Opencv convolution matrix gives unusual results - c++

Question

So I have a program that is trying to apply a simple 3x3 convolution matrix to an image.
This is the function that is doing the work:
Mat process(Mat image) {
int x = 2;
int y = 2;
Mat nimage(image); //just a new mat to put the resulting image on
while (y < image.rows-2) {
while (x < image.cols-2) {
nimage.at<uchar>(y,x) = //apply matrix to pixel
image.at<char>(y-1,x-1)*matrix[0]+
image.at<char>(y-1,x)*matrix[1]+
image.at<char>(y-1,x+1)*matrix[2]+
image.at<char>(y,x-1)*matrix[3]+
image.at<char>(y,x)*matrix[4]+
image.at<char>(y,x+1)*matrix[5]+
image.at<char>(y+1,x-1)*matrix[6]+
image.at<char>(y+1,x)*matrix[7]+
image.at<char>(y+1,x+1)*matrix[8];
//if (total < 0) total = 0;
//if (total > 255) total = 255;
//cout << (int)total << ": " << x << "," << y << endl;
x++;
}
x = 0;
y++;
}
cout << "done" << endl;
return nimage;
}
And the matrix looks like this
double ar[9] = {-1,0,0,
0,2,0,
0,0,0};
And the image that is used as input looks like this:
The desired output (I ran the same matrix on the input image in GIMP):
And the result is... weird:
I think this has to do with the data type I use when I set a pixel of the new image (nimage.at<uchar>(y,x) = ...), because whenever I change it I get a different, yet still incorrect result.

Answer 1

From the OpenCV documentation about the copy constructor of Mat, emphasis mine:
m – Array that (as a whole or partly) is assigned to the constructed matrix. No data is copied by these constructors. Instead, the header pointing to m data or its sub-array is constructed and associated with it. The reference counter, if any, is incremented. So, when you modify the matrix formed using such a constructor, you also modify the corresponding elements of m. If you want to have an independent copy of the sub-array, use Mat::clone().
So
Mat nimage(image); //just a new mat to put the resulting image on
doesn't actually create a new matrix; it creates a new Mat object, but that object still refers to the same matrix. From then on nimage.at(y,x) acts like image.at(y,x).
To copy the image, use
Mat nimage(image.clone()); //just a new mat to put the resulting image on