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 4×3 of it, but I have doubts on how to access that matrix in an efficient way.

Mat mat43= Mat::eye(4,3,CV_32FC1);  //this is a submatrix at position (4,4)

Do I need to change element by element?

Answer 1

One of the quickest ways is setting a header matrix pointing to the range of columns/rows you want to update, like this:

Mat aux = X.colRange(4,7).rowRange(4,8); // you are pointing to submatrix 4x3 at X(4,4)

Now, you can copy your matrix to aux (but actually you will be copying it to X, because aux is just a pointer):

mat43.copyTo(aux);

Thats it.