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be split independently, and selected for the CU if the
sum of the RD costs of four sub-CUs is smaller than
the RD cost of a large CU (Kim et al., 2012; Huang
et al., 2014). In intra mode, there are up to 35 possible
intra prediction modes in H.265/HEVC while only 9
modes are allowed in H.264/AVC. Note that the PU
can only be split into a square shape, and each PU
will choose the best matching prediction mode.
1.2 Related Work
In order to reduce the complexity of intra coding in
H.265/HEVC, there have been many algorithms pro-
posed in the literature focusing on fast intra mode de-
cision and fast CU size decision. The decision mak-
ing can be modified such as in (Piao et al., 2010) to
speed up the process. The algorithms used in (Yan
et al., 2012) and (Shen et al., 2013a) are based on the
correlation of adjacent prediction costs, which speeds
up the process but the extra computation load is still
high. In (Cho and Kim, 2013), an early CU split-
ting and pruning method is discussed and the method
in (Shen et al., 2013b) replaces the fixed tree-depth
range for each CU with an adaptive CU depth deci-
sion depending on the previously encoded slices and
neighbouring CUs. In (Zhao et al., 2011), a smaller
candidate set is proposed that does not lead to notice-
able quality loss, this compared various settings with
different mode candidates for the full Rate-Distortion
Optimized Quantization (RDOQ).
To reduce the complexity, (Choi and Jang, 2012)
proposed a tree pruning algorithm for fast CU deci-
sion making. It skips the further CU splitting if the
current CU chooses the SKIP mode. Such SKIP-
mode-based early termination cannot be applied to
the intra prediction. Moreover, some papers proposed
alternative thresholds condition: a local edge infor-
mation is obtained by calculating edge feature pa-
rameters in (da Silva et al., 2012) and (Jiang et al.,
2012) employed a gradient based method to speed
up CU decisions. According to (Min and Cheung,
2015; Jamali et al., 2015; Lim et al., 2015), the results
of HEVC intra coding based on different versions of
HMs are similar due to the fact that HEVC intra cod-
ing does not involve significant changes among these
versions. Therefore, it is acceptable to compare these
methods under different HM versions. Then, a gra-
dient based fast intra mode decision is proposed in
(Chen et al., 2013), which based on the gradient direc-
tions for each CU, selects only a small set of modes
for further intra prediction. Hence, a large portion of
modes are removed from the computation.
1.3 Contribution
In this paper, we propose a fast intra mode prediction
algorithm while maintaining the RD performance by
analysing picture feature statistics. An early termi-
nation technique for CU is applied in H.265/HEVC
for experiments. Corresponding to the required en-
coding efficiency and the quality of encoded picture,
we use the subsampling result of deviation values of
Luma/Chroma in different sizes for video sequences.
Since there are several Chroma-Subsampling in the
standard, we propose the weight of Luma/Chroma
will depend on the ratio of common Sampling sys-
tems (explained in Sec. 2.2). Further, to balance
the quality and performance, we define a thresh-
olds (T ) without using static decision thresholds, and
rely on a comparison between the Quantization Pa-
rameter (QP) and CU size of the coding tree un-
der test, which have to be updated at each recur-
sion. The use of adaptive thresholds is fundamental
in H.265/HEVC as the number of possible partition-
ing and coding modes is more appropriate then static
thresholds for each of them.
The rest of the paper is organized as follow: Sec. 2
provides details of the idea of the fast intra prediction
algorithm including the proposed weighed sampling
operator and adaptive thresholds. Sec. 3 gives the de-
tails of the concept about the thresholds and how to
achieve this in different depth layers. The simulation
results are given in Sec. 4, and Sec. 5 concludes the
paper.
2 PROPOSED METHOD
The intra mode prediction defined in the
H.265/HEVC standard allows 35 (including Pla-
nar, DC mode and 33 angular) predictions. Planar
and DC prediction modes can provide predictions
for image areas with smooth and gradually changing
content. The angular intra predictions are designed
to model different directional structures typically
present in picture (see Fig. 2).
2.1 Fast Intra Prediction Algorithm
As a range of sample methods, the angular predictions
are also useful in smooth PU blocks without high fre-
quency components for complex textures that cannot
be properly modelled with any of the directional pre-
dictors. It can easily perceive that a large size CU is
more suitable for some homogeneous or flat regions
in the image. For discriminating texture, CUs that are
not split into sub-CUs are expected to have smaller
Fast Intra Prediction Algorithm with Enhanced Sampling Decision for H.265/HEVC
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