python-2.7 - 在树莓派上使用 opencv4 videocapture 对象在 read() 2 次后冻结

Question

使用树莓派上的 opencv4 videocapture 对象在 read() 2 次后冻结。

细节：

 - Raspberrypi Jessie
 - python 2.7 + opencv 4 
 - python code can be run on windows10 (py2.7 and opencv4) without problem

这是基于示例的代码：

import cv2
ret, frame = cap.read()
cap = cv2.VideoCapture(0)
countDown = 20
while(countDown > 0 ):

    ret, frame = cap.read()
    print countDown , "readed"
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # cv2.imwrite (  "test" + str(countDown) +   ".png" , gray )
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
    countDown -= 1 

cap.release()

结果 ：

20 readed
19 readed
then hangs forever........