OpenCV submatrix access: copy or reference?

Question

If I extract a submatrix from a matrix using

cv::Mat A = cv::Mat::ones(4,4);

cv::Mat B = A( cv::Rect( 1, 1, 2, 2 ) );

Is "B" a copy of those values from "A" or does it reference to those values?

Could you provide an example of how to get

(1) a copy of the submatrix?

(2) a reference to the submatrix?

Answer 1

B is a copy of A's Mat-header, but references the same pixels.

so, if you manipulate B's pixels, A is affected, too.

(1) (a 'deep copy') would be:

cv::Rect r( 1, 1, 2, 2 );
cv::Mat A = cv::Mat::ones(4,4);
cv::Mat B = A(r).clone(); // now B has a seperate *copy* of the pixels

cv::Mat C; 
A(r).copyTo(C);           // another way to make a 'deep copy'

(2) (a 'shallow copy'), that's what you're doing above already:

cv::Mat B = A(r);         // B points to A's pixels