Conversion of 3x1 or 1x3 cv::Mat to cv::Point3d?

Question

I'm dealing with some code in which a do a lot of 3x3 matrix multiplications an also some translation of 3d points using rotation matrices, etc. I decided to use OpenCV core functionalities for the mathematical operations. The possibility to use the recent constructor added to the cv::Mat class to convert a cv::Point3d directly to a 3x1 cv::Mat reduces and simplifies the code greatly.

What I am wondering now is if there is a simple way to convert a 3x1 or 1x3 cv::Mat to an cv::Point3d? I always can do something like:

cv::Mat mat(3,1,CV_64FC1);
cv::Point3d p (mat.at<double>(0,0), mat.at<double>(1,0), mat.at<double>(2,0));

or

cv::Mat mat(3,1,CV_64FC1);
const double *data = mat.ptr<double>(0);
cv::Point3d p (data[0], data[1], data[2]);

I am very worried about the performance (avoid the 3 calls to at method).

Answer 1

cv::Point3d has a constructor which allows direct creation from cv::Mat:

cv::Mat mat(3,1,CV_64FC1);
cv::Point3d p(mat);

Another possibility you may not have considered is using cv::Matx instead of cv::Mat for your mathematical operations. I find it is easier to use, and offers more functionality, like multiplication of Point types without needing a conversion:

cv::Point3d p(1,2,3);
cv::Matx33d m = cv::Matx33d::eye();
cv::Point3d p2 = m * p;

cv::Matx is also statically allocated, rather than dynamically (like cv::Mat), in case you really need that extra little bit of performance. However, as in all performance-related advice: make sure what you're optimizing is actually a bottleneck by profiling.