Graphics Reference
In-Depth Information
pŒ0Œ0
D
.p Œ
1Œ0
C
2
dcVal
C
pŒ0Œ
1
C
2/ >> 2
(4.22)
For other boundary samples, a two-tap [3 1]/4 smoothing filter is applied with the
higher weight on the DC value:
pŒx Œ0
D
.p Œ x Œ
1
C
3
dcVal
C
2/ >> 2
for x
D
1:::N
1
(4.23)
pŒ0Œy
D
.p Œ
1Œy
C
3
dcVal
C
2/ >> 2 for y
D
1:::N
1
(4.24)
4.4
Intra Mode Coding
While increasing the number of intra prediction modes provides better prediction,
efficient intra mode coding is required to ensure that the selected mode is signaled
with minimal overhead. For luma component, three most probable modes are
derived to predict the intra mode instead of a single most probable one as in
H.264/AVC. Possible redundancies among the three most probable modes are also
considered and redundant modes are substituted with alternative ones to maximize
the signaling efficiency. For chroma intra mode, HEVC introduces a derived mode
which allows efficient signaling of the likely scenario where chroma is using the
same prediction mode as luma. The syntax elements for signaling luma and chroma
intra modes are designed by utilizing the increased number of most probable modes
for the luma component and the statistical behavior of the chroma component.
4.4.1
Prediction of Luma Intra Mode
HEVC supports a total of 33 angular prediction modes as well as planar and DC
prediction for luma intra prediction for all the PU sizes. Due to the large number of
intra prediction modes, H.264/AVC-like mode coding approach based on a single
most probable mode was not effective in HEVC. Instead, HEVC defines three most
probable modes for each PU based on the modes of the neighboring PUs. The
selected number of most probable modes makes it also possible to indicate one of
the 32 remaining modes by a CABAC bypassed fixed-length code, as distribution of
the mode probabilities outside of the set of most probable modes has been found to
be relatively uniform.
The selection of the set of three most probable modes is based on modes of two
neighboring PUs, one left and one to the above of the current PU. Let the intra
modes of left and above of the current PU be
A
and
B
, respectively. If a neighboring
PU is not coded as intra or is coded with pulse code modulation (PCM) mode, the
PU is considered to be a DC predicted one. In addition, B is assumed to be DC
