摘要： 针对屏幕中同时出现视频和文字图片时现有码率控制方法的局限性，提出一种新的码率控制方法。以视频缓存检验器和恒定码率因子控制模式为基础，通过视频窗口检测算法将屏幕内容划分为视频区域和非视频区域，并对不同的区域采取不同的码率控制算法。结合视频区域识别算法，新的码率控制方法分别对帧层和宏块层采用如下方法实现优化：对于帧层，若当前帧为P帧且在I帧周围一定范围内，则该帧通过采用P_SKIP模式降低非视频区域的帧率以补偿I帧的整体质量，提高屏幕视频序列的整体视觉效果；对于宏块层，则根据当前宏块所处区域(视频区域或非视频区域)对宏块量化参数及其波动范围作一定调整。实验结果表明，与x264原有的VBV+CRF模式相比，新的码率控制方法获得的主观评价评分要高40%。
关键词:
屏幕编码,
码率控制,
视频缓存检验器,
恒定码率因子,
视频区域识别算法,
P_SKIP模式
Abstract: With the quick development of modern distance education, screen coding for encoding screen content appears, a new rate control method is proposed for the characteristic of screen content that includes videos, text and pictures. It is based on the Video Buffer Verifier(VBV) and Constant Rate Factor(CRF) mode and divides the screen content into video area and non-video area using a video recognition algorithm which can recognize the position of the video window on the screen. Combining with the video recognition algorithm, the new rate control method respectively improves rate control method of the frame layer and the macro block layer through following method. For frame layer, if the current frame is a P-frame, and the P-frame is within a certain range around the I-frame, this frame lowers the frame rate of non-video area by adopting P_SKIP into the non-video area to enhance quality of the nearest I-frame so that improves the overall visual effect of the screen sequence. For macro block layer, the new rate control method adjusts the value of Quantization Parameter(QP) and limits its fluctuation range by the location of the current macro block, whether it is in video area or not. Experimental results prove that the new rate control method can score 40% more than the original VBV+CRF mode in the subjective assessment.
Key words:
screen coding,
rate control,
Video Buffer Verifier(VBV),
Constant Rate Factor(CRF),
video area recognition algorithm,
P_SKIP mode
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