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. 

Another good CV blog

Today I found another wonderful CV blog here. One post talks about BRIEF seems to be interesting.