title: Raspberry Pi Camera and OpenCV: can't open camera by index
body: <p>I've got a weird problem:</p>
  <p>I've installed the OpenCV lib on my Pi. I have a Pi Cam connexted to the Pi (i am able to list all video devices and am able to take a picture with raspistill)</p>
  <p>But when i try to take a video feed from opencv with python</p>
  <pre><code>from flask import Flask, render_template, Response
  import cv2
  
  app = Flask(__name__)
  
  cap = cv2.VideoCapture(1)
  </code></pre>
  <p>I get the error:</p>
  <pre><code>[ WARN:0] global /tmp/pip-wheel-qd18ncao/opencv-python/opencv/modules/videoio/src/cap_v4l.cpp (893) open VIDEOIO(V4L2:/dev/video0): can't open camera by index
  </code></pre>
  <p>I tried it with different index (from -1 up to 13) but nothing works.</p>
  <p>Any hints ?</p>