Compute Laplace of Gaussian kernel for a size and sigma using OpenCV

laplace_of_gaussian_2D.cpp

#include <opencv2/opencv.hpp>
#include <opencv2/imgproc.hpp>
#include <assert.h>

// Calculates Laplace of Gaussian kernel                                                                                                                                          
cv::Mat createLOGKernel(int ksize, double sigma)
{
    using namespace cv;
    using namespace std;

    assert(1 == ksize%2);

    Mat kernel(ksize, ksize, CV_64F);
    Mat X(ksize, ksize, CV_64F);
    Mat Y(ksize, ksize, CV_64F);
    int siz = (ksize-1)/2;
    double std2 = sigma*sigma;

    vector<double> seq(ksize);
    iota(begin(seq), end(seq), -siz);
    Mat seqX(1, ksize, CV_64F, seq.data());
    Mat seqY(ksize, 1, CV_64F, seq.data());
    repeat(seqX, ksize, 1, X);
    repeat(seqY, 1, ksize, Y);

    Mat XX, YY;
    multiply(X, X, XX);
    multiply(Y, Y, YY);

    Mat H = -(XX + YY) / (2.*std2);
    exp(H, H);

    double minVal, maxVal;
    minMaxIdx(H, &minVal, &maxVal);
    Mat mask = H < numeric_limits<double>::epsilon()*maxVal;
    H.setTo(0, mask);

    double sumh = sum(H)[0];
    if (sumh != 0) H /= sumh;

    Mat H1;
    multiply(H, (XX + YY - 2*std2) / (std2*std2), H1);
    H = H1 - sum(H1)[0] / (ksize*ksize);
    return H;
}