Graphics Reference
In-Depth Information
Tabl e 5. 1
Order in which motion data combinations of different merge candidates,
that have been already inserted in the merge list, are tested to create combined bi-
predictive merge candidates
om i ti nOr r
0123456789 0 1
x
0
;y
0
;t
0
from
.
01021203132 3
x
1
;y
1
;t
1
from
.
10202130313 2
included in the merge candidate list. In the event of a lost co-located picture, the
decoder could not derive the temporal candidates and hence not check whether it
would be redundant. The indexing of all subsequent candidates would be affected
by this.
Additional Candidates
For parsing robustness reasons, which will be explained in Sect.
5.2.2.3
, the length
of the merge candidate list is fixed. After the spatial and the temporal merge
candidates have been added, it can happen that the list has not yet the fixed length.
In order to compensate for the coding efficiency loss that comes along with the
non-length adaptive list index signaling, additional candidates are generated [
25
].
Depending on the slice type, up to two kind of candidates are used to fully populate
the list:
Combined bi-predictive candidates
Zero motion vector candidates
In bi-predictive slices, additional candidates can be generated based on the existing
ones by combining reference picture list 0 motion data of one candidate with and
the list 1 motion data of another one. This is done by copying x
0
;y
0
;t
0
from
one candidate, e.g. the first one, and x
1
;y
1
;t
1
from another, e.g. the second
one. The different combinations are predefined and given in Table
5.1
.
When the list is still not full after adding the combined bi-predictive candidates,
or for uni-predictive slices, zero motion vector candidates are calculated to complete
the list. All zero motion vector candidates have one zero displacement motion vector
for uni-predictive slices and two for bi-predictive slices. The reference indices are
set equal to zero and are incremented by one for each additional candidate until the
maximum number of reference indices is reached. If that is the case and there are
still additional candidates missing, a reference index equal to zero is used to create
these. For all the additional candidates, no redundancy checks are performed as it
turned out that omitting these checks will not introduce a coding efficiency loss [
21
].
