Graphics Reference
In-Depth Information
since it only depends on the reference picture list structure signaled in the slice
header. Note that the MV scaling is only performed when the current reference
picture and the candidate reference picture are both short term reference pictures.
mv
D
sign.
mv
cand
ScaleFactor/
..
j
mv
cand
ScaleFactor
jC
2
7
/
8/
(5.3)
ScaleFactor
D
clip.
2
12
;2
12
1; .tb
tx
C
2
5
/
6/
(5.4)
2
14
Cj
t
2
j
td
tx
D
(5.5)
For candidate B, the candidates B0 to B2 are checked sequentially in the same
way as A0 and A1 are checked in the first pass. The second pass, however, is only
performed when blocks A0 and A1 do not contain any motion information, i.e. are
not available or coded using intra-picture prediction. Then, candidate A is set equal
to the non-scaled candidate B, if found, and candidate B is set equal to a second,
non-scaled or scaled variant of candidate B. Since you could also end up in the
second pass when there still might be potential non-scaled candidates, the second
pass searches for non-scaled as well as for scaled MVs derived from candidates B0
to B2.
Overall, this design allows to process A0 and A1 independently from B0, B1,
and B2. The derivation of B should only be aware of the availability of both A0 and
A1 in order to search for a scaled or an additional non-scaled MV derived from B0
to B2. This dependency is acceptable given that it significantly reduces the complex
motion vector scaling operations for candidate B. Reducing the number of motion
vector scalings represents a significant complexity reduction in the motion vector
predictor derivation process.
Temporal Candidate
It can be seen from Fig.
5.4
b that only motion vectors from spatial neighboring
blocks to the left and above the current block are considered as spatial MVP
candidates. This can be explained by the fact that the blocks to the right and below
the current block are not yet decoded and hence, their motion data is not available.
Since the co-located picture is a reference picture which is already decoded, it is
possible to also consider motion data from the block at the same position, from
blocks to the right of the co-located block or from the blocks below. In HEVC,
the block to the bottom right and at the center of the current block have been
determined to be the most suitable to provide a good temporal motion vector
predictor (TMVP). These candidates are illustrated in Fig.
5.4
a where C0 represents
the bottom right neighbor and C1 represents the center block. Here again, motion
data of C0 is considered first and, if not available, motion data from the co-located
candidate block at the center is used to derive the temporal MVP candidate C. The
motion data of C0 is also considered as not being available when the associated
