python - How to correctly check if a camera is available?

Question

I am using OpenCV to open and read from several webcams. It all works fine, but I cannot seem to find a way to know if a camera is available.

I tried this code (cam 2 does not exist):

import cv2
try:
    c = cv2.VideoCapture(2)
except:
    print "Cam 2 is invalid."

But this just prints a lot of errors:

VIDEOIO ERROR: V4L: index 2 is not correct!
failed to open /usr/lib64/dri/hybrid_drv_video.so
Failed to wrapper hybrid_drv_video.so
failed to open /usr/lib64/dri/hybrid_drv_video.so
Failed to wrapper hybrid_drv_video.so
GStreamer Plugin: Embedded video playback halted; module v4l2src0 reported: Internal data stream error.
OpenCV Error: Unspecified error (GStreamer: unable to start pipeline
) in cvCaptureFromCAM_GStreamer, file /builddir/build/BUILD/opencv-3.2.0/modules/videoio/src/cap_gstreamer.cpp, line 832
VIDEOIO(cvCreateCapture_GStreamer(CV_CAP_GSTREAMER_V4L2, reinterpret_cast<char *>(index))): raised OpenCV exception:

/builddir/build/BUILD/opencv-3.2.0/modules/videoio/src/cap_gstreamer.cpp:832: error: (-2) GStreamer: unable to start pipeline
 in function cvCaptureFromCAM_GStreamer

OpenCV Error: Unspecified error (unicap: failed to get info for device
) in CvCapture_Unicap::initDevice, file /builddir/build/BUILD/opencv-3.2.0/modules/videoio/src/cap_unicap.cpp, line 139
VIDEOIO(cvCreateCameraCapture_Unicap(index)): raised OpenCV exception:

/builddir/build/BUILD/opencv-3.2.0/modules/videoio/src/cap_unicap.cpp:139: error: (-2) unicap: failed to get info for device
 in function CvCapture_Unicap::initDevice

CvCapture_OpenNI::CvCapture_OpenNI : Failed to enumerate production trees: Can't create any node of the requested type!
<VideoCapture 0x7fa5b5de0450>

No exception is thrown. When using c.read() later, I do get False, but I would like to do this in the initialisation phase of my program.

So, how do I find out how many valid cameras I have or check if a certain number 'maps' to a valid one?

Answer 1

Using cv2.VideoCapture( invalid device number ) does not throw exceptions. It constructs a <VideoCapture object> containing an invalid device - if you use it you get exceptions.

Test the constructed object for None and not isOpened() to weed out invalid ones.

---

For me this works (1 laptop camera device):

import cv2 as cv 

def testDevice(source):
   cap = cv.VideoCapture(source) 
   if cap is None or not cap.isOpened():
       print('Warning: unable to open video source: ', source)

testDevice(0) # no printout
testDevice(1) # prints message

Output with 1:

Warning: unable to open video source:  1

Example from: https://github.com/opencv/opencv_contrib/blob/master/samples/python2/video.py lines 159ff

cap = cv.VideoCapture(source)
    if 'size' in params:
        w, h = map(int, params['size'].split('x'))
        cap.set(cv.CAP_PROP_FRAME_WIDTH, w)
        cap.set(cv.CAP_PROP_FRAME_HEIGHT, h)
if cap is None or not cap.isOpened():
    print 'Warning: unable to open video source: ', source