Access violation reading location OpenCV Canny function

Question

I have a problem with my project in OpeCV. I want to get picture, which is a result of subtraction two pictures, and then get marked edges with using a Canny Edge Detector, but only what I get is a exception.

Here is a print screen with exception:

This is my code:

#pragma once
#include <opencv2/core/core.hpp>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/opencv.hpp>
#include <iostream>
#include <string>

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    Mat pierwszy = imread("one.jpg", IMREAD_COLOR);
    Mat drugi = imread("two.jpg", IMREAD_COLOR);
    Mat wynik;
    subtract(pierwszy, drugi , wynik);
    imwrite("../../images/wynik.jpg", wynik);
    Canny(wynik, wynik, 33, 100);
    namedWindow("Display window", WINDOW_AUTOSIZE);
    imshow("Display window", wynik);
    waitKey(0);
    return 0;
}

Mat's objects are the same size.

Answer 1

To use Cany Edge Detector we need make image on gray scale, like Miki said. Now I use function cvtColor with COLOR_BGR2GRAY and my code looks like this:

#pragma once
#include <opencv2/core/core.hpp>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/opencv.hpp>
#include <iostream>
#include <string>

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    Mat pierwszy = imread("one.jpg", IMREAD_COLOR);
    Mat drugi = imread("two.jpg", IMREAD_COLOR);
    Mat wynik;
    subtract(pierwszy, drugi , wynik);
    imwrite("../../images/wynik.jpg", wynik);
    Mat wynik_gray;
    cvtColor(wynik, wynik_gray, COLOR_BGR2GRAY);
    Canny(wynik_gray, wynik_gray, 33, 100);
    namedWindow("Display window", WINDOW_AUTOSIZE);
    imshow("Display window", wynik_gray);
    waitKey(0);
    return 0;
}