为什么cv2.rectangle没有返回图像？

Question

我正试图在我用python中的opencv2从我的笔记本电脑摄像头录制的帧上绘制矩形。

但是，从cv2.rectangle函数返回的图像是None。为什么？

import numpy as np
import cv2

# details of rectangle to be drawn.
x, y, h, w = (493, 305, 125, 90)

cap = cv2.VideoCapture(0)

while 1:
  ret, frame = cap.read()

  if not ret or not frame:
    # camera didn't give us a frame.
    continue

  # attempt to draw a rectangle.
  img = cv2.rectangle(frame, (x, y), (x + w, y + h), (0, 255, 0), 2)

  # This prints out None every time! why?
  print str(img)

  if not img:
    continue

  # We never get here since img is None. :/
  cv2.imshow('img', img)

我试过添加检查，如你所见，如果从相机的帧是空的。我也试过确保矩形真的适合框架。

我确信我的相机是工作的，因为我可以成功地imshow帧。

Answer 1

rectangle不返回任何内容

[编辑：]在opencv2.4.x中，但它确实在opencv3.0/python中返回图像

另外请注意，非常流行的py_tutrorials是3.0版的，所以不要搞混；）

---

import numpy as np
import cv2

# details of rectangle to be drawn.
x, y, h, w = (493, 305, 125, 90)

cap = cv2.VideoCapture(0)

while 1:
  ret, frame = cap.read()

  if not ret or not frame:
    # camera didn't give us a frame.
    continue

  # draw a rectangle.
  cv2.rectangle(frame, (x, y), (x + w, y + h), (0, 255, 0), 2)

  # show it ;)
  cv2.imshow('img', frame)