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IEEE CONSUMER ELECTRONICS MAGAZINE
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JULY 2012
encoder resembles that of the H.264/AVC. So far, only one
profile (main) has been specified for HEVC, and more
profiles and a number of levels are being considered. The
key features of the latest version of HEVC are described in
detail in the following sections.
PICTURE PARTITIONING
Similar to the conventional video coding standards, HEVC is a
block-based hybrid-coding scheme. One of the major
contributors to the higher compression performance of HEVC
is the introduction of larger block structures with flexible
subpartitioning mechanisms. The basic block in HEVC is
known as the largest coding unit (LCU) and can be recursively
split into smaller coding units (CUs), which in turn can be split
into small prediction units (PUs) and transform units (TU).
These concepts are explained in the following subsections.
CODING UNITS
In H.264/AVC, each picture is partitioned into 16 # 16
macroblocks, and each macroblock can be further split
into smaller blocks (as small as 4 # 4) for prediction [2].
Other standards, such as MPEG-2 and H.263, were more
rigid regarding block sizes for motion compensation and
transforms. Such a rigid structure may not perform well
for all kinds of content; large blocks will generally work
better for smooth regions of a picture, whereas edges and
texture regions will often benefit from smaller block
sizes. As the picture resolution of videos increases from
standard definition to HD and beyond, the chances are
that the picture will contain larger smooth regions, which
can be encoded more effectively when large block sizes
are used. This is the reason that HEVC supports larger
encoding blocks than H.264/AVC, while it also has a
more flexible partitioning structure to allow smaller
blocks to be used for more textured and—in general—
uneven regions.
In HEVC, each picture is partitioned into square picture
areas called LCUs that can be as large as 64 # 64. The
LCU notion in HEVC is generally similar to that of a
macroblock in the previous coding standards. LCUs can be
further split into smaller units called CUs, which are used as
the basic unit for intra- and intercoding. The size of CUs
can be as large as that of LCUs or can be recursively split
into four equally sized CUs and become as small as 8 # 8,
depending on the picture content. Because of recursive
quarter-size splitting, a content-adaptive coding tree
structure comprised of CUs is created in HEVC [3].
Figure2 shows an example of partitioning a 64 # 64 LCU
to various sizes of CUs.
32 × 32
16 × 16
8 × 8
64 × 64
FIGURE 2. Partitioning of a 64
#
64 LCU to various sizes of CU.
3/4 × 2N
1/4 × 2N
1/4 × 2N
3/4 × 2N
2N
2N
(a)
(b)
2N
2N
N
N
N
N
2N
2N
2N
2N
FIGURE 3. (a) Symmetric and (b) asymmetric PUs.