Performing simple inverse and multiplication operations on Mat matrices

Question

I have detected a number of points from 2 images. I am trying to find a transformation matrix between these 2 images. So I need to do an inverse of the matrix coordinates of one matrix and then multiply it to the coordinates of the other Mat matrix. I used very simple opencv methods but I get this error https://i.stack.imgur.com/Rqp7v.jpg. I do not understand why. Here is my code http://pastebin.com/Tef42E2Q. Can anybody guide me here please?

Answer 1

You should make sure at least two things before Mat inverse.

1. The Matrix should be square.

2. The Matrix should be non-singular, that is determinant should be non-zero.

Eg:

    Mat A(3,3,CV_32FC1);
    A=(Mat_<float>(3,3)<< 0,2,3,\
                          4,5,6,\
                          7,8,9);
    cout<<A <<endl;

    if(determinant(A)!=0){
    Mat B=A.inv();
    cout<<B <<endl;
    }

Also see the answer Mat.inv() yielding all zeroes in opencv

Edit:-

Here is some bug I found in your code

The piece of code

    Mat matrix_f(2,3,CV_32F);
    matrix_f=Mat(coordinates_f);

should changed to

    Mat matrix_f(2,3,CV_32F);
    matrix_f=Mat(coordinates_f);
    matrix_f=matrix_f.reshape(1,2);

because later you are going to multiply with a 3X3 Mat, So we will make it's rows to 3

And next is change the lines

    Mat new_left(3,3,CV_32F);
    new_left=Mat(coordinates_l_new);

to

      Mat new_left(3,3,CV_32F);
      new_left=Mat(coordinates_l_new);
      new_left=new_left.reshape(1,3);

as you are going to find the inverse of new_left, it should be square matrix.

And finally make sure the Mat is non-singular by finding determinant

 if(determinant(new_left)!=0) {
        Mat T(3,3,CV_32F);
        T=matrix_f * (new_left.inv());
  }