Graphics Reference
In-Depth Information
PU belongs to a CTU beyond the current CTU row. This minimizes the memory
bandwidth requirements to store the co-located motion data. In contrast to the
spatial MVP candidates, where the motion vectors may refer to the same reference
picture, motion vector scaling is mandatory for the TMVP. Hence, the same scaling
operation from Eq. (
5.3
) as for the spatial MVPs is used whereas td is defined as
the POC difference between the co-located picture and the reference picture of the
co-located candidate block.
While the temporal direct mode in H.264/AVC always refers to the first reference
picture in the second reference picture list, list 1, and is only allowed in bi-predictive
slices, HEVC offers the possibility to indicate for each picture which reference
picture is considered as the co-located picture. This is done by signaling in the slice
header the co-located reference picture list and reference picture index as well as
requiring that these syntax elements in all slices in a picture should specify the
same reference picture.
Since the temporal MVP candidate introduces additional dependencies, it might
be desirable to disable its usage for error robustness reasons. In H.264/AVC there
is the possibility to disable the temporal direct mode for bi-predictive slices in
the slice header (
direct_spatial_mv_pred_flag
). HEVC syntax extends
this signaling by allowing to disable the TMVP at sequence level or at picture
level (
sps/slice_temporal_mvp_enabled_flag
). Although the flag is
signaled in the slice header, it is a requirement of bitstream conformance that its
value shall be the same for all slices in one picture. Since the signaling of the picture-
level flag depends on the SPS flag, signaling it in the PPS would introduce a parsing
dependency between SPS and PPS. Another advantage of this slice header signaling
is that if you want to change only the value of this flag and no other parameter in
the PPS, there is no need to transmit a second PPS.
5.2.1.2
AMVP Motion Data Signaling
In general, motion data signaling in HEVC is similar as in H.264/AVC. An inter-
picture prediction syntax element,
inter_pred_idc
, signals whether reference
list 0, 1 or both are used. For each MCP obtained from one reference picture list, the
corresponding reference picture (t ) is signaled by an index to the reference picture
list,
ref_idx_l0/1
,andtheMV(x; y) is represented by an index to the MVP,
mvp_l0/1_flag
, and its MVD. The MVD syntax is further detailed in Chap.
8
.A
newly introduced flag in the slice header,
mvd_l1_zero_flag
, indicates whether
the MVD for the second reference picture list is equal to zero and therefore not
signaled in the bitstream. When the motion vector is fully reconstructed, a final
clipping operation assures that the values of each component of the final motion
vector will always be in the range of
2
15
to 2
15
1,inclusive.
