Graphics Reference
In-Depth Information
8.6.5.2
Coding of Sign
To reduce storage cost of the coefficients, as already noted above, the trans-
form coefficient data is grouped for every 4
4 subblock and the sign bins
are bypass coded and signaled before
coeff_abs_level_remaining
bins.
Before
coeff_abs_level_remaining
is added, the partial value of the
coefficient level can be represented with 4 bits. Thus, CABAC in HEVC only
requires storage of 4
4
4 bits for each subblock (as compared to 8
8
9 bits for
a 4
4 transform block in H.264/AVC), and the reconstructed transform coefficient
level can be immediately written out once
coeff_abs_level_remaining
is
parsed.
To improve coding efficiency, the optional
sign bit hiding
(SBH) technique can
be used [
24
]. SBH is a technique to hide one bit such as, e.g., a sign of a non-zero
coefficient in a group of non-zero coefficients. For this, the encoder quantizes the
coefficients in the group such that the sum of their absolute level values is even or
odd for the sign bit to be hidden having value 0 or 1, respectively. This inherently
lossy coding technique is based on the idea that in a group of quantized coefficients,
it is likely that there is at least one coefficient level for which the value can be
increased or decreased by 1 with only marginally increased rate-distortion cost.
This is, e.g., the case, when the unquantized coefficient was close to a quantization
decision threshold, such that quantizing the coefficient to the next lower or next
higher possible quantized value are both similarly good decisions.
SBH is enabled by
sign_data_hiding_enabled_flag
in the PPS and
if it is enabled, it applies to each 4
4 subblock for which the number of non-
zero coefficients exceeds a certain threshold. This threshold was chosen in HEVC
to be a value of 3 and the sign bit to be hidden is that of the last significant scan
position in the reverse scanning pattern of each subblock. The condition for SBH
can be checked while parsing the significance map and thus, SBH does not have
a significant impact on the entropy decoding throughput. Average bit rate savings
between 0.6 and 0.9 % were reported for SBH at common test conditions [
104
].
8.6.5.3
Summary of Absolute Level and Sign Coding in HEVC
Figure
8.25
summarizes the last four out of up to five scan passes required for
parsing the absolute levels and signs for every non-zero 4
4 subblock in HEVC.
8.6.6
Comparison of HEVC and H.264/AVC
Tab le
8.15
summarizes the differences in transform coefficient coding between
HEVC and H.264/AVC as well as across different transform block sizes. In terms
of throughput and memory related aspects, HEVC requires 3
fewer contexts
(121 vs. 359) than H.264/AVC for transform coefficient coding. Note, however,
