def update(levels):
    vis = np.zeros((cvImg.height, cvImg.width, 3), np.uint8)
    levels = levels - 3
    cv2.drawContours( vis, contours, (-1, 3)[levels <= 0], (128,255,255),
        3, cv2.CV_AA, hierarchy, abs(levels) )
    cv2.imshow('contours', vis)