Question

I have an OpenCV function which return type is Mat.
How do I convert that to a 2 dimensional float array (** float)?

Probably very simple but I have not been able to do it myself.

Answer 1

A quick look at the documentation for the Mat class doesn’t reveal any obvious ‘convert to float**‘ operator, but you could probably do it by hand:

 Mat mat = (Mat_<float>(3,3) << 1, 0, 0, 0, 1, 0, 0, 0, 1);

 // allocate and initialize a 2d float array
 float **m = new float*[mat.Rows];
 for (int r = 0; r < mat.Rows; ++r)
 {
    m[r] = new float[mat.Cols];
    for (int c = 0; c < mat.Cols; ++c)
    {
       m[r][c] = mat.at(r, c);
    }
 }

 // (use m for something)

 // don't forget to clean up!
 for (int r = 0; r < mat.Rows; ++r)
 {
    delete[] m[r];
 }
 delete[] m;

If you’re not dead-set on using float**, you could use std::vector or boost::multi_array to avoid the awkward memory allocation/deallocation and reduce the potential for leaks.

You might also have some luck using Mat::ptr<float>(n) to get a float* to the nth row of a matrix, but if you don’t copy the data out, I’m not sure what guarantees you’ll have about how long that pointer will remain valid.