Question

I am developing an application that requires multiple webcams. In order to make sure that the correct webcam is used for each part of the application, I created some udev rules that SYMLINK the webcam to a specific name, depending on the serial number.

This works great, and I can access the camera by that name using VLC and a variety of other applications.

But when I try to access the camera by that name (or the non-syminked name given by linux) using OpenCV and python, I can’t read a frame from the camera and my program hangs. The camera is opened successfully. I’ve created a sample application in C++ to test if it was perhaps a python/opencv related bug, but the same thing happens in C++ too.

Here is my C++ test application that doesn’t work:

#include <iostream>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

int main (int argc, const char * argv[])
{
    VideoCapture cap("/dev/my_custom_name");
    if (!cap.isOpened())
        return -1;

    cout << "Opened..." << endl;

    Mat img;
    namedWindow("video capture", CV_WINDOW_AUTOSIZE);
    while (true)
    {
        cout << "Trying..." << endl;
        cap >> img;
        cout << "Got" << endl;
        imshow("video capture", img);
        if (waitKey(10) >= 0)
            break;
    }
    return 0;
}

I get the Opened... and Trying... messages, but not the Got message.

Any ideas on how to resolve this issue?

(This is all on linux btw).

Thanks

Answer 1

I figured this out. When I opened the capture in VLC, I noticed that it preixed the filename with v4l2://. When I did the same in my application, it worked!

So to reference above, "/dev/my_custom_name" should become "v4l2:///dev/my_custom_name".