Graphics Reference
In-Depth Information
3.1
Introduction
The High Efficiency Video Coding (HEVC) standard is designed along the suc-
cessful principle of block-based hybrid video coding. Following this principle, a
picture is first partitioned into blocks and then each block is predicted by using
either intra-picture or inter-picture prediction. While the former prediction method
uses only decoded samples within the same picture as a reference, the latter uses
displaced blocks of already decoded pictures as a reference. Since inter-picture
prediction typically compensates for the motion of real-world objects between
pictures of a video sequence, it is also referred to as motion-compensated prediction.
While intra-picture prediction exploits the spatial redundancy between neighboring
blocks inside a picture, motion-compensated prediction utilizes the large amount
of temporal redundancy between pictures. In either case, the resulting prediction
error,whichisformedbytakingthedifference between the original block and
its prediction, is transmitted using transform coding, which exploits the spatial
redundancy inside a block and consists of a decorrelating linear transform, scalar
quantization of the transform coefficients and entropy coding of the resulting
transform coefficient levels.
Figure
3.1
shows a block diagram of a block-based hybrid video encoder with
some characteristic ingredients of HEVC regarding its novel block partitioning
concept. This innovative feature of HEVC along with its specific key elements
will be one of the main subjects of this chapter. In a first step of this new block
partitioning approach, each picture in HEVC is subdivided into disjunct square
blocks of the same size, each of which serves as the root of a first block partitioning
quadtree structure, the coding tree, and which are therefore referred to as coding tree
blocks (CTBs). The CTBs can be further subdivided along the coding tree structure
into coding blocks (CBs), which are the entities for which an encoder has to
decide between intra-picture and motion-compensated prediction. The partitioning
of pictures into CTBs and the partitioning of CTBs into CBs are described in
Sects.
3.2.1
and
3.2.2
, respectively. Both sections highlight the similarities and
differences of the CTB/CB partitioning to the macroblock partitioning that is used in
older video coding standards. The partitioning of CBs for the purpose of prediction
is to a large degree independent of the coding tree structure and will be described in
Sect.
3.2.3
. Transform coding of the prediction residual at the CB level relies on the
second, nested block partitioning quadtree structure, the so-called residual quadtree,
and will be described along with its resulting transform blocks (TBs) in Sect.
3.2.4
.
Finally, Sect.
3.2.5
presents an experimental evaluation of some aspects of the block
partitioning concept of HEVC and compares the HEVC design to that of older video
coding standards.
In the second part of this chapter, the focus is on aspects of picture partitioning
for the purposes of packetization and parallel processing. Section
3.3.1
describes the
segmentation of a picture into slices, as already known from previous video coding
standards, and the novel, optional fragmentation of a slice into slice segments.
Aspects of high-level parallelization are discussed in Sect.
3.3.2
along with the
