说明：  dwt数字水印对二、三层中的中频、高频小波系数进行操作。因为第二、三 层的小波系数是上一层低频系数进一步分解得到的结果
(dwt digital watermarking, three intermediate frequency, high-frequency wavelet coefficients. Second, the wavelet coefficients of the three is a layer of low-frequency coefficients further decomposition of the results obtained)