Graphics Reference
In-Depth Information
the context selection logic was highlighted in [ 90 ]. Accordingly, all dependencies
on the neighbors were removed and the context is selected based on the binIdx (i.e.,
whether it is the first or second bin) [ 82 , 91 ].
To maximize the impact of fast bypass coding, the bypass coded bins for both
the horizontal (x) and vertical (y) components of mvd are grouped together in
HEVC [ 67 ]. For instance, for a motion vector difference of mvd.x; y/ D .2; 2/,
the coding order is 1111 0000 , where underlined values are bypass coded. Without
bypass grouping, the coding order is 11 001 1 00 . If four bypass bins can be processed
in a single cycle, enabling bypass grouping reduces the number of cycles required
to process the motion vector by one.
In HEVC, reference indices ref_idx_l0 and ref_idx_l1 are coded with
truncated unary regular coded bins, which is the same as for H.264/AVC; the
maximum length of the truncated unary binarization, cMax, is dictated by the
reference picture list size. However, in HEVC only the first two bins are regular
coded [ 71 ], whereas all bins are regular coded in H.264/AVC. In both HEVC and
H.264/AVC, the regular coded bins of the reference indices for different reference
picture lists share the same set of contexts. The inter-prediction direction (list 0, list
1 or bi-directional) is signaled using inter_pred_idc with custom binarization.
8.5.2
Intra Prediction Mode Coding
Similar to motion data coding, a most probable mode (MPM) is calculated for intra
mode coding. In H.264/AVC, the minimum mode of the top and left neighbors is
used as MPM. prev_intra4x4_pred_mode_flag (or prev_intra8x8_
pred_mode_flag ) is signaled to indicate whether the most probable mode is
used. If the MPM is not used, the remainder mode rem_intra4x4_pred_mode
_flag (or rem_intra8x8_pred_mode_flag ) is signaled.
In HEVC, additional MPMs are used to improve coding efficiency. A candidate
list of most probable modes with a fixed length of three is constructed based on the
left and top neighbors. The additional candidate modes (DC, planar, vertical) can
be added if the left and top neighbors are the same or unavailable. Note that the top
neighbors outside the current CTU are considered unavailable in order to avoid the
need for a line buffer. 14 The prediction flag prev_intra_pred_mode_flag
is signaled to indicate whether one of the most probable modes is used. If an MPM
is used, a most probable mode index ( mpm_idx ) is signaled to indicate which
candidate to use. It should be noted that in HEVC, the order in which the coefficients
of the residual are parsed (e.g., diagonal, vertical or horizontal) depends on the
reconstructed intra mode (i.e., the parsing of the TU data that follows depends on
list construction and intra mode reconstruction). Thus, the candidate list size was
limited to three for reduced computation to ensure that it would not affect entropy
decoding throughput [ 22 , 83 ].
14 For more details on MPM list construction please refer to Chap. 4 .
 
Search WWH ::




Custom Search