在c++中绘制图像的频谱(fftw, OpenCV)

I'm trying to create a program that will draw a 2d greyscale spectrum of a given image. I'm using OpenCV and FFTW libraries. By using tips and codes from the internet and modifying them I've managed to load an image, calculate fft of this image and recreate the image from the fft (it's the same). What I'm unable to do is to draw the fourier spectrum itself. Could you please help me? Here's the code (less important lines removed):I'm trying to create a program that will draw a