title: OpenCV find image frequencies
body: <p>I am trying to find the dominant frequencies in an image using OpenCV. I saw the dft example in the OpenCV tutorial but how can I find the dominant frequencies of an image with periodic structures? I want to extract the spectral informations of the lowest harmonics, copy them to a new image and do reverse dft. A sample image is attached to this question.</p>
  
  <p><img src="https://i.stack.imgur.com/465R8.jpg" alt="enter image description here"></p>