Graphics Reference
In-Depth Information
H.264/AVC standardization process by using a design of a family of multiplication-
free binary arithmetic coders, which later became known as the
modulo coder
(M coder) [
43
,
45
]. The main innovative features of this design are given by
a table-based interval subdivision coupled with the above-mentioned FSM-based
probability estimation as well as a fast bypass coding mode. The former, which is
also the basis of what is called the
regular coding mode
of the M coder, will be
briefly reviewed next, followed by a short discussion of the latter aspect.
8.2.3.1
Regular Coding Mode
The basic idea of the M-coder approach of interval subdivision is to quantize the
range of possible interval widths induced by renormalization into a small number of
K cells. To further simplify matters, a uniform quantization with K
D
2
is assumed
to be performed, resulting in a set
W
Df
W
0
;W
1
;
;W
K1
g
of representative
interval widths. Together with the representative set of LPS-related probability
values of the FSM given by
P
Df
p
0
;p
1
;
;p
N 1
g
, this quantization enables
the approximation of the multiplication on the right-hand side of Eq. (
8.3
) by means
of a table of K
N pre-calculated product values
f
W
k
p
n
j
0
k<K
I
0
n<N
g
in a suitable chosen integer precision. The entries of the corresponding
2-D lookup table
TabRangeLPS
are addressed by the (probability) state index n and
the quantization cell index k.R/ related to the given value of the interval range R.
Computation of k.R/ is easily carried out by a concatenation of a bit shift and a bit-
masking operation, where the latter can be interpreted as a
modulo operation
using
the operand K
D
2
, hence the naming of the family of coders.
In the context of H.264/AVC, the optimal empirical choice of the free parameters
D
2 and N
D
64 was determined under the constraint of a maximum table size
of 2
N
256 bytes for the lookup table
TabRangeLPS
with each of its entries
being represented with 8 bits. This specific M-coder design of using a lookup table
TabRangeLPS
with 4
64 entries was also adopted for HEVC. Please note that by
choosing a value of
D
0, the 2-D table
TabRangeLPS
degenerates to a 1-D table,
where for all possible values of R only one single representative interval width value
W is used for each of the N product values p
n
R,where0
n<N. This choice
is equivalent to the subinterval division operation performed in the Q coder and its
derivatives of QM and MQ coder, as has been standardized in JBIG, JPEG, and
JPEG2000. Thus, the M-coder design can be interpreted as a generalization of the
Q-coder family.
2
Compared to the QM/MQ coder, the M coder, being configured as
in H.264/AVC and HEVC, achieves an increase in throughput of 18 %, while at the
same time it provides bit-rate savings of 2-4 %, when evaluated in the CABAC
environment of H.264/AVC [
43
]. Interestingly, the throughput improvements of
2
Please note that apart from the interval subdivision aspect there are some subtle technical
differences between (and also within) the coder families, such as concerning, e.g., probability
estimation, conditional exchange, carry-over handling, and termination.
