Accessing Beaglebone python opencv usb camera but the display showing black screen

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I am facing issue while accessing USB camera using beagle-bone black wireless. Firstly the error is "select timeout" exception which was resolved by this post

Now I am facing the black screen in output.

Here is the testing code I am using

from cv2 import *
# initialize the camera
cam = VideoCapture(0)   # 0 -> index of camera
print "Cam capture"
cam.set(3,320)
cam.set(4,240)
print "Cam set"
s, img = cam.read()
print "Cam read"
if s:    # frame captured without any errors
    namedWindow("cam-test",CV_WINDOW_AUTOSIZE)
    imshow("cam-test",img)
    while True:
        key = waitKey(30)
        if key == ord('q') :
                destroyWindow("cam-test")

I have already check that video0 in /dev directory.