I want to count number of people going up or down using a reference line let's say in the middle of that video. Now, How do I actually implement it using python and openCV.. I saw a lot of videos showing the people counter but no one has the method or instructions on how to exactly do that.. I don't need code.. Plz just tell me the method..
Take a look at the detailed breakdown here, as pointed out in comments by leaf, but basically you can use the OpenCV2 built-in methods to perform pedestrian detection. OpenCV ships with a pre-trained HOG + Linear SVM model that can be used to perform pedestrian detection in both images and video streams.
To separate the Up & Down counters I would split each frame on the vertical line before running the detection on each half separately. You can count the number of people going in the given direction in a single frame by a simple
len(contours) while processing that frames direction half.
To track the total number of people going in a given direction you will need to detect the motion of each contour across the frame and only add a new entry to the count when a new contour is created near the entry edge of the direction frame - of course this could be confused by people sprinting through the frame, moving the opposite direction to the expected running up the down or vice-versa and entering the frame then backing out.