Updating a submatrix of Mat in OpenCV

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旧巷少年郎
旧巷少年郎 2021-02-01 18:02

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

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  • 2021-02-01 18:12

    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.

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  • 2021-02-01 18:15

    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;            
    
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