  The High Efficiency Video Coding (HEVC/H.265) standard doubles the
compression efficiency of the widely used H.264/AVC standard. For practical
applications, rate control (RC) algorithms for HEVC need to be developed. Based
on the R-Q, R-${\rho}$ or R-${\lambda}$ models, rate control algorithms aim at
encoding a video clip/segment to a target bit rate accurately with high video
quality after compression. Among the various models used by HEVC rate control
algorithms, the R-${\lambda}$ model performs the best in both coding efficiency
and rate control accuracy. However, compared with encoding with a fixed
quantization parameter (QP), even the best rate control algorithm [1] still
under-performs when comparing the video quality achieved at identical average
bit rates. In this paper, we propose a novel generalized
rate-distortion-${\lambda}$ (R-D-${\lambda}$) model for the relationship
between rate (R), distortion (D) and the Lagrangian multiplier (${\lambda}$) in
rate-distortion (RD) optimized encoding. In addition to the well designed
hierarchical initialization and coefficient update scheme, a new model based
rate allocation scheme composed of amortization, smooth window and consistency
control is proposed for a better rate allocation. Experimental results
implementing the proposed algorithm in the HEVC reference software HM-16.9 show
that the proposed rate control algorithm is able to achieve an average of BDBR
saving of 6.09%, 3.15% and 4.03% for random access (RA), low delay P (LDP) and
low delay B (LDB) configurations respectively as compared with the
R-${\lambda}$ model based RC algorithm [1] implemented in HM. The proposed
algorithm also outperforms the state-of-the-art algorithms, while rate control
accuracy and encoding speed are hardly impacted.
