Entropy Coding in HEVC - High Efficiency Video Coding (HEVC) - page 227

block sizes of neighboring CBs, thus the context selection for split_cu_flag
depends on the relative depth of the top and left neighboring CBs compared to that of
the current CB. Note that in H.264/AVC the partitioning information is sent together
with other data as aggregated syntax elements mb_type and sub_mb_type with
different ranges of allowed values and hence different binarization schemes for
different slices. 8 This kind of aggregating different information in one single syntax
element is mostly due to historical reasons, reflecting the circumstances that earlier
video coding standards (including H.264/AVC) were designed under the regime
of VLC-based entropy coding, where alphabet extensions are used to circumvent
the lower bound of 1 bit per symbol. Thus, by allowing the signaling of a coding
quadtree structure with a one-bin syntax element, i.e., the split_cu_flag
at each node, HEVC is much more flexible and allows many more coding and
prediction block structures than H.264/AVC, even when choosing a CTB size of
16 16 luma samples and ignoring the fact that HEVC doesn't allow for inter-
predicted 4 4 luma blocks, as discussed in Chap. 3 .

8.4.2 Prediction Mode and Prediction Block Structure

In P and B slices, a cu_skip_flag is sent for each CU to indicate whether
all associated CBs are coded using skip mode, i.e., by using the so-called merge
mode for inter-picture prediction (as explicitly described in Sect. 8.5 ) and not
sending any residual data. To leverage spatial correlation of neighboring CUs, the
context of the cu_skip_flag depends on whether the top and left neighboring
CUs are also skipped. For every non-skipped CU, a regular coded flag called
pred_mode_flag is sent to indicate the prediction mode, i.e., the decision
whether the CU is either intra coded or inter coded. 9
Every non-skipped CU may be further subdivided into PUs, as shown for the
example in Fig. 8.6 . The syntax element part_mode indicates if and how each CU
is partitioned for the purpose of prediction. The choice of prediction block structures
for each CU depends on whether the CU is intra-coded or inter-coded; accordingly,
part_mode is binarized and coded differently for intra-coded CUs and inter-coded
CUs, as shown in Fig. 8.7 .
Intra-coded CUs can have a single PU (referred to as PART_2Nx2N) equal to the
size of the CU, or be subdivided into four smaller PUs (referred to as PART_NxN).
PART_NxN, however, is only allowed when the CU size is equal to the minimum
allowed CU size. If the CU size is greater than the minimum allowed CU size,

8 In H.264/AVC, mb_type and sub_mb_type are used to represent the following equivalent
information in HEVC: split_cu_flag , pred_mode_flag , part_mode , pcm_flag ,
inter_pred_idc , coded block pattern ( cbf_luma , cbf_cr , cbf_cb ) and intra prediction
mode for 16 16 intra-coded PU.

9 pred_mode_flag is not sent for CUs in I slices since they are all intra-coded.