Counting a point that intersect a line in opencv python

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Hello everyone, I am working in vehicle counting with opencv and python programming. I already complete step: 1. Detect moving vehicle with BackgroundSubtractorMOG2 2. Draw rectangle on it, then poin a centroid of that 3. Draw a line (to indicate of the counting)

if that centroid accros/intercept with the line I want count that 1. but in my code sometime it add sometime no. Here the line code:

cv2.line(frame,(0,170),(300,170),(200,200,0),2)

and there the centroid:

if w > 20 and h > 25: 
            cv2.rectangle(frame, (x,y), (x+w,y+h), (180, 1, 0), 1)

            x1=w/2      
            y1=h/2
            cx=x+x1
            cy=y+y1
            centroid=(cx,cy)

            cv2.circle(frame,(int(cx),int(cy)),4,(0,255,0),-1)

my counting code:

if cy==170:   
 counter=counter+1

Can anyone help me. please. for your advice thankyou!