Graphics Reference
In-Depth Information
high-definition (HD) video content, with typical picture resolutions of 1280
720 or
1920
1080 luma samples, they have been primarily designed for video resolutions
ranging from QCIF (176
144 luma samples) to standard definition (720
480 or
720
576 luma samples). Due to the popularity of HD video and the growing interest
in Ultra HD (UHD) formats [
13
] with resolutions of, for example, 3840
2160 or
even 7680
4320 luma samples, HEVC [
18
] has been designed with a focus on
high resolution video. However, for such large picture resolutions, restricting the
largest block size that can be used for signaling prediction parameters to 16
16
luma samples as in prior video coding standards is inefficient in rate-distortion
sense [
4
,
26
,
37
]. For typical HD or UHD video content, many picture areas that
can be described by the same motion parameters are much larger than blocks of
16
16 luma samples. Signaling a coding mode for each 16
16 macroblock
would already require a substantial amount of the target bit rate. Furthermore, due
to the increased spatial correlation between neighboring samples in high-resolution
video, using transform sizes larger than 16
16 for coding the residual signal can
also be advantageous for many image parts. The support of larger block sizes for
intra-picture prediction, motion-compensated prediction and transform coding was
one of the key aspects in many proposals for HEVC, for example [
25
,
27
,
44
].
Even though the coding of HD and UHD video was one important aspect in
the HEVC development, the standard has been designed to provide an improved
coding efficiency relative to its predecessor H.264
j
MPEG-4 AVC for all existing
video coding applications. While increasing the size of the largest supported block
size is advantageous for high-resolution video, it may have a negative impact on
coding efficiency for low-resolution video, in particular if low-complexity encoder
implementations are used that are not capable of evaluating all supported sub-
partitioning modes. For this reason, HEVC includes a flexible mechanism for
partitioning video pictures into basic processing units of variable sizes.
As already mentioned, in HEVC, each picture is partitioned into square-shaped
coding tree blocks (CTBs) such that the resulting number of CTBs is identical
for both the luma and chroma picture components (assuming a non-monochrome
video format).
2
Consequently, each CTB of luma samples together with its two
corresponding CTBs of chroma samples and the syntax associated with these sample
blocks is subsumed under a so-called coding tree unit (CTU). A CTU represents
the basic processing unit in HEVC and is in that regard similar to the concept
of a macroblock in prior video coding standards. The luma CTB covers a square
picture area of 2
N
2
N
luma samples. In the 4:2:0 chroma sampling format, each
of the two chroma CTBs covers the corresponding area of 2
N 1
2
N 1
chroma
samples of one of the two chroma components. The parameter N is transmitted in
the sequence parameter set and can be chosen by the encoder among the values
N
D
4, 5, and 6, corresponding to CTU sizes of 16
16, 32
32,and64
64
2
The profiles defined in version 1 of the HEVC standard [
18
] only support video in the 4:2:0
chroma sampling format; monochrome video is not supported in these profiles.
