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In-Depth Information
mode when the above neighboring PU is outside the CTU to avoid introduction of
an additional line buffer for intra mode reconstruction.
If
A
is not equal to
B
, the first two most probable modes denoted as
MPM
[0] and
MPM
[1] are set equal to
A
and
B
, respectively, and the third most probable mode
denoted as
MPM
[2] is determined as follows:
If neither of
A
or
B
is planar mode,
MPM
[2] is set to planar mode.
Otherwise, if neither of
A
or
B
is DC mode,
MPM
[2] is set to DC mode.
Otherwise (one of the two most probable modes is planar and the other is DC),
MPM
[2] is set equal to angular mode 26 (directly vertical).
If
A
is equal to
B
, the three most probable modes are determined as follows. In the
case they are not angular modes (
A
and
B
are less than 2), the three most probable
modes are set equal to planar mode, DC mode and angular mode 26, respectively.
Otherwise (
A
and
B
are greater than or equal to 2), the first most probable mode
MPM
[0] is set equal to
A
and two remaining most probable modes
MPM
[1] and
MPM
[2] are set equal to the neighboring directions of
A
and calculated as:
MPM Π1
D
2
C
..A
2
1
C
32/ % 32 /
(4.25)
MPM Π2
D
2
C
..A
2
C
1/ % 32 /
(4.26)
where % denotes the modulo operator (i.e.,
a
%
b
denotes the remainder of
a
divided
by
b
). Figure
4.8
summarizes the derivation process for the three most probable
modes
MPM
[0],
MPM
[1] and
MPM
[2] from the neighboring intra modes
A
and
B
.
The three most probable modes
MPM
[0],
MPM
[1] and
MPM
[2] identified as
described above are further sorted in an ascending order according to their mode
number to form the ordered set of most probable modes. If the current intra
prediction mode is equal to one of the elements in the set of most probable modes,
only the index in the set is transmitted to the decoder. Otherwise, a 5-bit CABAC
bypassed codeword is used to specify the selected mode outside of the set of most
probable modes as the number of modes outside of the set is equal to 32. The
bitstream syntax design for signaling intra modes is described in Sect.
4.4.3
.
4.4.2
Derived Mode for Chroma Intra Prediction
To enable the use of the increased number of directionalities in the chroma
intra prediction while minimizing the signaling overhead, HEVC introduces the
INTRA_DERIVED mode to indicate the cases when a chroma PU uses the same
prediction mode as the corresponding luma PU. More specifically, for a chroma PU
one of the five chroma intra prediction modes: planar, angular 26 (directly vertical),
angular 10 (directly horizontal), DC or derived mode is signaled.
