Graphics Reference
In-Depth Information
The number of luma intra mode candidates entering full RDO is determined
according to the corresponding PU sizes as eight for PUs of size 4
4and8
8
PU, and three for other PU sizes. For those luma intra mode candidates as well
as luma intra modes which are a part of the most probable mode set, intra sample
prediction and transform coding are performed to obtain both the number of required
bits and the resulting distortion. Finally, the luma intra mode which minimizes the
rate-distortion cost is selected. For the chroma intra encoding, all possible intra
chroma modes are evaluated based on their rate-distortion costs as the number of
intra chroma modes is much smaller than that of luma modes. It has been reported
that this kind of technique can provide negligible coding efficiency loss (less than
1 % increase in bit rate at aligned objective quality) compared to a full rate-distortion
search while reducing the encoding complexity by a factor of three [
8
].
In the literature, several fast intra coding algorithms have been studied in the
context of HEVC. One popular approach for achieving this purpose is to reduce
the number of block sizes to be tested in the HEVC intra coding. Shen et al. [
13
]
proposed a fast CU size decision and mode decision algorithm for HEVC intra
coding based on the previous decisions in spatially nearby blocks. It results to about
21 % encoding time decrease with negligible loss of coding efficiency. In [
4
], the
fast CU splitting and pruning decisions according to a Bayes decision rule for the
HEVC intra coding was proposed and about 50 % encoding time reduction was
reported with 0.6 % bit rate increases.
Another approach is to reduce the number of intra mode candidates based
on local image characteristics. Zhao et al. [
19
] proposed forced use of the most
probable mode with a reduced number of luma intra mode candidates in the rate-
distortion optimization stage. Around 20 % encoding time reduction was achieved
with this approach and the method was also decided to be included in the HEVC
reference software. In [
3
,
5
,
14
], it has been shown that reducing the intra mode
candidates based on directionalities of the source and neighboring blocks about 20-
30 % encoding time reduction can be achieved.
4.6
Coding Efficiency and Decoder Complexity
4.6.1
Coding Efficiency
Coding efficiency of HEVC intra coding has been reported to exceed significantly
that of earlier video and still picture codecs. For example, Hanhart et al. [
6
]
evaluated both objective (PSNR) and subjective (MOS) quality of HEVC intra
coding, JPEG and JPEG 2000 using the JPEG XR test set. Test material in this case
consisted solely of camera captured photographic content. The study concluded that
HEVC outperformed legacy still picture codecs with most significant differences
at bit rates below 1.00 bpp. Table
4.6
summarizes the average bit rate reductions
HEVC obtained over other codecs in this study.
