How to keep tracking the same object among others

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What I have? I have a code that detects the biggest object in the video.

How do I do it? I am using knn filter, findControus, contourArea and boundingRect to detect and draw a rectangle around of the biggest object.

My problem: Sometimes my object changes the speed, then another object gets more 'background subtraction difference' and it misses the real target.

I was thinking how can I keep following only this guy. I tried to use a keyboard input to tell to my code who is the right guy, and then keep following it. But it didn't work.

The reason that I need to track only one is because I need to make an analysis based on its coordinate.

This is my code:

maximumArea = 0
        bestContour = None
        for contour in contours:
            currentArea = cv2.contourArea(contour)
            if currentArea > maximumArea:
                bestContour = contour
                maximumArea = currentArea
        if bestContour is not None:
            x,y,w,h = cv2.boundingRect(bestContour)
            cv2.rectangle(orig_img, (x,y),(x+w,y+h), (0,0,255), 3)        
            print("The X, Y coordinate is: %s , %s  " % (x, y))
            f = open("hello.txt", "a")
            f.write("\n" "%s %s" % (x, y)) 
            f.close()