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

Answer 2

Another solution, which is available in Linux is to use the /dev/videoX device in the VideoCapture() call. The devices are there when the cam is plugged in. Together with glob(), it is trivial to get all the cameras:

import cv2, glob

for camera in glob.glob("/dev/video?"):
    c = cv2.VideoCapture(camera)

Of course a check is needed on c using isOpened(), but you are sure you only scan the available cameras.

Answer 3

Here is a "NOT working" solution to help you prevent tumbling over in this pitfall:

import cv2 as cv
import PySpin

print (cv.__version__)

# provided by Patrick Artner as solution to be working for other cameras than
#                                                  those of Point Grey (FLIR).

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

# ... PySpin / Spinnaker (wrapper/SDK libary) ...

system   = PySpin.System.GetInstance()
cam_list = system.GetCameras()

cam = ''
cam_num = 0

for ID, cam in enumerate(cam_list):
    # Retrieve TL device nodemap
    if ID == cam_num:
        print ('Got cam')
        cam = cam

        cam.Init()

        # ... CV2 again ...

        for i in range(10):
            testDevice(i) # no printout

Answer 4

You can try this code:

from __future__ import print_function
import numpy as np
import cv2

# detect all connected webcams
valid_cams = []
for i in range(8):
    cap = cv2.VideoCapture(i)
    if cap is None or not cap.isOpened():
        print('Warning: unable to open video source: ', i)
    else:
        valid_cams.append(i)

caps = []
for webcam in valid_cams:
    caps.append(cv2.VideoCapture(webcam))

while True:
    # Capture frame-by-frame
    for webcam in valid_cams:
        ret, frame = caps[webcam].read()
        # Display the resulting frame
        cv2.imshow('webcam'+str(webcam), frame)
    k = cv2.waitKey(1)
    if k == ord('q') or k == 27:
        break

# When everything done, release the capture
for cap in caps:
    cap.release()

cv2.destroyAllWindows()