目前傳統的位元速率控制方法可以控制編碼後的總位元數大小和目標位元數大小相近,但由於沒有參考人類視覺的特性,於人眼視覺上較重要的區域並無法保證能有較佳的影像品質。因此,本論文提出以使用者感興趣區域(Region-of-Interest; ROI)為主的位元速率控制方法,此方法將分配較多的位元資源於ROI區域的編碼上,但依舊保證能達到精確的位元控制結果。此種使用者感興趣區域為主的位元速率控制方法特別適用於視訊電話以及視訊監視系統等應用上。本論文的位元速率控制方法有以下幾個特點:首先ROI區域的選擇來自基於人眼視覺的特徵的圖(Saliency Map);此外,ROI區域的影像品質並非固定,而可以由使用者決定;最後考慮連續畫面間ROI/非ROI區域的畫面品質平滑性、ROI/非ROI區域交接處品質的連續性等限制下,量化參數則被適當調整,以達到較佳的視覺效果。實驗結果證實本論文所提出的位元速率控制方法不管是在主觀品質或客觀品質的評估上皆有不錯的表現,同時能保證壓縮完的位元數與目標位元數相近。
Conventional rate control schemes focus on making output bit rate approach a target value and are deficient in ensuring a higher quality of ROI (Region-of-Interest) than others in a frame. In this paper, we propose a new scheme for H.264/AVC, aiming to allocate more bit resource for the encoding of ROI and still maintain the accuracy of the output bit rate. ROI-based rate control algorithms can find their specific advantages in video telephony and video surveillance applications. Our proposed scheme is based on the one implemented in H.264/AVC JM software, but enhanced with several features: ROI determination with saliency map, tunable quality factor for ROI, two-channel (ROI & non-ROI) rate control, and QP adjustment with constraints from temporal and spatial domains, as well as from ROI/non-ROI adjacency boundaries. Experiment results show both advantages in objective PSNR and subjective evaluations for ROI, while making the output bit rate accurate as before.