網路有限的頻寛下，為使解碼端能得到好的編碼影像品質，位元率控制機制扮演重要角色。H.264/AVC參考編碼器JM中的位元率控制主要依JVT-G012文件提案進行實作。JVT-G012位元率控制演算法並未對場景切換進行處理，當遇到場景切換時會造成編碼位元數量大增，此時緩衝區容易滿溢，後續FRAME可用位元數急速下降，造成整體編碼品質下降。本論文利用連續兩張畫面間16x16模式運動估計求得之SAD (Sum of Absolute Differences)比值以進行場景切換之偵測。當偵測到是場景切換畫面後，我們將目前GOP (Group of Pictures)剩餘的畫面當作新的GOP並提出依畫面複雜度和緩衝區狀態的場景切換適合之量化參數。實驗結果指出，與JM17.2版本模擬比較，我們所提出的方法平均PSNR能提高大約1.07dB並使用較少的緩衝區。Under the constraint of limited channel rates, the decoder client might get better video quality by using rate control. The rate control of H.264/AVC reference software JM is implemented according to the technical document JVT-G012. However, the method in JVT-G012 does not particularly deal with scene changes, resulting in coded bits significantly increased. This situation can cause the buffer overflow and even influence the quality of remaining frames. In this thesis, we first propose a scene change detection method by utilizing the SAD ratio of consecutive two frames after performing 16x16 motion estimation. After scene change is detected, we regard the remaining frames in current GOP as a transition GOP and propose a suitable quantization parameter to the first I-frame in transition GOP based on frame complexity and buffer fullness status.Compared with the method in JM17.2, the proposed method can improve average PSNR by 1.07 dB with less buffer size.
