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compensate the coding performance loss caused by the fixed list
size, additional candidates are assigned to the empty positions in
the candidate list. In this process, the index is coded in truncated
unary codes of a maximum length, where the maximum length is
transmitted in slice header for the skip mode and merge mode
and fixed to 2 for the inter mode.
For the inter mode, a zero vector motion candidate is added to
fill the empty positions in the AMVP candidate list after the
derivation and pruning of the two spatial motion candidates and
the one temporal motion candidate. As for the skip mode and
merge mode, after the derivation and pruning of the four spatial
motion candidates and the one temporal motion candidate, if the
number of available candidates is smaller than the fixed
candidate list size, additional candidates are derived and added
to fill the empty positions in the merging candidate list.
Two types of additional candidates are used to fill the
merging candidate list: the combined bi-predictive motion
candidate and the zero vector motion candidate. The combined
bi-predictive motion candidates [21] are created by combining
two original motion candidates according to a predefined order.
After adding the combined bi-predictive motion candidates, if
the merging candidate list still has empty position(s), zero
vector motion candidates are added to the remaining positions.
III. THE ENHANCED CODING TOOLS FOR MOTION CANDIDATE
This section describes our proposed methods for improving
the MV coding efficiency. Four improvements including a
priority-based derivation algorithm of spatial motion candidates,
a priority-based derivation algorithm of temporal motion
candidates, a surrounding-based candidate list, and parallel
derivation of the candidate list are described as follows.
A. Priority-based Derivation Algorithm of Spatial Motion
Candidates
To improve the coding efficiency of the inter mode, a
priority-based scheme is proposed to derive each spatial motion
candidate based on a predefined priority. In the previous
scheme for deriving spatial motion candidate, only MVs with
the same reference list and the same reference index as the
current block are considered as available spatial motion
candidates. In the proposed priority-based scheme, the spatial
motion candidate can be derived from the MV with a different
list or a different reference picture to increase the chance of
availability for the spatial motion candidate [22][23]. The
spatial candidate is derived based on the following ordered
steps:
1. The MV from the same reference list and the same
reference picture
2. The MV from the other reference list and the same
reference picture
3. The scaled MV from the same reference list and a different
reference picture
4. The scaled MV from the other reference list and a different
reference picture
time
k l mji
picture id
current
block
neighboring
block b
time
k l mji
picture id
current
block
1
2
3
4
1’
2’
3’
4’
neighboring
block b
(b)
Fig. 2. Examples of deriving spatial motion candidate, where the numbers
denote the priority for the derivation of the spatial motion candidate.
Fig. 1. Illustration of deriving AMVP or merging candidate list.