Graphics Reference
In-Depth Information
t: Reference picture index:
x y: Spatial displacement:
x
y
t
0
= 2
t
1
= 4
Prior Decoded Pictures as Reference
Current Picture
Fig. 5.1
Inter-picture prediction concept and parameters using a translational motion model
While intra-picture prediction exploits the correlation between spatially neighboring
samples, inter-picture prediction makes use of the temporal correlation between
pictures in order to derive a motion-compensated prediction (MCP) for a block of
image samples.
For this block-based MCP, a video picture is divided into rectangular blocks.
Assuming homogeneous motion inside one block and that moving objects are larger
than one block, for each block, a corresponding block in a previously decoded
picture can be found that serves as a predictor. The general concept of MCP based on
a translational motion model is illustrated in Fig.
5.1
. Using a translational motion
model, the position of the block in a previously decoded picture is indicated by
a motion vector .x; y)wherex specifies the horizontal and y the vertical
displacement relative to the position of the current block. The motion vectors
.x; y) could be of fractional sample accuracy to more accurately capture the
movement of the underlying object. Interpolation is applied on the reference pictures
to derive the prediction signal when the corresponding motion vector has fractional
sample accuracy. The previously decoded picture is referred to as the reference
picture and indicated by a reference index t to a reference picture list. These
translational motion model parameters, i.e. motion vectors and reference indices, are
further referred to as motion data. Two kinds of inter-picture prediction are allowed
in modern video coding standards, namely uni-prediction and bi-prediction.
In case of bi-prediction, two sets of motion data (x
0
;y
0
;t
0
and
x
1
;y
1
;t
1
) are used to generate two MCPs (possibly from different pictures),
which are then combined to get the final MCP. Per default, this is done by averaging
but in case of weighted prediction, different weights can be applied to each MCP,
e.g. to compensate for scene fade outs. The reference pictures that can be used in
bi-prediction are stored in two separate lists, namely list 0 and list 1. In order to
limit the memory bandwidth in slices allowing bi-prediction, the HEVC standard
restricts PUs with 4
8 and 8
4 luma prediction blocks to use uni-prediction only.
Motion data is derived at the encoder using a motion estimation process. Motion
