I ve made an example on how to perform FFT operation using FFTW in OpenCV. It simply loads an image, compute FFT & IFFT, then display the final result. The resulted image has no difference with the original, as it should.

Wave文件读取、播放、实时显示波形、对其作FFT和DCT等功能-Wave document read, play, real-time display of waveform, its function as FFT and DCT