title: gstreamer critical error when trying to capture video using webcam python opencv
body: <p>i'm trying to take a video with webcam using opencv and python with a simple code</p>
  
  <pre><code>import numpy as np
  import cv2
  
  cap = cv2.VideoCapture(0)
  print('cap.isOpened')
  if cap.isOpened():
      print ('cap is opened')
      while(True):
          re,img=cap.read()
          cv2.imshow("video output", img)
          k = cv2.waitKey(10)&amp;0xFF
          if k==27:
              break
  cap.release()
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>it's working fine if i try to play an existing video such as .mp4 file. but when i try using a webcam i got an error</p>
  
  <blockquote>
    <p>GStreamer-CRITICAL **: gst_element_get_state: assertion 'GST_IS_ELEMENT (element)' failed
    cap.isOpened</p>
  </blockquote>
  
  <p>for more information i'm using odroid xu4 with ubuntu 16.04, webcam i use logitech c170 ( it work properly in webcamtest and using guvcview) thought it doesn't workon cheese and camorama.</p>
  
  <p>need help about this please..</p>