Signal Processing - September 2016 - 166

Table 1. The interprediction modes for each type of macroblock partition.
MBPart

Mode

P_16x16

Skip

Forward
Prediction

√

Backward
Prediction

√

P_16x8

√

P_8x16

√

P_8x8

√

B_Skip

√

B_16x16

√

B_16x8
B_8x16
B_8x8

√

MV2

Reference Frame

Current Frame

Figure 3. An illustration of multiple hypothesis.

MV1

MV2

Dist2
Dist1
Forward
Current
Backward
Reference
Frame
Reference

Figure 4. An illustration of symmetrical prediction.

Symmetrical
Prediction

√

MV1

Figure 5. The neighboring macroblock partitions for MVP.

forward predictor is the predicted
motion vector derived by the process of motion vector prediction,
and the motion vector (MV2) of the
second forward predictor is derived
166

Multiple
Hypothesis

by the motion estimation constrained with the first predictor.
Only the motion vector difference
of the second predictor and the
interprediction residuals are transmitted in the bitstream.
5) Symmetrical prediction: Symmetrical
mode averages one forward predictor and a backward predictor to get
the final interpredictor for the
macroblock partition, as shown in
Figure 4. The motion vector (MV1)
of the forward predictor is derived
by forward motion estimation. The
backward motion vector (MV2) is
derived by scaling the forward motion vector. The scaling factor is
decided by the distance (Dist1) between the forward reference frame
and the current frame and the distance (Dist2) between the backward
reference frame and the current
frame. Only the motion vector difference of the forward predictor and
the interprediction residuals are
transmitted in the bitstream.

Motion vector prediction
A motion vector is predicted, and only
the motion vector difference (between
the motion vector and its prediction)
is coded into bitstream. To predict the
motion vector of the current macroblock
partition (given by E), its four neighboring macroblock partitions of left,
above, left-above, and right-above (their
motion vectors are given by A, B, D, and
C, respectively) are used, as shown in
Figure 5. If there is no motion vector
IEEE Signal Processing Magazine

September 2016

for one macroblock partition (intramacroblock partition) or the macroblock
partition has not been reconstructed, the
motion vector of this macroblock partition is set as a zero vector. When the
partitions A, B, C, and D are all unavailable, the predictor of E is set as a zero
vector. When only one of A, B, C, or D is
available, the predictor of E is set as that
available motion vector. Otherwise, if C
is unavailable, then it is replaced with D,
and A, B, and C are used to predict E by
the following process.
First, the signs of each horizontal
component of A, B, and C are checked.
If the sign of one motion vector
(denoted by X) is different from the
other two, then this motion vector X
is excluded from the motion vector
prediction process, and the predictor
of the horizontal component is the
averaging of the horizontal component of the other two motion vectors.
Otherwise, the Euler distance of the
horizontal component of each pair of
neighboring motion vectors is calculated, the motion vector pair with the
smallest Euler distance is selected, and
the predictor of the horizontal component is the average of the horizontal component of the selected motion
vector pair. The vertical component of
E is predicted in the same way as the
horizontal component.

Subpel interpolation
Quarter-pel motion compensation is
adopted for the luma component. A
two-dimensional separable Lanczos filter is used to generate the subpel position values [11], as shown in Figure 6.
Three one-dimensional filters, F1, F2,
and F3, are used to generate the subpel
values as follows:
a x, y = / F1 (z) * A x + z, y, b x, y
z

= / F2 (z) * A x + z, y, c x, y
z

= / F3 (z) * A x + z, y

(1)

d x, y = / F1 (z) * A x, y + z, h x, y
z

= / F2 (z) * A x, y + z, n x, y
z

= / F3 (z) * A x, y + z
z

(2)

Table of Contents for the Digital Edition of Signal Processing - September 2016