def draw(self, img, pixmapper, bounds):
        '''draw the trail'''
        for p in self.points:
            (px,py) = pixmapper(p)
            (width, height) = image_shape(img)
            if px >= 0 and py >= 0 and px < width and py < height:
                cv2.circle(img, (px,py), 1, self.colour)
draw the trail