title: Drawing spectrum of an image in C++ (fftw, OpenCV)
body: <p>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):</p>
  
  <pre><code>/* Copy input image */
  
  /* Create output image */
  
  /* Allocate input data for FFTW */
  in   = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * N);
  dft  = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * N);
  
  /* Create plans */
  plan_f = fftw_plan_dft_2d(w, h, in, dft, FFTW_FORWARD, FFTW_ESTIMATE);
  
  /* Populate input data in row-major order */
  for (i = 0, k = 0; i &lt; h; i++) 
  {
      for (j = 0; j &lt; w; j++, k++)
      {
          in[k][0] = ((uchar*)(img1-&gt;imageData + i * img1-&gt;widthStep))[j];
          in[k][1] = 0.;
      }
  }
  
  /* forward DFT */
  fftw_execute(plan_f);
  
  /* spectrum */
  for (i = 0, k = 0; i &lt; h; i++)
  {
      for (j = 0; j &lt; w; j++, k++)
          ((uchar*)(img2-&gt;imageData + i * img2-&gt;widthStep))[j] = sqrt(pow(dft[k][0],2) + pow(dft[k][1],2));
  }       
  
  cvShowImage("iplimage_dft(): original", img1);
  cvShowImage("iplimage_dft(): result", img2);
  cvWaitKey(0);
  
  /* Free memory */
  </code></pre>
  
  <p>}</p>
  
  <p>The problem is in the "Spectrum" section. Instead of a spectrum I get some noise. What am I doing wrong? I would be grateful for your help.</p>