两个常用的数字水印评价函数：信躁比比（SNR）和峰值信躁比（PSNR）-two commonly used digital watermarking evaluation function : Oh habibi letter (SNR) and peak letter impatient than (PSNR)

这是一个求图像PSNR的子函数，用的时候直接调用很方便-This is a PSNR image function, when used directly call is very convenient