  Due to the excellent compression and high real-time performance, x265 is
widely used in practical applications. Combined with CU-tree based
pre-analysis, x265 rate control can obtain high rate-distortion (R-D)
performance. However, the pre-analysis information is not fully utilized, and
the accuracy of rate control is not satisfactory in x265 because of an
empirical linear model. In this paper, we propose an improved cost-guided rate
control scheme for x265. Firstly, the pre-analysis information is further used
to refine the bit allocation. Secondly, CU-tree is combined with the
lambda-domain model for more accurate rate control and higher R-D performance.
Experimental results show that compared with the original x265, our method can
achieve 10.3\% BD-rate gain with only 0.22\textperthousand bitrate error.
