Graphics Reference
In-Depth Information
4.3.2
DC Prediction
In the case of DC prediction, the predicted sample values are populated with a
constant value representing the average of the reference samples immediately left
and to the above of the block to be predicted. The DC predicted luminance blocks
of size 16
16 and smaller go through a light filtering process to soften the left and
above edges of the block as described in Sect.
4.3.4
.
4.3.3
Planar Prediction
While angular prediction provides good approximations for structures with sig-
nificant edges, it can create some visible contouring in smooth image areas.
Similarly, some blockiness can typically be observed in smooth image areas when
DC prediction is applied at low or medium bit rates. The planar prediction of
HEVC is designed to overcome these issues by having the capability to generate a
prediction surface without discontinuities on the block boundaries. This is achieved
by averaging a horizontal and vertical linear prediction on sample basis as follows:
pŒxŒy
D
.p
h
ŒxŒy
C
p
v
ŒxŒy
C
N/ >> .log
2
.N/
C
1/
(4.18)
The horizontal prediction
p
h
[
x
][
y
] and the vertical prediction
p
v
[
x
][
y
]for
location [
x
][
y
] are calculated as follows:
p
h
ŒxŒy
D
.N
1
x/
pŒ
1Œy
C
.x
C
1/
pŒN Œ
1
(4.19)
p
v
ŒxŒy
D
.N
1
y/
pŒx Œ
1
C
.y
C
1/
pŒ
1ŒN
(4.20)
Figure
4.7
illustrates derivation of predicted sample values in the case of planar
mode. The top-right reference sample
p
[
N
][
1] is used as the right reference for
all the horizontal filtering. Similarly, the bottom-left reference sample
p
[
1][
N
]is
used as the bottom reference for all the vertical operations. Final prediction value
for each sample is obtained by averaging the horizontal and vertical predictions.
4.3.4
Post-processing for Predicted Samples
Some of the prediction modes can generate discontinuities for the predicted sample
values on the boundaries of the prediction blocks. Such cases are most evident for
the DC prediction and for the directly horizontal and vertical angular predictions.
In the case of DC prediction, such discontinuity can occur on both top and left
boundary of the block as all the predicted samples are replaced with a single value.
