Complexity Correlation-Based CTU-Level Rate Control with Direction Selection for HEVC

Rate control is a crucial consideration in high-efficiency video coding (HEVC). The estimation of model parameters is very important for coding tree unit (CTU)-level rate control, as it will significantly affect bit allocation and thus coding performance. However, the model parameters in the CTU-level rate control sometimes fails because of inadequate consideration of the correlation between model parameters and complexity characteristic. In this study, we establish a novel complexity correlation-based CTU-level rate control for HEVC. First, we formulate the model parameter estimation scheme as a multivariable estimation problem; second, based on the complexity correlation of the neighbouring CTU, an optimal direction is selected in five directions for reference CTU set selection during model parameter estimation to further improve the prediction accuracy of the complexity of the current CTU. Third, to improve their precision, the relationship between the model parameters and the complexity of the reference CTU set in the optimal direction is established by using least square method (LS), and the model parameters are solved via the estimated complexity of the current CTU. Experimental results show that the proposed algorithm can significantly improve the accuracy of the CTU-level rate control and thus the coding performance; the proposed scheme consistently outperforms HM 16.0 and other state-of-the-art algorithms in a variety of testing configurations. More specifically, up to 8.4% and on average 6.4% BD-Rate reduction is achieved compared to HM 16.0 and up to 4.7% and an average of 3.4% BD-Rate reduction is achieved compared to other algorithms, with only a slight complexity overhead.