Save spectrum of DFT using OpenCV - c++

Question

I am using Visual Studio 2013 and OpenCV library. I display discrete Fourier transform and it works well. I'd like to save the displayed image which is the spectrum, but the saved one is black even when I use JPEG quality=100.
#include "opencv2/core/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/opencv.hpp"
#include <iostream>
#include <cmath>
using namespace cv;
using namespace std;
static void help(char* progName)
{
cout << endl
<< "This program demonstrated the use of the discrete Fourier transform (DFT). " << endl
<< "The dft of an image is taken and it's power spectrum is displayed." << endl
<< "Usage:" << endl
<< progName << " [image_name -- default lena.jpg] " << endl << endl;
}
int main(int argc, char ** argv)
{
help(argv[0]);
const char* filename = argc >= 2 ? argv[1] : "lena.jpg";
Mat I = imread(filename, CV_LOAD_IMAGE_GRAYSCALE);
if (I.empty())
return -1;
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_<float>(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
// => 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);
// crop the spectrum, if it has an odd number of rows or columns
magI = magI(Rect(0, 0, magI.cols & -2, magI.rows & -2));
// rearrange the quadrants of Fourier image so that the origin is at the image center
int cx = magI.cols / 2;
int cy = magI.rows / 2;
Mat q0(magI, Rect(0, 0, cx, cy)); // Top-Left - Create a ROI per quadrant
Mat q1(magI, Rect(cx, 0, cx, cy)); // Top-Right
Mat q2(magI, Rect(0, cy, cx, cy)); // Bottom-Left
Mat q3(magI, Rect(cx, cy, cx, cy)); // Bottom-Right
Mat tmp; // swap quadrants (Top-Left with Bottom-Right)
q0.copyTo(tmp);
q3.copyTo(q0);
tmp.copyTo(q3);
q1.copyTo(tmp); // swap quadrant (Top-Right with Bottom-Left)
q2.copyTo(q1);
tmp.copyTo(q2);
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("spectrum magnitude", magI);
cv::Mat gs_bgr(magI.size(), CV_8UC1);
cv::cvtColor(magI, gs_bgr, CV_RGB2GRAY);
imwrite("orig.png", gs_bgr);
waitKey();
return 0;
}

Answer 1

the outcome of the dft (magI) is a float Mat, but you can only save uchar images with imwrite.
since you normalized the image to [0..1] , the resulting uchar - grayscale img only has 0 and 1 values, which will look pretty black indeed.
also, applying cv::cvtColor(magI, gs_bgr, CV_RGB2GRAY); to a 1chan float img seems broken.
instead of that, try:
Mat gray;
magI.convertTo(gray, CV_8U, 255); // upscale to [0..255]