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Mat::at throws error: Assertion failed ((unsigned)i0 < (unsigned)size.p[0]) in opencv\build\include\opencv2\core\mat.inl.hpp, line 1100

Hello guys! I am new to OpenCV and I have tried to write a program that takes as input the images my camera captures live and modify some pixels. Right now I was trying to invert the values of a grayscale image when this error appeared. I think I got sth wrong because I have used a line of code before and it worked just fine. Can you please help me out?

Here is the code:

    while(1){
    VideoCapture cap(0);
    if (!cap.isOpened())
       return 1;
    Mat src,dst,gray;
    cap.read(src);
    cvtColor(src, gray, COLOR_BGR2GRAY);
    dst = gray;

    for (int y = 0; y < dst.cols; y++) {
        for (int x = 0; x < dst.rows; x++) {
            dst.at<uchar>(y, x) = 255 - dst.at<uchar>(y, x);
        }
    }
    if (waitKey(30) == 's')
        break;
    }

I have used the line: "dst.at<uchar>(y, x) = 255 - dst.at<uchar>(y, x);" before and it worked just fine. What could be wrong?

Error: Assertion failed ((unsigned)i0 < (unsigned)size.p[0]) in cv::Mat::at, file ..\opencv\build\include\opencv2\core\mat.inl.hpp, line 1100