How to keep tracking one object forever?

edit

Hey.

I am trying to track ants, it is really hard because ants change its form and I am using grayscale image. It means, mosse and camshit didnt work on my project.

I have to track 1 special one(not a random), because I need to take its position and make some analysis. So I need a way to select one ant and track it forever, I tried everything, but nothing worked.

Well, I can set the the ant position manually, or by mouse, it is not a problem, the problem is how to keep tracking the same ant after know its first position.

I am using knn filter on my code to make it easier, because I will have all my ants white, and the background in black.

It is my code:

import numpy as np
import cv2

class ColourTracker:
    def __init__(self):
        cv2.namedWindow("Background")
        cv2.namedWindow("Frame")

        self.capture = cv2.VideoCapture('video.avi')
        self.knn = cv2.createBackgroundSubtractorKNN()

    def run(self):
        while True:
            f, orig_img = self.capture.read()

            if( not f ):
                break;

            fore = self.knn.apply( orig_img )
            back = self.knn.getBackgroundImage()
            kernel = np.ones((5,5),np.uint8)
            fore = cv2.erode( fore, kernel )
            fore = cv2.dilate( fore, kernel )
           # fore = fore[r:r+h, c:c+w]
            image, contours, hiearchy = cv2.findContours( fore, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE )



            maximumArea = 0
            bestContour = None
            for contour in contours:
                currentArea = cv2.contourArea(contour)
                if currentArea > maximumArea:
                    bestContour = contour
                    maximumArea = currentArea

                    x,y,w,h = cv2.boundingRect(bestContour)
                    cv2.rectangle(orig_img, (x,y),(x+w,y+h), (0,0,255), 3)        

            cv2.imshow( "Background", back )
            cv2.imshow( "Frame", orig_img )

            k = cv2.waitKey(24) & 0xff 
            print k
            if k == 27:
                break
if __name__ == "__main__":
    colour_tracker = ColourTracker()
    colour_tracker.run()