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SIG
SIG
SIG ALG1 ALG1 ALG2
s
s
s
s
s
ALRem
ALRem
ALRem
regular coded
bypass coded
Fig. 8.23
Grouping
same
regular
coded
bins
and
bypass
bins
to
increase
throughput.
s
D coeff_sign_flag
provided that the values of the corresponding syntax elements are inferred to be
equal to 0, when not explicitly signaled. Note that the flags SIG, ALG1, and ALG2
represent the first and the optional second and third bin indices of the TrU part of
z , respectively. ALRem corresponds to the concatenation of the TRk and EGk part
of z with all of its bin values being bypass coded and with a maximum bin string
length of 32 [ 12 ]. Only the values of the three flags are regular coded. However,
due to the adaptation rules for B 0 , ALG2 can occur only once in each subblock,
while the occurrence of ALG1 is restricted to 8 scan positions per subblock at the
maximum [ 16 ]. Together with the maximum of 16 SIG flags per subblock, only up
to 25 regular coded bins can occur in each subblock (without accounting for CSBF).
Thus, the maximum number of regular coded bins per 4 4 transform (sub-) block
is reduced by a factor of about 9.6 relative to the corresponding maximum number
of 16 14 C 15 D 239 regular coded bins for H.264/AVC CABAC (including SIG
bins but without accounting for LAST) [ 46 ]. This change provides obviously the
most substantial reduction to the (worst case) number of regular coded bins in the
entire revision of CABAC.
The rationale behind processing SIG, ALG1, ALG2, and ALRem with individual
syntax elements rather than as conventional bin indices of the adaptive binarization
of z is given by the fact that all values of one syntax element in each subblock
are grouped together and signaled in separate scan passes. This grouping provides
essentially three advantages. First, bins in the coefficient level binarization that
use the same context selection logic are grouped together to reduce the amount
of speculative context selection computations, as shown in Fig. 8.23 . Second, by
grouping bypass coded bins together, the throughput advantages of bypass bins
are maximized [ 87 ]. Third, the storage for (partially reconstructed) coefficient data
during the parsing process at the decoder can be reduced, as further explained
in Sect. 8.6.5.2 below. Note that the reordering of bins has no impact on coding
efficiency.
Context modeling for coding of the regular coded bins of the absolute level is
restricted to the three flags SIG, ALG1, and ALG2. Since context model selection
for the SIG flag has already been introduced in Sects. 8.6.4.1 and 8.6.4.3 , we will
focus in the following on the two flags ALG1 and ALG2. For each of both flags, six
sets of context models are provided: four sets for subblocks of the luma component
and two sets for subblocks of the chroma component. Since only up to one ALG2
flag per subblock is encoded/decoded, each of the six ALG2 related sets contains
only one context model. For the ALG1 flag, each set consists of four context models
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