Updating a submatrix of Mat in OpenCV

Question

I am working with OpenCV and C++. I have a matrix X like this

Mat X = Mat::zeros(13,6,CV_32FC1);

and I want to update just a submatrix 4x3 of i

Answer 1

First, you have to create a matrix that points to the original one:

Mat orig(13,6,CV_32FC1, Scalar::all(0));

Mat roi(orig(cv::Rect(1,1,4,3))); // now, it points to the original matrix;

Mat otherMatrix = Mat::eye(4,3,CV_32FC1);

roi.setTo(5);                // OK
roi = 4.7f;                  // OK
otherMatrix.copyTo(roi);     // OK

Keep in mind that any operations that involves direct attribution, with the "=" sign from another matrix will change the roi matrix source from orig to that other matrix.

// Wrong. Roi will point to otherMatrix, and orig remains unchanged
roi = otherMatrix;