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In-Depth Information
significant bins. Note that for k
D
0, the truncated Rice is equal to the truncated
unary binarization.
k-th order Exp-Golomb code is proved to be a robust, near-optimal prefix-free
code for geometrically distributed sources with unknown or varying distribution
parameter. Each codeword consists of a unary prefix of length l
N
C
1 and a suffix
of length l
N
C
k,wherel
N
Db
log
2
..N >> k/
C
1/
c
[
46
].
Fixed-length code uses a fixed-length bin string with length
d
log
2
.cMax
C
1/
e
and with most significant bins signaled before least significant bins.
The binarization process is selected based on the type of syntax element. In some
cases, binarization also depends on the value of a previously processed syntax ele-
ment (e.g., binarization of
coeff_abs_level_remaining
depends on the pre-
viously decoded coefficient levels) or slice parameters that indicate if certain modes
are enabled (e.g., binarization of partition mode, so-called
part_mode
, depends
on whether asymmetric motion partition is enabled). The majority of the syntax
elements use the binarization processes as listed above, or some combination of
them (e.g.,
cu_qp_delta_abs
uses TrU (prefix) + EG0 (suffix) [
98
]). However,
certain syntax elements (e.g.,
part_mode
and
intra_chroma_pred_mode
)
use custom binarization processes.
During the HEVC standardization process, special attention has been put on the
development of an adequately designed binarization scheme for absolute values of
transform coefficient levels. In order to guarantee a sufficiently high throughput,
the goal here was the maximization of bypass-coded bins under the constraint
of not sacrificing coding efficiency too much. This was accomplished by making
the binarization scheme adaptive based on previously coded transform coefficient
levels. More details on that are given in Sect.
8.6.5
.
8.2.2
Context Modeling, Probability Estimation
and Assignment
By decomposing each non-binary syntax element value into a sequence of bins,
further processing of each bin value in CABAC depends on the associated coding-
mode decision, which can be either chosen as the regular or the bypass mode (as
described in Sect.
8.2.3
). The latter is chosen for bins, which are assumed to be uni-
formly distributed and for which, consequently, the whole regular binary arithmetic
encoding (and decoding) process is simply bypassed. In the regular coding mode,
each bin value is encoded by using the regular binary arithmetic coding engine,
where the associated probability model is either determined by a fixed choice, based
on the type of syntax element and the bin position or bin index (binIdx) in the
binarized representation of the syntax element, or adaptively chosen from two or
more probability models depending on the related side information (e.g., spatial
neighbors as illustrated in Fig.
8.1
, component, depth or size of CU/PU/TU, or
position within TU). Selection of the probability model is referred to as context
