Yet another issue with calcOpticalFlowPyrLK

I remember that I encountered once the problem of 

OpenCV Error: Assertion failed ((npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0) 

when I was using calcOpticalFlowPyrLK. It turned out that the format of features passed to calcOpticalFlowPyrLK should be Point2f, instead of keypoint. Here is an example of how to use it.