位元率控制演算法在現今的影像編碼及有限頻寬的串流應用中扮演著相當重要的角色。然而，大多數的研究都著重於提升影像編碼的效率，而忽略了影像品質的穩定度。許多位元率控制法因為編碼參數的初始值不理想，造成影像初期的品質有劇烈的變動。另外，利用畫面間參考關係而使用的階層化編碼架構雖然可以有效的提升編碼效率，但也帶來畫面間更大的品質差異。
這篇論文提出一個目標為穩定影像視覺效果的兩階段位元率控制演算法，首先將影像降取樣為四分之一的大小，並使用兩個不同的拉格朗日乘數( ) 值進行兩次編碼。從降取樣影像的編碼結果預測原始影像的lnR-ln 模型並計算一個固定的 值，應用在原始影像編碼的所有畫面中。另外我們也採用一個量化參數(QP) 的調整策略，利用編碼降取樣影像時記錄的位元率與失真資訊，來調整編碼原始影像時使用的QP 值，以進一步讓畫面間品質更加穩定並降低位元率誤差。實驗結果顯示，這篇論文所提出的兩階段位元率控制方法相較於HEVC 的測試軟體HM16.9，BDBR 為8.28%，平均可以降低96.76% 的品質變動程度。

While most researches of rate control focus on the coding efficiency, the fluctuation of video quality is seldom considered. Many rate control schemes suffer from un-reliable initialization of coding parameters, and lead to seriously inconsistent qual-ity. Besides, the hierarchical structure for frame references introduces more quality fluctuations, although it improves the coding efficiency significantly. This thesis presents a two pass rate control method that aims for a consistent visual quality. In the first pass the video is down-sampled by four times, and then encoded two times using two different Lagrange multiplier ( ) values. The results of encoding down-sampled video is used to build the log R-ln model second pass to use and derive a fixed value which is applied for all frames in the original video. A quantization parameter (QP) adjustment policy is adopted to maintain a consistent quality and lower the bitrate error. Experimental results show that the proposed two pass rate control method has 8.28% BDBR and can reduce the fluctuation of video quality to be averagely 96.76% less than that encoded by the HEVC Test Model (HM16.9).

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