example_12-01.cpp

// Example 12-1. Using cv::dft() and cv::idft() to accelerate
// the computation of convolutions 

#include <iostream>
#include <opencv2/opencv.hpp>

using std::cout;
using std::endl;

int main(int argc, char** argv) {
    if (argc != 2) {
        cout    << "\nExample 12-1. Using cv::dft() and cv::idft() to accelerate the"
                << "\n computation of convolutions"
                << "\nFourier Transform\nUsage: "
                << argv[0] << " <path/imagename>\n" << endl;
        return -1;
    }

    cv::Mat A = cv::imread(argv[1], 0);

    if (A.empty()) {
        cout << "Cannot load " << argv[1] << endl;
        return -1;
    }

    cv::Size patchSize(100, 100);
    cv::Point topleft(A.cols / 2, A.rows /2);
    cv::Rect roi(topleft.x, topleft.y, patchSize.width, patchSize.height);
    cv::Mat B = A(roi);

    int dft_M = cv::getOptimalDFTSize(A.rows + B.rows - 1);
    int dft_N = cv::getOptimalDFTSize(A.cols + B.cols - 1);

    cv::Mat dft_A = cv::Mat::zeros(dft_M, dft_N, CV_32F);
    cv::Mat dft_B = cv::Mat::zeros(dft_M, dft_N, CV_32F);

    cv::Mat dft_A_part = dft_A(cv::Rect(0, 0, A.cols, A.rows));
    cv::Mat dft_B_part = dft_B(cv::Rect(0, 0, B.cols, B.rows));

    A.convertTo(dft_A_part, dft_A_part.type(), 1, -mean(A)[0]);
    B.convertTo(dft_B_part, dft_B_part.type(), 1, -mean(B)[0]);

    cv::dft(dft_A, dft_A, 0, A.rows);
    cv::dft(dft_B, dft_B, 0, B.rows);

    // set the last parameter to false to compute convolution instead of correlation
    //
    cv::mulSpectrums(dft_A, dft_B, dft_A, 0, true);
    cv::idft(dft_A, dft_A, cv::DFT_SCALE, A.rows + B.rows - 1);

    cv::Mat corr = dft_A(cv::Rect(0, 0, A.cols + B.cols - 1, A.rows + B.rows - 1));
    cv::normalize(corr, corr, 0, 1, cv::NORM_MINMAX, corr.type());
    cv::pow(corr, 3.0, corr);

    B ^= cv::Scalar::all(255);

    cv::imshow("Image", A);
    cv::imshow("ROI", B);

    cv::imshow("Correlation", corr);
    cv::waitKey();

    return 0;
}