Save spectrum of DFT using OpenCV

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]