Graphics Reference
In-Depth Information
while the low nibble of InitValue represents the variable offsetIdx, from which the
slope m and offset n of the linear model are derived using [
29
]
m
D
slopeIdx
5
45
n
D
.offsetIdx << 3/
16:
Given the values of m and n, exactly the same initialization procedure as in
H.264/AVC is performed for derivation of the parameters of each probability model
[
46
,
63
]. Note that the 8-bit design allows to cut in half the amount of storage needed
for context initialization tables. Further restriction to two instead of three table
entries for P and B slice types reduces the memory requirements for those tables
in HEVC by at least another 12.5-15 % relative to those of an 8-bit equivalent of
H.264/AVC. Since there are 134 contexts for I slices and 154 for each of both slice
types P and B, a total amount of 442 bytes of memory is needed for storage of all
context initialization tables in HEVC.
8.7.2
Context Training
The main purpose of the context initialization tables is to bridge the learning
phase starting from a uniform distribution, i.e., the case of no prior knowledge of
the statistics of the given bin distributions, towards the well-adapted phase of the
probability estimator. Assuming that after processing of a number of N
bins, the
probability estimator that starts from p
D
0:5 reaches such a well-adapted state, the
bins for each probability model were tracked for N
bins for each test sequence of
a training set at a particular QP and for a particular slice type. As a result, a model
probability p
;QP
was estimated from the relative frequency obtained after coding
the first N
bins for each probability model. This training procedure was performed
separately for each QP and each of the three slice types. To finally determine the
pair of parameters .m; n/ that describe the assumed linear relationship between QP
and model probability p
;QP
, a simple linear regression was applied for each slice
type. Note that a choice of N
D
50 was assumed to be appropriate.
8.7.3
Context Memory for Wavefront Parallel Processing
and Dependent Slices
For improving the parallelization and low-delay capabilities beyond the use of
regular slices, as known from H.264/AVC, a partitioning of pictures into tiles,
wavefronts and dependent slices have been introduced in HEVC. Since the use of
regular slices implies in particular that the corresponding CABAC bitstream must
be independently parsable, re-initialization of the CABAC probability models is
required at the beginning of each regular slice. Although the initialization procedure,
