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()
