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c++arrayscastingopencv3.0mat

How to cast CV_64F type Matrix to 1D Array with Double elements

I couldn't find an answer to this question but I believe it should be easily done.

Here I have following data structures;

Mat MyMatrix = Mat(3, 1, CV_64F, &targetArray) //Some 3x1 data in it
// Some process...

array <double ,3> MyArray
MyMatrix.convertTo(MyArray, double, 0 , DBL_MAX)

I want to convert MyMatrix (which I guarantee to be 3x1) to an 1D Array (Array elements should be double). How can I do that with C++ and opencv3.0.1?

Solution

  • You cannot convert a cv::Mat to a std::array. You can only copy the data into an array.

    Given a 

    Mat m = Mat(3, 1, CV_64F);
// fill with some value

    you can use memcpy:

    array<double, 3> a; 
memcpy(a.data(), m.ptr<double>(0), 3*sizeof(double));

    or std::copy:

    array<double, 3> b;
copy(m.begin<double>(), m.end<double>(), b.begin());

    or, since it's only 3 elements, the array constructor:

    array<double, 3> c = {m.at<double>(0), m.at<double>(1), m.at<double>(2)};

    or, obviously, with a loop:

    array<double, 3> d;
for (int i = 0; i < d.size(); ++i) { d[i] = m.at<double>(i); }

    Conversion is instead possible with std::vector:

    vector<double> e = m;

    Note that, instead of a matrix with only 3 values, you can use Vec3d, or Matx31 or Matx13. Using Mat1d would however simplify the notation and make the code less verbose.