IEEE Consumer Electronics Magazine - September/October 2022 - 13

novel splits binary tree (BT) and ternary tree
(TT). The binary-tree (BT) divides a CU in two
sub-CUs while the ternary-tree (TT) divides a CU
in three sub-CUs with the ratio 1:2:1. Both BT and
TT can split a CU horizontally or vertically.
Intra-Coding Tools
Intra-coding principle takes advantage of spaFigure
2. VVC encoder block diagram.
These seven blocks are briefly described in this
section. The luma forward mapping step is normative
and relies on the Luma Mapping with
Chroma Scaling (LMCS) tool, which is described
in the " In-Loop Filters " section. For more exhaustive
description of VVC HLS15
or VVC coding
tools, and profiles, the reader may refer to the
overview paper.9
Picture Partitioning
The first step of the picture partitioning block
splits the picture into blocks of equal size, named
coding tree unit (CTU). The maximum CTU size is
128 128 samples in VVC and is composed of one
or three coding tree blocks depending on
whether the video signal is monochrome or contains
three-color components. The CTUs are then
processed in raster scan order from top left to
bottom right. In order to adapt the prediction
block size to the local activity of the samples,
each CTU is then recursively split into smaller
rectangular coding units (CUs), according to the
multitype tree (MTT) partitioning scheme. The
multi-type tree (MTT) partitioning16 is an extension
of the quad-tree (QT) partitioning adopted
in HEVC. The blocks resulting from the partitioning
process are named CUs and may have a size
between 64 64 and 4 4. In intra-slice, the luma
and chroma components can be recursively split
according to their own coding trees (separate
luma and chroma coding trees). As in HEVC, QT
divides a CU in four equal sub-CUs. In addition,
VVC allows a rectangular shape for CU with its
September/October 2022
tial correlation existing in local image texture.
To decorrelate them, a series of coding tools are
provided in VVC, which are tied with partitioning
and a set of intra-prediction modes (IPMs).17 For
the block partitioning, in addition to the principles
explained in the previous section, a new
tool called dual tree is introduced, which allows
separate partitioning trees for luma and chroma
channel types. As opposed to the single-tree partitioning
used in inter-coding, the intra-specific
dual-tree tool offers a higher level of freedom for
coding decisions of chroma blocks.
The set of Intra-prediction modes (IPMs) inVVC
has been extended to 67 modes, compared to 35 in
HEVC.This set consists of twomodes of dc and planar
for modeling homogeneous textures, as well as
65 directional modes for modeling angular textures.
The IPMs are coded in VVC through a most
probable modes (MPMs) list of six IPMs, while a
three MPMs list is used in HEVC. For square shape
blocks, these directional modes cover the same
range of directional angles, with twice precision as
in HEVC. Moreover, thanks to a new tool in VVC
called wide-angle intra-prediction, the set of
65 angular directional IPMs are adaptively shifted
for nonsquare blocks. This tool assigns additional
directional modes to the longer side of nonsquare
blocks. By doing so, prediction directions with
angle greater than 45 relative to the pure horizontal
or vertical modes are also possible.
Intra-prediction references are improved in
VVC to more efficiently manage noisy and less
correlated neighboring samples. A reference
selection tool, called multiple reference lines
(MRL), has been introduced that allows the
encoder to choose among three reference lines
and explicitly signal the best one. Once the reference
line is selected, the reference sample
smoothing and the reference sample interpolation
are two mechanisms in VVC to denoise reference
samples. The choice between these two
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