Draw arrows (vectors) with opencv / python

def cvArrow(img, pt1, pt2, color, thickness=1, lineType=8, shift=0):
    cv2.line(img,pt1,pt2,color,thickness,lineType,shift)
    vx = pt2[0] - pt1[0]
    vy = pt2[1] - pt1[1]
    v  = math.sqrt(vx ** 2 + vy ** 2)
    ux = vx / v
    uy = vy / v
    #The width of the arrow
    w = 5
    h = 10
    ptl = (int(pt2[0] - uy*w - ux*h), int(pt2[1] + ux*w - uy*h))
    ptr = (int(pt2[0] + uy*w - ux*h), int(pt2[1] - ux*w - uy*h))
    #Draw the tip of the arrow
    cv2.line(img,pt2,ptl,color,thickness,lineType,shift)
    cv2.line(img,pt2,ptr,color,thickness,lineType,shift)

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