Graphics Reference
In-Depth Information
of the standard it is expected that the HEVC still imaging features will be
further extended towards 4:4:4 color formats and high dynamic range photography
requiring support for bit-depths above 8 bits per sample.
The key characteristics of the HEVC Main Still Picture profile can be summa-
rized as follows:
HEVC Main profile intra coding tools are used
-
Chroma format is limited to 4:2:0
-
Bit depth is limited to 8 bits per sample
-
Coding tree block size can vary from 16
16 to 64
64
Bitstream contains only one picture
Decoded picture buffer is of size one picture
4.8
Summary of Differences from H.264/AVC
Both HEVC intra coding and H.264/AVC intra coding are based on spatial sample
prediction followed by transform coding [
18
]. However, the intra coding methods
in HEVC are more elaborated in number of ways. Firstly, the set of supported
prediction block sizes is extended up to 32
32 to be aligned with the HEVC
coding structures and to improve reconstruction of smooth image areas. Secondly,
the number of available directional modes is extended from 8 to 33 to improve
modelling of directional textures. In HEVC the whole range of directional predictors
is made available for both luma and chroma blocks, while in H.264/AVC the number
of available directional prediction modes for chroma is limited to two (namely the
directly horizontal and vertical modes). Also sample continuities along the block
boundaries are considered more by redefining the planar mode and applying post-
processing for directly horizontal, vertical and the DC mode. Another advantage
of HEVC is its ability to pad the missing reference samples and allow usage of all
the prediction modes independent of availability of certain reference samples. The
HEVC intra mode coding also uses an approach different from that of H.264/AVC.
Due to the large number of intra modes in HEVC, the luma intra mode is signaled
by using three most probable modes and the selected luma mode is always made
available as one of the candidate modes for the corresponding chroma blocks.
Tab le
4.8
summarizes key differences of intra prediction in H.264/AVC and HEVC.
Lainema et al. [
8
] provides further analysis on contribution of individual intra
prediction tools to the overall coding efficiency.
