Graphics Reference
In-Depth Information
Coding
Tree Unit
(CTU)
Divide into
coding units
(CU) with
quad tree
skip
Divide into
transform units
(TU) with quad
tree
Fig. 8.10
Illustration of residual quadtrees (one for each CU) used to signal transform units for
residual coding of CUs. Note that the same relationships and comments as given in Fig.
8.6
apply
here as well
allowed for 4
4 TUs and only if the corresponding functionality is enabled by
the
transform_skip_enabled_flag
in the PPS. Signaling of this mode is
performed by using the
transform_skip_flag
, which is coded using a single
fixed context model.
8.6.3
Coded Block Flags
At the top level of the hierarchy of significance flags, as already explained in
Sect.
8.3.1
, coded block flags (CBFs) are signaled for the RQT root, i.e., at the CU
level in the form of the
rqt_root_cbf
and for subsequent luma and chroma
TBs in the form of
cbf_luma
and
cbf_cb
,
cbf_cr
, respectively.
rqt_root
_cbf
is only coded and transmitted for inter-predicted CUs that are not coded in
merge mode using a single PU (PART_2Nx2N)
15
; for that a single context model
is used. While signaling of
cbf_luma
is only performed at the leaf nodes of the
RQT, provided that a non-zero
rqt_root_cbf
was signaled before, the chroma
CBFs
cbf_cb
and
cbf_cr
are transmitted at each internal node as long as a
corresponding non-zero chroma CBF at its parent node occurred. For coding of both
15
Intra-predicted CUs typically have nonzero residual, so
rqt_root_cbf
is not used.
