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 functions on the image quality evaluation. Including psnr PSNR, mean square error rms, is the mean square error nmse and so on. Can be used to image denoising and compression ima