Inter-Picture Prediction in HEVC - High Efficiency Video Coding (HEVC) - page 125

5.2.2.3 Merge Motion Data Signaling and Skip Mode

The motion data signaling scheme using the merge mode is quite simple. For each
PU coded in inter-picture prediction mode, a so called merge_flag indicates
that block merging is used to derive the motion data. The merge_idx further
determines the candidate in the merge list that provides all the motion data needed
for the MCP. Therefore, instead of all the syntax elements needed for AMVP
based motion data coding described in Sect. 5.2.1.2 , only a flag and a list index are
transmitted. This difference can be seen when comparing the input to the AMVP
and the merge motion data coding block in Fig. 5.2 .
Besides this PU-level signaling, the number of candidates in the merge list is
signaled in the slice header. Since the default value is five, it is represented as a
difference to five ( five_minus_max_num_merge_cand ). That way, the five
is signaled with a short codeword for the 0 whereas using only one candidate,
is signaled with a longer codeword for the 4. Regarding the impact on the merge
candidate list construction process, the overall process remains the same although
it terminates after the list contains the maximum number of merge candidates. In
the initial design, the maximum value for the merge index coding was given by the
number of available spatial and temporal candidates in the list. When e.g. only two
candidates are available, the index can be efficiently coded as a flag. But, in order
to parse the merge index, the whole merge candidate list has to be constructed to
know the actual number of candidates. Assuming unavailable neighboring blocks
due to transmission errors, it would not be possible to parse the merge index
anymore. Fixing the number of merge candidates improves the parsing robustness
by decoupling the parsing and the merge candidate list construction while sacrificing
coding efficiency. Populating the list with the additional merge candidates presented
in Sect. 5.2.2.2 compensates again for that loss while keeping the parsing robustness.
A crucial application of the block merging concept in HEVC is its combination
with a skip mode. In previous video coding standards, the skip mode was used to
indicate for a block that the motion data is inferred instead of explicitly signaled and
that the prediction residual is zero, i.e. no transform coefficients are transmitted.
This mode is well suited to code static image regions where the prediction error
tends to be very small. In HEVC, at the beginning of each CU in an inter-picture
prediction slice, a skip_flag is signaled that implies the following:
￿ the CU only contains one PU (2N 2N partition type)

￿ the merge mode is used to derive the motion data ( merge_flag equal to 1)

￿ no residual data is present in the bitstream

5.2.2.4 Coding Efficiency of HEVC Merge and Skip Mode

In this section, the coding efficiency of the HEVC merge and skip modes is
analyzed. This is done experimentally by disabling the merge mode as well as
the skip mode, i.e. removing merge_flag , merge_index and skip_flag