一种基于x265的帧级基础量化参数确定方法

A Method for Determining Basic Quantization Parameters at Frame Level Based on x265

为了提高视频编码的率失真性能,x265编码器中的前瞻模块为码率控制部分做了许多决策,如帧类型决策、编码单元CU的量化参数(Quantized Parameter,QP)偏移等。然而,x265中忽略了初始帧基础量化参数PQ0的确定,导致所有后续以初始帧为参考帧的帧图像质量下降。同时,帧级QP只由长期历史帧复杂度决定,导致对最近的帧变化不敏感。针对上述问题,提出一种充分利用前瞻模块来修正初始帧基础P_(Q0)以及通过下采样SATD值改进P帧基础QP的算法。实验结果表明,在默认预设条件下,提出的算法使得率失真性能BD-rate(PSNR)与BD-rate(SSIM)分别降低了14.33%与16.22%,同时满足比特约束要求,有效提升了编码质量。

In order to improve the rate distortion performance of video coding, the look ahead module in the x265 encoder makes many decisions for the rate control part, such as frame type decision, Quantization Parameter(QP) offset of the encoding unit CU, etc. However,the determination of the basic quantization parameter P_(Q0)of the initial frame is ignored in x265, resulting in the degradation of the image quality of all subsequent frames with the initial frame as the reference frame. At the same time, the frame level QP is only determined by the long-term history frame complexity, which makes it insensitive to the latest frame changes. To solve the above problems, this paper proposes an algorithm that makes full use of the look ahead module to modify the initial frame base P_(Q0)and improves the P frame base QP by down sampling the SATD value. The experimental results show that under the default preset conditions, the proposed algorithm reduces the rate distortion performance of BD rate(PSNR) and BD rate(SSIM) by 14.33% and 16.22%, respectively, while meeting the bit constraint requirements, and effectively improving the coding quality.