Graphics Reference
In-Depth Information
prediction modes provide predictions for image areas with smooth and gradually
changing content. Planar and DC predictions are also useful in creating “neutral”
prediction blocks with no high frequency components for complex textures that
cannot be properly modeled with any of the directional predictors that the angular
intra prediction is able to generate. In order to further improve the prediction quality,
some of the angular modes and the DC prediction mode include a light post-filtering
operation to enhance continuity of the prediction signal at block boundaries.
4.3.1
Angular Prediction
Angular intra prediction in HEVC is designed to efficiently model different
directional structures typically present in video and image content. The set of
available prediction directions has been selected to provide a good trade-off between
encoding complexity and coding efficiency for typical video material. The sample
prediction process itself is designed to have low computational requirements and
to be consistent across different block sizes and prediction directions. This has
been found especially important as the number of block sizes and prediction
directions supported by HEVC intra coding far exceeds those of previous video
codecs, such as H.264/AVC. In HEVC there are four effective intra prediction block
sizes ranging from 4
4to32
32 samples, each of which supports 33 distinct
prediction directions. A decoder must thus support 132 combinations of block size
and prediction direction.
4.3.1.1
Angle Definitions
HEVC defines a set of 33 angular prediction directions at 1/32 sample accuracy as
illustrated in Fig.
4.5
. In natural imagery, horizontal and vertical patterns typically
occur more frequently than patterns with other directionalities. Small differences
for displacement parameters for modes close to horizontal and vertical directions
take advantage of that phenomenon and provide more accurate prediction for
nearly horizontal and vertical patterns [
10
]. The displacement parameter differences
become larger closer to diagonal directions to reduce the density of prediction
modes for less frequently occurring patterns.
Tab le
4.2
provides the exact mapping from indicated intra prediction mode to
angular parameter
A
. That parameter defines the angularity of the selected prediction
mode (how many 1/32 sample grid units each row of samples is displaced with
respect to the previous row).
4.3.1.2
Reference Row Extension for the Negative Prediction Directions
In order to simplify the sample prediction process, the reference samples above the
block
p
[
x
][
1] and left of the block
p
[
1][
y
] are placed in a one dimensional (1-D)
