说明：  一个二维图像的fft变换程序，用到fftw库，经过测试，fftw可以说是目前我已知的最快的离散傅立叶变换库，相当强悍
(A two-dimensional images of fft transform procedures, use fftw library, after testing fftw. I can say it is the fastest Discrete Fourier Transform library,i known. very tough)