How to convert mxArray to Point2f ? OpenCV interface

In order to use the calcOpticalFlowPyrLK function provided by OpenCV libraries through the MATLAB/OpenCV interface, the third and fourth arguments are required to be 2D arrays - the third populated with 2D keypoint locations (in single floats) to tracked forward by the function and keypoints that were tracked forward's location to be stored in the fourth argument.

From the main enviroment, the array of 2D keypoints locations are singles - How do I convert them from mxArray of singles to a format that the cpp environment will accept - a 2D array of single-precision floating-points?

A function that is the opposite of ocvMxArrayFromPoints2f would be ideal - ocvPoints2fFromMxArray. I have attempted to use ocvStructToKeyPoints, but an error

error: (-215) (npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0 in function cv::calcOpticalFlowPyrLK

..

Error in testopticalFlowLKOCV (line 38) opticalFlowLKOCV(original, distorted, validPtsOriginal)

was thrown. The cpp used was

    //convert structures to keypoints
        ocvStructToKeyPoints(prhs[2], keypoints);

    //Calculate forward optical flow for prev_location
        if (keypoints.size() > 0)
        {
             calcOpticalFlowPyrLK(*img_prev, *img_current, keypoints, keypoints1, status, err);

            }
        else 
        {
             mexErrMsgTxt("Keypoints are empty");

            }

I assumed the cause of the error was due to https://github.com/uscauv-legacy/old-uscauv-ros-pkg/issues/47,

i.e. the third argument given to the function is incorrect, it is not a 2D array but rather a keypoint object - and thus there were no 2D locations

Answer 1

[Gerard Walsh]

The function ocvMxArrayToMat_single() worked successfully if the third argument is sent through as an array (mxArray) of singles from MATLAB command window