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Efficient Bit Allocation and CTU level Rate Control for HEVCPicture Coding Symposium, 2013, IEEE

Junjun Si, Siwei Ma, Wen Gao

Insitute of Digital Media, Peking University, Beijing, China

2 Overview

Introduction Analysis and Proposed Bit Allocation Scheme Proposed CTU Level Rate Control Scheme Experimental Results Conclusion

3 Overview

Introduction Analysis and Proposed Bit Allocation Scheme Proposed CTU Level Rate Control Scheme Experimental Results Conclusion

4 Introduction

The ultimate aim of video coding is to provide the best video quality at the receiver side, subject to the available channel bandwidth.

Given a bit budget, the best coding quality can be achieved by optimal bit allocation and accurate rate control schemes.

The schemes is to meet the bit allocation budget by setting an appropriate QP for each coding unit of the video.

In this paper, CTU level rate control algorithm along with an efficient bit allocation scheme is proposed to generate higher and smoother coding quality for HEVC encoder.

5 Introduction

Step1 Step2

6

Several classic R-D models and related work will be discussed in this section.

1. ρ-domain model [2]

2. Quadratic model (Q-domain) [3]

3. R-domain model [1]

4. Exponential model [4][5]

Introduction

[1] B. Li, H. Li, L. Li, and et al. “Rate Control by Rr-lambda Model for HEVC,” in Joint Collaborative Team on Video Coding (JCT-VC) 11th Meeting, JCTVC-K0103, Shanghai, China, 2012.[2] Z. He, Y.K. Kim and S.K. Mitra, “Low Delay Rate Control for DCT Video Coding via ρ-domain Source Modeling,” IEEE Transactions on Circuits and Systems for Video Technology, vol.11, pp.928–940, 2001.[3] T. Chiang and Y. Q. Zhang, “A New Rate Control Scheme Using Quadratic Rate-Distortion Modeling” IEEE Transactions on Circuits System and Video Technology, vol.7, pp 246-250, Feb.1997.[4] L. Tian, Y. Sun, Y. Zhou, and et al., “Analysis of Quadratic R-D Model in H.264/AVC Video Coding,” 17th IEEE International Conference on Image Processing, pp. 2853-2856, China, 2010.[5] Y. Zhou, Y. Sun, Z. Feng, and et al., “New Rate-Distortion Modeling and Efficient Rate Control for H. 264/AVC video coding,” Signal Processing: Image Communication, vol. 24, pp 345-356, 2009.

7 Introduction

1. ρ-domain model

2. Quadratic model

After observing the relationship between Qstep and quantization, the equation is modified by [8]

ρ is the percentage of zeros among the quantized transform coefficient.

8 Introduction

3. R-domain model

4. Exponential model

intra-picture complexity

similar to MAD, may lead to the model error

9 Overview

Introduction Analysis and Proposed Bit Allocation Scheme Proposed CTU Level Rate Control Scheme Experimental Results Conclusion

10 Before Proposed Bit Allocation Scheme

Current HEVC bit allocation problem

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11 Proposed Bit Allocation Scheme

Thus the computational complexity to generate the operational rate distortion curves, it cannot work well for practical video applications.

Three levels’ simple but efficient bit allocation scheme for HEVC is proposed, which are GOP, frame, CTU levels.

12 Proposed Bit Allocation Scheme – GOP level

Proposed method

R : target bitrate

F : frame rate

N : GOP size

HM 10.0

If , then

If , then

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13 Proposed Bit Allocation Scheme – frame level

By Table1, taking low delay configuration as an example, frames in depth 0 are critical in the reference relationship.

Therefore, it’s reasonable to allocate more bits to these frames so that they can be coded as high quality.

14 Proposed Bit Allocation Scheme – frame level

Proposed method For picture depth 0 and I – frame

1. is a constant, set to 0.15

2. is a constant, set to 0.3(RA) and 0.45(LD)

3. is the number of frames of depth 0 in the GOP.

