Page 158 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 5.3. E+ciency of Warping-Based Methods at Very Low Bit Rates 135

BMA-HO This is the same as BMA but with half-pel accuracy and over-
lapped motion compensation. Half-pel accuracy was obtained using a re-
(nement stage around the full-pel motion vector. Overlapping windows of
32 × 32 and a bilinear weighting function, Equation (4:33), were used for
overlapped motion compensation. Border blocks were handled by assum-
ing “phantom” blocks outside the frame boundary, with motion vectors
equal to those of the border blocks.
WBA This is a warping-based algorithm. Node points were placed at the
centers of 16 × 16 blocks in the current frame. This formed a regular
(xed mesh with square patches. In order for the mesh to cover the whole
frame area, node points were also placed on the borders.
Backward node tracking was used to map the current node points to their
matches in the reference frame. A continuous method was used to pro-
duce a continuous motion (eld. To ensure that the number of transmitted
motion vectors is the same as that of the BMA, no motion vectors were
transmitted for the border node points. Instead, each border node was
assigned the motion vector of the closest inner node. However, to ensure
that the borders of the current frame were mapped to the borders of the
reference frame, border nodes at the corners of the frame were assigned
zero motion vectors, the vertical component of a top or a bottom border
nodal vector was set to zero, and the horizontal component of a left or a
right border nodal vector was set to zero. The mesh geometry and nodal
motion vectors are illustrated in Figure 5.3.

horizontal component is
equal to inner vector
corner nodes have
zero vectors vertical component is zero
I
d
,
0
) 0 , 0 ( ( x ) O
I
( x d I ,d I )
outer y
node
A
inner
node
block-motion
vector inner nodes are at the 4 patches affected
centers of BMA blocks by node A
(a) BMA blocks (b) WBA patches
Figure 5.3: BMA blocks and WBA patches