说明：  几个关于图像质量评价的函数。包括psnr峰值信噪比，均方误差rms，正则均方误差nmse等。可用于去噪图像和压缩图像的质量评价
(several image quality evaluation of the function. Including psnr PSNR, mean square error rms, it is mean-square error nmse other. Images can be used for de-noising and compression of image quality evaluation)