For remaining picture

HM 10.0 For picture level :

1. All pictures with same is one possible choice

2. Hierarchical bit allocation is another choice

15 Proposed Bit Allocation Scheme – CTU level

Proposed method Assume that target bitrate for

this frames is T :

’ is update with the actual coding bits m of the CTU :

HM 10.0 For basic unit :

16 Effectiveness of The Proposed Bit Allocation Scheme

The proposed bit allocation scheme is implemented on HM10.0.

Table2 below are the experimental result

17 Overview

Introduction Analysis and Proposed Bit Allocation Scheme Proposed CTU Level Rate Control Scheme Experimental Results Conclusion

18 Proposed CTU Level Rate Control Scheme

In authors’ previous work[1][2], they proposed a frame level rate control scheme. At CTU level the QP for each CTU is decided with a reaction factor based on the frame level QP.

Besides, a CTU level QP adjustment strategy is designed. The SATD value of each CTU in the frame is calculated and store for CTU level QP adjustment.

[1] J. Si, S. Ma, W. Gao, And M. Yang. ―Adaptive Rate Control For HEVC,‖ ITU-T Sg16 Contribution, Jctvc- I0433, Geneva, Ch, Apr.-May 2012 [2] J. Si, S. Ma, X. Zhang, W. Gao, ―Adaptive Rate control for High Efficiency video Coding,‖ in Proceedings of SPIE Conference on Visual Communications and Image Processing, VCIP, San Diego, USA, Nov.2012

19

CTU level rate control : Instead of applying the rate distortion model, authors proposed to use a feedback strategy to generate CTU QP based on the coding status (reaction factor , can be applied on both frame and CTU level).

SATD means Sum of Absolute Transformed Difference

CTU Level Rate Control

20 CTU Level Rate Control

With reaction factor, the relation between target bitrate and actual coding status can be updated.

Meanwhile, with reaction factor of both frame and CTU, the quantization scale can be expressed as :

is the quantization scale and can be mapped to QP :

21 CTU Level Rate Control – Compared with HM 10.0

Proposed method HM 10.0

Different level may have different parameter

Target bit-rate corresponding to different level

22 QP Adjustment Strategy

In CTU level rate control, it’s very likely that few bits can be remained for CTUs in the posterior part of the frame, especially under the low bitrate conditions.

As a result, QP for these CTUs increase dramatically to a very large value and the CTUs are consequently encoded in poor quality.

If the target bitrate for uncoded CTUs is less than threshold (0 in their experiment), QP adjustment strategy need to be applied.

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23 QP Adjustment Strategy– Compared with HM 10.0

Proposed method CTU QP clipping

HM 10.0 CTU QP clipping

24 QP Adjustment Strategy

The proposed QP adjustment scheme is implemented on HM10.0.

Table3 below are the experimental result

25 Overview

Introduction Analysis and Proposed Bit Allocation Scheme Proposed CTU Level Rate Control Scheme Experimental Results Conclusion

26 Experimental Design

Experiments are designed and implemented on HM10.0.

The R-D performance of proposed rate control scheme is compared with JCTVC-K0103 (Configuration is Table4 below).

27 Experimental Result – Rate Distortion Performance

The authors employ the JCTVC-K0103 rate control as anchor.

28 Experimental Result – Smoothness of the Coding Quality

Another advantage of the proposed rate control scheme is it helps to smooth the coding quality and reduce the quality fluctuation.

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29 Overview

Introduction Analysis and Proposed Bit Allocation Scheme Proposed CTU Level Rate Control Scheme Experimental Results Conclusion

30 Conclusion

The proposed allocation method takes the hierarchical coding structure of HEVC into consideration.

For CTU level rate control, QP determination at CTU level is based on the frame level QP with feedback of the coding status of CTUs.

Besides, a CTU level QP adjustment strategy is proposed to improve the coding performance just like HM10.0.