goodFeaturesToTrack Interval for Robust Tracking

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Hi, About the optical flow tutorial http://docs.opencv.org/trunk/doc/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.html

Referring to sample OpenCV source code ...\samples\python2\lk_track.py There is a statement: "So actually for a robust tracking, corner points should be detected in particular intervals. OpenCV samples comes up with such a sample which finds the feature points at every 5 frames"

Why is this the case? Can somebody explain why for a robust tracking, we need to find the feature points at every particular interval?

Thank you.