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In-Depth Information
32x32 merge estimation region (MER)
PU0
PU2
PU3
Potentially available candidate (different MER)
PU5
PU1
PU4
PU6
Unavailable candidate (same MER)
PU8
Unavailable candidate (not coded yet)
PU7
PU9
Fig. 5.7
Example of a CTU with a 64
64 luma CTB, when motion estimation of for PUs inside
a32
32 motion estimation region is carried out independently, enabling the possibility to do it in
parallel
5.2.2.5
Merge Estimation Regions for Parallel Merge Mode Estimation
The way the merge candidate list is constructed introduces dependencies between
neighboring blocks. Especially in embedded encoder implementations, the motion
estimation stage of neighboring blocks is typically performed in parallel or at least
pipelined to increase the throughput. For AMVP, this is not a big issue since the
MVP is only used to differentially code the MV found by the motion search. The
motion estimation stage for the merge mode, however, would typically just consist
of the candidate list construction and the decision which candidate to choose, based
on a cost function. Due to the aforementioned dependency between neighboring
blocks, merge candidate lists of neighboring blocks cannot be generated in parallel
and represent a bottleneck for parallel encoder designs. Therefore, a parallel merge
estimation level was introduced in HEVC that indicates the region in which merge
candidate lists can be independently derived by checking whether a candidate block
is located in that merge estimation region (MER). A candidate block that is in
the same MER is not included in the merge candidate list. Hence, its motion data
does not need to be available at the time of the list construction. When this level
is e.g. 32, all prediction units in a 32
32 area can construct the merge candidate
list in parallel since all merge candidates that are in the same 32
32 MER, are not
inserted in the list. Figure
5.7
illustrates that example showing a CTU partitioning
with seven CUs and ten PUs. All potential merge candidates for the first PU0 are
available because they are outside the first 32
32 MER. For the second MER, merge
candidate lists of PUs 2-6 cannot include motion data from these PUs when the
merge estimation inside that MER should be independent. Therefore, when looking
at a PU5 for example, no merge candidates are available and hence not inserted in
