slice macroblocks
• Partition C contains coded residual data for inter
coded macroblocks (forward and bi-directional)
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Transport of H.264
• A coded H.264 video sequence
consists of a series of NAL units,
each containing an RBSP
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RBSP type
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Parameter set
• sequence parameter set contains
parameters to be applied to a
complete video sequence
• picture parameter set contains
parameters which are applied to one
or more decoded pictures within a
sequence
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• Thank You!!
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