18

I'm trying to create 3 real-time capture frames with webcams into a USB hub into my laptop. Using the "camera" app on Windows, I can change the camera source one at a time and confirm that all 3 webcams are working. However, my OpenCV Python code can only ever find two.

(Quick notes on the USB - it's a USB 3.0 hub, laptop port is USB 3, and I even have an active USB female-to-male cable going into the laptop, so given this and the Windows app working, I generally trust the hardware.)

Below I did some raw testing of cv2.VideoCapture(src) with the results below: 

cams_test = 10
for i in range(0, cams_test):
    cap = cv2.VideoCapture(i)
    test, frame = cap.read()
    print("i : "+str(i)+" /// result: "+str(test))

That first argument, test, returns True/False depending on if the frame can be read. Results:

i : 0 /// result: True
i : 1 /// result: True
i : 2 /// result: False
i : 3 /// result: False
i : 4 /// result: False
i : 5 /// result: False
i : 6 /// result: False
i : 7 /// result: False
i : 8 /// result: False
i : 9 /// result: False

As with other sample code I tested, only 2 webcams can be registered and show frames in Python. And the Windows 10 camera app lets me scroll between all 3 working and connected webcam feeds.

I know I can create multiple, like 3+, cv2.imshow() frames if I use the caps that work. My project involves doing this to show realtime USB webcam feeds on the laptop from multiple cameras.

Any help and advice appreciated; also potentially interested in (Python-based) alternative solutions. Cheers.

JDS
  • 16,388
  • 47
  • 161
  • 224
  • Are all cameras of the same model? Have you tried rearranging camera ports? – zindarod Apr 05 '18 at 04:35
  • Yes, all same model cameras. The USB hub they're connected to is located somewhere a little difficult to reach so I haven't tried rearranging the ports unfortunately. – JDS Apr 06 '18 at 05:16
  • Maybe this C++ code could bring you closer to a solution: https://github.com/studiosi/OpenCVDeviceEnumerator – handle Apr 10 '18 at 13:31
  • Try to use the camera id `-1` for all devices (to get first/next available device?). This use does not seem to be documented for [VideoCapture](https://docs.opencv.org/3.4.1/d8/dfe/classcv_1_1VideoCapture.html#a5d5f5dacb77bbebdcbfb341e3d4355c1), but seemed to help [elsewhere](https://stackoverflow.com/questions/21792909/cv2-videocapture-open-always-returns-false). Or try to increase your test range, there are offsets to the id that specify the interface. – handle Apr 10 '18 at 13:37
  • @handle Yes I tried that, I really like the interface where it lets me choose, problem is OpenCV crashes (Windows 10) with the -1. Pretty sure it's the `VideoCapture()` function that crashes. What might the offsets be? How could I determine automatically? This is pretty rough, can't believe this isn't a standard feature. – JDS Apr 10 '18 at 21:15
  • The link to VideoCapture in my last comment mentions the offsets, but not the use of -1. Sorry, I can't really help. I was hoping you might "see" all three devices and get either the correct IDs or device strings to open them. – handle Apr 10 '18 at 21:53
  • Have you tested without the hub? Experience with USB says hubs are frequent source of confusion – jamesson Apr 13 '18 at 16:42
  • Also; some way to get Python to enumerate connected usb devices from Python? might be halpful – jamesson Apr 13 '18 at 16:44
  • 1
    Did you consider trying a different [videoIO backend](https://docs.opencv.org/3.4.1/d0/da7/videoio_overview.html)? It might be defaulting to VFW, so perhaps DirectShow might yield a better result? – Dan Mašek Apr 24 '18 at 17:08

4 Answers4

6

having two video input devices does not automatically mean that their IDs are 0 and 10. Try to access the all cameras by increasing to about 500 or more instead of 10

cams_test = 500
for i in range(0, cams_test):
    cap = cv2.VideoCapture(i)
    test, frame = cap.read()
    if test:
        print("i : "+str(i)+" /// result: "+str(test))

I think you are only testing

kishea
  • 637
  • 1
  • 6
  • 17
5

Do your webcams support USB3? Based on my practical experience with webcams and OpenCV, most common cameras only transmit on the USB2 standard, at 480 MBit/s. The exact details of how many cameras you can support on one USB channel depend on the resolution, framerate, and video compression technology. In my experience, with 1080p color video at 30 fps and H.264 encoding, only one camera stream fit onto a single USB controller.

An easy test to see if you are bandwidth limited would be to modify your program to explicitly close each stream after it is tested (cap.release()), so that your test program only opens one camera at a time, like the Windows camera app. If that works, then you may very well be looking at a bandwidth limitation.

When debugging video capture issues on Windows, it can also be useful to open the camera streams with VLC. VLC can enumerate all of the cameras available from DirectShow, which is the same API that OpenCV's VideoCapture uses by default. You can also start multiple VLC instances to attempt simultaneous streams.

If you click on "Advanced Options" and then check "Device Properties," VLC will show which settings (exposure, focus, etc...) can be tweaked through DirectShow. These settings are usually accessible in OpenCV through VideoCapture.set().

Jack C.
  • 744
  • 3
  • 7
  • Thanks for your response. The webcams are older/cheaper and don't mention 'supporting' USB3. However I read that USB3 does all the things that USB2 does and just extends it. I'll try out the `cap.release()` idea. – JDS Apr 08 '18 at 02:46
  • USB3 is backwards compatible. https://superuser.com/questions/437687/is-usb-3-0-backwards-compatible – zindarod Apr 08 '18 at 07:45
  • @zindarod that's what I thought – JDS Apr 09 '18 at 02:29
  • @TarunLalwani Problem definitely still exists. Haven't found a good solution. Happy to use a different library than OpenCV at this point, as long as it's python based. – JDS Apr 17 '18 at 17:44
  • Try defining this environment variable `OPENCV_VIDEOCAPTURE_DEBUG=1` and see if any logs show up? – Tarun Lalwani Apr 17 '18 at 19:00
2

1) about cv2... Well, is there any other instances/alternatives? maybe that third cam is binded to other instance instead of cv2 ?

2) did you try to start loop (range) from -1 instead of 0?

3) If none of the method helps you, I advise to go to Device Manager and temporarily DISABLE the individual webcam and then run your script again and see what changes. After that ENABLE webcam and see what changed. ( in worst scenario, instead of ENABLE/DISABLE, you might insall/resintall the particular webcam and then windows might resolve them correctly).

T.Todua
  • 53,146
  • 19
  • 236
  • 237
1

First,we have to test your webcam can or cannot be connect to OPENCV.

connect only one webcam,and run your program with cv2.VideoCapture(0).Parameter 0 mean there is a single camera connected. if all your webcam is ok,then it will be problem of index,there is some code which will find the deviceID assigned to the cam.

and if some webcam unfortunately fail,then it will be hard.

In this case you have to create your own function that grabs using the SDK and write the image into a cv::Mat.

obgnaw
  • 3,007
  • 11
  • 25