Rate-distortion optimization (RDO) can effectively improve the compression performance for encoders

which is an important topic in video coding. In this paper

the Lagrange multiplier based RDO method is reviewed comprehensively from several aspects including independent RDO

dependent RDO

bit allocation for rate control

etc. First

an example is employed to explain how the constrained optimization problem is converted into an unconstrained optimization problem based on the Lagrange multiplier method

and the relationship between Lagrange multiplier and quantization parameter is analyzed according to relevant references. Second

the rate-distortion dependencies in the process of video coding are analyzed

and the dependent RDO methods presented in recent years are classified and discussed. Third

the effect of the rate-distortion dependencies on bit allocation for rate control is discussed. Finally

the features and performance of different RDO methods are tabulated for analysis and comparison

and the existing challenges and the research works deserving further exploration are presented.