OpenCV无法获取VideoCaptu

Question

我有以下Python代码，用于加载视频文件并将其显示在窗口中：

cap = cv2.VideoCapture('file.mp4')
cap.open('file.mp4')
while(cap.isOpened()):
    ret, frame = cap.read()
    cv2.imshow("My Video Window", frame)

cap.release()
cv2.destroyAllWindows()

但是，当我运行它时，我得到以下错误：

^{pr2}$

有什么问题吗？ 谢谢！在

如果我加上：

gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

我得到：

OpenCV Error: Assertion failed (scn == 3 || scn == 4) in ipp_cvtColor, file /tmp/opencv3-20160626-61731-v18vfg/opencv-3.1.0/modules/imgproc/src/color.cpp, line 7456
Traceback (most recent call last):
  File "file.py", line 66, in <module>
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.error: /tmp/opencv3-20160626-61731-v18vfg/opencv-3.1.0/modules/imgproc/src/color.cpp:7456: error: (-215) scn == 3 || scn == 4 in function ipp_cvtColor