1. / Fast block partitioning method in HEVC Intra coding for UHD video /
Hello everybody, my name is Nicolas Dhollande, from b<>com and today I’m going to present you a “Fast block partitioning method in HEVC Intra
coding for UHD video”, a work achieved by Xavier Ducloux, Olivier Le Meur, Christine Guillemot, and myself.
Authors: Nicolas Dhollande, Xavier Ducloux, Olivier Le Meur, Christine Guillemot
11/28/2018

2. High Efficiency Video Coding (HEVC)
Problem addressed
High Efficiency Video Coding (HEVC)
Latest video standard (2013)
Joint Coding Team on Video Coding (JCT-VC)
HEVC bitrate savings over MPEG.4/AVC with the same perceived quality:
64% in UHD, 62% in HD [Tan and al, JCTVC-Q0204]
HEVC will be key for UHD deployment
Goal: limit as far as possible the encoding complexity with a limited quality loss
HEVC: 2-4x more encoding power than MPEG.4/AVC
UHD: 4-5x more encoding power than HD
We’ll speak about the latest video standard …
At the beginning, the goal of the standardization was to halve the bitrate required… it turns out that HEVC achieves …
HEVC necessary for the UHD deployement, given that UHD contains 8 times more information to encode, it allows to transmit UHD with 10-20Mbits .. but also to alleviate the recent video traffic growth over Internet.
the encoding power is … and up to …

3. HEVC vs previous standard MPEG.4/AVC:
Problem addressed
HEVC vs previous standard MPEG.4/AVC:
Coding Tree Unit (composed of 64x64 to 8x8 sizes CU’s)
more accurate Intra coding (35 luma predictions) within a PU
extended transforms (32x32 to 4x4) with a QT structure
The codec brings new features compared to MPEG.4/AVC…
A Slice is divided in 64x64 size Coding Tree Units, which are divided in a quad-tree of Coding Units, ranged from the 64x64 size to the 8x8 size.
In Intra, CU contains 1 PU, or 4 sub –PU for the minimum CU size…
In a PU, 35 prediction patterns ...
Instead of using either 4x4 or 8x8 transforms, HEVC enables to divide a CU in a quadtree of TU, with sizes 4x4 to 32x32.

4. Highest speed-up potential with UHD !
Problem addressed
The quad-tree structure leads to intensive computation when the encoder builds the CTU
Depth first partitioning process: 85 CU / 341 Intra PU to be evaluated
Just a few CU coded in the syntax
What if the encoder knows the optimal PU quad-tree in advance …
The determination of the optimal quad-tree structure in a CTU is heavy in term of encoding power.
In the HEVC reference software, a depth-first partitioning strategy is employed … consists in evaluating 85 CU for a CTU
Imagine that the encoder knows in advance the optimal PU quad-tree, the potential of encoding time speed-up is more than 6x in UHD, and up to 4x in HD format
This motivates
Video sequences
Intra only (AI)
Speed-up ratio
EBU set UHD
6.33
Class B 1080p
3.82
Highest speed-up potential with UHD !
Encoding of the optimal PU quad-tree vs reference

5. Typical algorithms for fast Intra partitioning:
Problem addressed
Typical algorithms for fast Intra partitioning:
Intuition that flat regions are divided in large blocks, and sharp/complex regions are divided in small blocks:
Judgement process based on the variance complexity on the downscaled content [Tian & Goto, 2012]
Judgement process based on the entropy [Zhang, 2013]
Modes inference from a lower resolution pre-coding pass. [Dhollande, 2014]
Typically, papers propose to use pre-analysis of the content in order to skip the less probable blocks from the depth first search
Another paper, propose to first encode a downscaled version of the video content, and then infers the modes in the target encoder.
In this paper, we focus on the first category which uses pre-analysis of the content, and we simply use the variance for the judgement process.

