import cv2


def on_mouse(event, x, y, flags, param):
    rect_start = (0, 0)
    rect_end = (0, 0)
    # 鼠标左键按下，抬起，双击
    if event == cv2.EVENT_LBUTTONDOWN:
        rect_start = (x, y)
    elif event == cv2.EVENT_LBUTTONUP:
        rect_end = (x, y)
    cv2.rectangle(img, rect_start, rect_end, (0, 255, 0), 2)


img = cv2.imread("lena.jpg")
cv2.namedWindow('test')
cv2.setMouseCallback("test", on_mouse)

while (1):

    cv2.imshow("test", img)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
cv2.destroyAllWindows()