Convert Armadillo Mat <-> OpenCV

// convert an OpenCV multi-channel matrix to Armadillo cube. A copy is made
template <typename T, int NC>
Cube<T> to_arma(const cv::Mat_<cv::Vec<T, NC>> &src)
{
  vector<cv::Mat_<T>> channels;
  Cube<T> dst(src.cols, src.rows, NC);
  for (int c = 0; c < NC; ++c)
    channels.push_back({src.rows, src.cols, dst.slice(c).memptr()});
  cv::split(src, channels);    
  return dst;
}

// convert an OpenCV matrix to Armadillo matrix. NOTE: a copy is made
template <typename T>
Mat<T> to_arma(const cv::Mat_<T> &src)
{
  Mat<T> dst(src.cols, src.rows);
  src.copyTo({src.rows, src.cols, dst.memptr()});
  return dst;
}

// convert an Armadillo cube to OpenCV matrix. NOTE: a copy is made
template <typename T>
cv::Mat to_cvmat(const Cube<T> &src)
{
  vector<cv::Mat_<T>> channels;
  for (size_t c = 0; c < src.n_slices; ++c)
  {
    auto *data = const_cast<T*>(src.slice(c).memptr());
    channels.push_back({int(src.n_cols), int(src.n_rows), data});
  }
  cv::Mat dst;
  cv::merge(channels, dst);
  return dst;
}

// convert an Armadillo matrix to OpenCV matrix. NOTE: no copy is made
template <typename T>
cv::Mat_<T> to_cvmat(const Mat<T> &src)
{
  return cv::Mat_<double>{int(src.n_cols), int(src.n_rows), const_cast<T*>(src.memptr())};
}