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代码说明：

  二维傅里叶变换,对图像进行二维傅里叶变换处理(Two-Dimensional Fast Fourier Transform The purpose of this project is to develop a 2-D FFT program "package" that will be used in several other projects that follow. Your implementation must have the capabilities to: (a) Multiply the input image by (-1)x+y to center the transform for filtering. (b) Multiply the resulting (complex) array by a real function (in the sense that the the real coefficients multiply both the real and imaginary parts of the transforms). Recall that multiplication of two images is done on pairs of corresponding elements. (c) Compute the inverse Fourier transform. (d) Multiply the result by (-1)x+y and take the real part. (e) Compute the spectrum.)