Graphics Reference
In-Depth Information
the M coder can be largely attributed to its unique bypass functionality, as being
reviewed in the next subsection, while its use of a larger lookup table for interval
subdivision generates the main effects in coding-efficiency gain; however, this
increased table size can also adversely affect the overall throughput gain of the
M coder.
8.2.3.2
Bypass Coding Mode
As already mentioned, most of the throughput improvements of the M coder relative
to the Q-coder technology can be attributed to its second innovative feature, which
is given by a bypass of the probability estimation for approximately uniform
distributed bins. In addition, the interval subdivision is substituted by a hard-
wired equipartition in this so-called bypass coding mode. In this way, the whole
encoding/decoding process (including renormalization) can be realized by nothing
more than a bit shift, a comparison, and for half of the symbols an additional
subtraction.
Bypass coding has become an even more important feature during the HEVC
standardization process. While in H.264/AVC bypass coding was mainly used
for signs and least significant bins of absolute values of quantized transform
coefficients, in HEVC the majority of possible bin values is handled through the
bypass coding mode. As noted above, this is also a consequence of carefully
designed binarization schemes, which already serve as a kind of near-optimal prefix-
free codes of the corresponding syntax elements.
8.2.3.3
Fast Renormalization
One of the major throughput bottlenecks in any arithmetic encoding and decoding
process is given by the renormalization procedure. Renormalization in the M coder
is required whenever the new interval range R after interval subdivision no longer
stays within its admissible domain. Each time a renormalization operation must be
carried out, one or more bits can be outputted at the encoder or, equivalently, have to
be read by the decoder. This process, as it is specified in H.264/AVC and HEVC, is
performed bit-by-bit and is controlled by some conditional branches to check each
time if further renormalization loops are required. Both conditional branching and
bitwise processing, however, constitute considerable obstacles to a sufficiently high
throughput.
As a mitigation of this problem, a fast renormalization policy for the M coder was
proposed in [
48
]. By replacing the conventionally bitwise performed operations in
the regular coding mode with byte-wise or word-wise processing, a considerably
increased decoder throughput of around 25 % can be achieved. The corresponding
non-normative, fully standard-compliant changes were integrated into the reference
software implementations of both H.264/AVC and HEVC. For more details, please
refer to [
47
,
48
].