6. Our approach: a fast Intra partitioning scheme for the Intra coding
Off-line training
Low complexity encoder
A fast Intra partitioning scheme consists of an off-line learning of thresholds in the one hand, and a low complexity encoder which uses these thresholds on the other hand …

7. Let S be a Bernoulli random variable with parameter P:
Motivations
Let S be a Bernoulli random variable with parameter P:
P: proba. that a PU is further split by the ref. encoder
σ: variance of the luminance pixels into the current PU
d: PU depth into the CTU {0,1,2,3}
qp: quantization parameter {22,27,32,37}
Let define S, a Bernoulli random variable which can take 2 values: split or not split…
P is the probability that the current block is further split given the variance, the PU depth and the QP.
Below is an example of this conditional probability with QP equal to 32 and for the PU size 32x32 (d=1)
sigma is represented with a logarithm scale
The probability to split in red increases with the sigma value…
Block partitioning probability in 32x32-size PU given σ within a qp=32 reference UHD encoding

8. Two thresholds on the variance for the splitting decision:
Off-line training
σ well associates the local signal characteristics with the HEVC Intra block sizes
Two thresholds on the variance for the splitting decision:
ThL such that:
ThH such that:
is the probability level used to select ThL and ThH from the conditional probabilities.
It can be stated that sigma well associates…
(PL) ?
(1-PL)
High proba of non splitting
High proba of splitting
ThL
ThH

9. Off-line training PL=90% ThL ThH ThL ThH d=1 qp=22 d=1 qp=37 ThL ThH
Accuracy++
Speed-up++
ThL
ThH
ThL
ThH
d=1 qp=22
d=1 qp=37
ThL
ThH
ThL
ThH
d=3 qp=22
d=0 qp=37

10. Off-line training PL=60% ThL ThH ThL ThH d=1 qp=22 d=1 qp=37 ThL ThH
Accuracy++
Speed-up++
ThL
ThH
ThL
ThH
d=1 qp=22
d=1 qp=37
We’ll show with experimental results that an high PL value favours the accuracy whereas a low PL value favour the encoding time speed-up
ThL
ThH
ThL
ThH
d=3 qp=22
d=0 qp=37

11. A reference encoding is performed for each qp.
Off-line training
A reference encoding is performed for each qp.
Conditional probabilities
are calculated
for each d and qp.
Depending on the PL parameter,
a couple (ThL, ThH) is chosen for each
d and qp.
To summarize,

12. Proposed scheme for the low complexity encoder
PU classification:
Our fast partitioning scheme:
Variant 1:
unconstrained CMP class
(reference partitioning)
Variant 2:
constrained CMP class
(1 split only)
Next PU level

13. Our approach: a fast Intra partitioning scheme for the Intra coding
Summary
Problem addressed
Our approach: a fast Intra partitioning scheme for the Intra coding
Motivations
Off-line training
Proposed scheme for the low complexity encoder
Results
Strengths of our algorithm / conclusion

14. Settings Performance measure Results HM test model v11.0
All Intra (AI) – Main profile
4 quantization parameters: 22,27,32,37
10 EBU UHD-1 sequences for the off-line training
750 frames each
Performance measure
Encoding time speed-up ratio = TREF / TALGO Vs
Average BD-rate

15. Performance of the low complexity encoder in UHD:
Results
Performance of the low complexity encoder in UHD:
In function of the application we want to address, PL value allow us to obtain different operating points.

16. Performance with others sequences and resolutions
Results
Performance with others sequences and resolutions

17. Our approach: a fast Intra partitioning scheme for the Intra coding
Summary
Problem addressed
Our approach: a fast Intra partitioning scheme for the Intra coding
Motivations
Off-line training
Proposed scheme for the low complexity encoder
Results
Strengths of our algorithm / conclusion

18. Strengths of our algorithm / conclusion
Simple fast Intra partitioning scheme which skips the less probable PU from the decision process.
High accuracy of our early terminations
Speed-up of 2x with no loss
Strong complexity reduction in UHD
About 6x against a limited loss of 5%
No subjective quality degradation
Possible improvements:
On-line learning of thresholds (each N frames)
Improved Intra modes pre-selection using the texture orientations
Fast RQT construction
We have shown that the threshold database we have generated is consistent enough for most of the UHD sequences.
However, the fast partitioning can be combined with others kinds of fast algorithms …

19. / nicolas.dhollande@b-com.com /