if len(sys.argv) > 1:
  #根据控制台输入的图像，进行导入
  fn = cv.samples.findFile(sys.argv[1])
  print('loading %s ...' % fn)
  img = cv.imread(fn)
  if img is None:
    print('Failed to load fn:', fn)
    sys.exit(1)

small = img
for _i in range(3):
  #图像降采样
  small = cv.pyrDown(small)

#鼠标响应事件
def onmouse(event, x, y, flags, param):
  h, _w = img.shape[:2]
  h1, _w1 = small.shape[:2]
  x, y = 1.0*x*h/h1, 1.0*y*h/h1
  zoom = cv.getRectSubPix(img, (800, 600), (x+0.5, y+0.5))
  cv.imshow('zoom', zoom)

cv.imshow('preview', small)
cv.setMouseCallback('preview', onmouse)
cv.waitKey()