对二维信号X（m,n）进行离散小波变换，注：m、n均为偶数，可以得到低频部分的，大小为垂直方向的，水平方向的和对角线方向的小波分解系数-Two-dimensional signal X (m, n) discrete wavelet transform NOTE: m, n are even, you can get the low-frequency portion, the size of the vertical direction, the wavelet coefficients in the horizontal direction and diagonal directions