为使视频数据适应异构网络和不同终端设备，基于最新视频编码标准 H．264的跳帧转码技术成为当前的研究热点。针对在 H．264跳帧转码过程中，对所有帧间候选模式进行运动估计而耗费巨大计算资源等问题，在分析相邻帧对应宏块编码特性的基础上，提出一种基于宏块时域相关性的快速模式选择算法。该算法首先统计不同特性视频序列相邻帧对应宏块编码模式的相关性；选取相关性高的模式作为编码模式可选集合，以减少模式选择遍历数目；在对当前宏块进行编码时，根据前一帧对应宏块的模式从上述集合中选取模式进行遍历，以实现率失真优化算法的提前终止。实验结果表明，该算法在控制比特率增长4％的情况下，缩减编码时间近25％，说明算法在保证视频图像具有较好质量的前提下，能够有效降低算法的时间复杂度。%For accommodating video data to heterogeneous networks and different terminal devices,the frame-skipping transcoding technology based on the newest video coding standard H.264 has become the research focus.To solve the problem of huge consumption of computational resource caused by applying motion estimation to all interframe candidate modes during H.264 frame-skipping transcoding process,we propose a fast mode decision algorithm,which is based on temporal correlation of macro-blocks,according to the analysis of character of adjacent frames corresponding to the macro-blocks coding.First,the algorithm summarises the correlation of the adjacent frames in video sequence with different characters in corresponding to macro-blocks coding modes;then it chooses those modes with high correlation as the selective set of coding modes to reduce the number of traversals in mode selection.Finally,when coding the current macro-block, based on the mode of the corresponding macro-block in prior frame,it chooses the candidate modes from the set to traverse,so as to achieve the early-termination of the RDO (rate distortion optimisation ).Experimental results show that the proposed algorithm can reduce approximately 25% coding time with 4% bit rate increasing,which indicates that the algorithm can efficiently reduce the complexity of computational time and simultaneously maintain a high quality of transcoded pictures as well.
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