多重視訊編碼器在單一視訊源情形下之恆定畫質位元率控制演算法

由於傳輸頻寬以及儲存空間的限制，壓縮過的影像訊號必需控制在一定的大小之內，於是在視訊壓縮系統之中，位元率控制演算法便扮演了一個很重要的角色。根據編碼所得到的位元流，位元率控制演算法可以分為兩種，第一種是固定頻寬之演算法，第二種則是固定品質之演算法，一般來說，由於沒有固定頻寬之限制，使用固定品質之位元率控制演算法通常可以得到較好的視訊品質。在一般需要即時編碼的系統中，因為缺乏目前這張畫面的資訊，恆定的畫面品質是很難達到的。而在本論文之中，我們針對一數位監視系統設計一即時的恆定畫質位元率控制演算法，我們所提出的這個演算法沒有缺乏目前這張畫面資訊的缺點，因此能達到恆定畫質之目的。此監視系統由一高解析度之視訊源、一基層編碼器及一重點區域編碼器所構成。由視訊源所擷取的全景影像由基層編碼器使用較低的解析度進行編碼，使用者感興趣之區域的影像則由重點區域編碼器在原來的解析度進行編碼。這兩個編碼器所編碼得到的位元流是可以獨立解碼的，重點區域之位元流不需要基層位元流即可進行解碼。由於基層畫面以及重點區域畫面有部分內容是相似的，只是在不同的解析度，因此我們可以預期，這兩個位元流的畫質變化趨勢是非常相似的，於是我們便利用基層編碼器在編碼時所得到的失真值來計算需要分配多少的位元給重點區域編碼器的這張畫面，以使這張畫面能和之前的畫面擁有相同的畫質。利用兩者的關聯性，不僅能得到恆定的畫質，同時也提升了整體的視訊品質。

關鍵字

視訊編碼；位元率控制；位元分配；恆定品質

並列摘要

Because of the limited channel bandwidth and storage spaces, rate control component is an important part in a video coding system, which controls the output bit-rate to satisfy the given constraints. Though not a part of the video coding standard, it receives a lot of attention and many different algorithms have been proposed. In this thesis, we consider the rate control algorithm for a video coding system for surveillance applications. This video coding system consists of one "base encoder" that encodes a down-sampled full-view version of the input video sequence, and one "region of interest (ROI) encoder" that encodes the region of interest of the input video at the original resolution. It is important to make these two different video bitstreams independently decodable. Therefore, two independent video encoders are used to encode them separately. The inter-relationship between the base and the ROI sequence is explored, and is applied to improve the rate control algorithm for the ROI encoder. Since the two sequences contain the same content at different resolution, the quality trends of these two sequences should be very similar. Based on this observation, a constant-quality rate control algorithm for the ROI encoder is proposed. The current frame in the ROI sequence can be encoded to have similar quality as the previous frames by allocating bits according to the distortion of the current and previous frames in the base sequence. Simulation results show that the proposed algorithm can achieve significant reduction in the quality variation. In addition, the overall quality is improved and the bit-rate is saved comparing to the rate control algorithm in JM 8.4.

並列關鍵字

video coding ； H.264/AVC ； bit allocation ； rate control ； constant quality

參考文獻

[1] ISO/IEC JTC1 IS 11172, Coding of Moving Picture and Coding of Continous Audio for Digital Storage Media up to 1.5Mbps, ISO/IEC, 1992.

[4] ITU-T Recommendation H.261, Video Codec for Audiovisual Services at p×64 kbits, ITU-T, Mar. 1993.

[5] ITU-T Recommendation H.263, Video Coding for Low Bit Rate Communication, ITU-T, May 1996.

[8] T. Wiegand and B. Girod, “Lagrange multiplier selection in hybrid video coder control,” Proceeding of Internation Conference on Image Processing, pp. 542–545, Oct. 2001.

[10] W. K. Pratt, Digital Image Processing, New York:Wiley, 3rd edition, 2001.

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多重視訊編碼器在單一視訊源情形下之恆定畫質位元率控制演算法

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