title: How to do inverse DFT in OpenCV
body: <p>I'm trying to implement inverse DFT using OpenCV in C++
  I downloaded complete dft example in docs.opencv.org and just adjust couple of lines to inverse.</p>
  
  <p>my DFT code is like this</p>
  
  <pre><code>Mat DFT(const char* filename)
  {
      Mat I = imread(filename, CV_LOAD_IMAGE_GRAYSCALE);
      if (I.empty())
      {
          Mat emty(7, 7, CV_32FC2, Scalar(1, 3));
          return emty;
      }
  
      Mat padded;                            //expand input image to optimal size
      int m = getOptimalDFTSize(I.rows);
      int n = getOptimalDFTSize(I.cols); // on the border add zero values
      copyMakeBorder(I, padded, 0, m - I.rows, 0, n - I.cols, BORDER_CONSTANT, Scalar::all(0));
  
      Mat planes[] = { Mat_&lt;float&gt;(padded), Mat::zeros(padded.size(), CV_32F) };
      Mat complexI;
      merge(planes, 2, complexI);         // Add to the expanded another plane with zeros
  
      dft(complexI, complexI);            // this way the result may fit in the source matrix
  
      // compute the magnitude and switch to logarithmic scale
      // =&gt; log(1 + sqrt(Re(DFT(I))^2 + Im(DFT(I))^2))
      split(complexI, planes);                   // planes[0] = Re(DFT(I), planes[1] = Im(DFT(I))
      magnitude(planes[0], planes[1], planes[0]);// planes[0] = magnitude
      Mat magI = planes[0];
  
      magI += Scalar::all(1);                    // switch to logarithmic scale
      log(magI, magI);
  
  
  
      normalize(magI, magI, 0, 1, CV_MINMAX); // Transform the matrix with float values into a
      // viewable image form (float between values 0 and 1).
  
      imshow("Input Image", I);    // Show the result
      imshow(filename, magI);
      //   waitKey();
  
      return magI;
  }
  </code></pre>
  
  <p>and did IDFT. By just fix dft to idft. But the output was just look like noise. What did I do wrong? I thought the dft and idft is just same....</p>
  
  <pre><code>Mat IDFT(Mat src)
  {
      Mat I = src;
      Mat padded;                            //expand input image to optimal size
      int m = getOptimalDFTSize(I.rows);
      int n = getOptimalDFTSize(I.cols); // on the border add zero values
      copyMakeBorder(I, padded, 0, m - I.rows, 0, n - I.cols, BORDER_CONSTANT, Scalar::all(0));
  
      Mat planes[] = { Mat_&lt;float&gt;(padded), Mat::zeros(padded.size(), CV_32F) };
      Mat complexI;
      merge(planes, 2, complexI);         // Add to the expanded another plane with zeros
  
      dft(complexI, complexI, DFT_INVERSE);            // this way the result may fit in the source matrix
  
      // compute the magnitude and switch to logarithmic scale
      // =&gt; log(1 + sqrt(Re(IDFT(I))^2 + Im(IDFT(I))^2))
      split(complexI, planes);                   // planes[0] = Re(IDFT(I), planes[1] = Im(IDFT(I))
      magnitude(planes[0], planes[1], planes[0]);// planes[0] = magnitude
      Mat magI = planes[0];
  
      magI += Scalar::all(1);                    // switch to logarithmic scale
      log(magI, magI);
  
  
      normalize(magI, magI, 0, 1, CV_MINMAX);
  
      imshow("forged map", magI);
  
  
      return magI;
  }
  </code></pre>