for cnt in contours:
  (x,y,w,h) = cv2.boundingRect(cnt)
  
  if w > 20 and h > 20:
    cv2.rectangle(img, (x,y), (x+w,y+h), (0,255,0), 2)
    x1=w/2
    y1=h/2
    cx=x+x1
    cy=y+y1
    a.append([cx,cy])