cannot open camera with ubuntu opencv c++

Question

I have a problem I can not open camera 0 of my pc

here is the code that I use:

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

using namespace cv;
using namespace std;

int main(int, char**)
{
    VideoCapture cap(0); // open the default camera
    if(!cap.isOpened()) { // check if we succeeded
        cout << "cannot open camera "<< endl;
        return -1;
    }
    Mat edges;
    namedWindow("edges",1);

    for(;;){
        Mat frame;
        cap >> frame; // get a new frame from camera
        cvtColor(frame, edges, COLOR_BGR2GRAY);
        GaussianBlur(edges, edges, Size(7,7), 1.5, 1.5);
        Canny(edges, edges, 0, 30, 3);
        imshow("edges", edges);
        if(waitKey(30) >= 0) break;
    }
    return 0;
}

it is displaying

can not open camera

because isOpened returns false

Answer 1

Please make sure that the camera is correctly detected. You can do so by executing:

    $ls /dev/video*

I have also found that other programs give you a more detailed output eg ffmpeg with V4L Execute eg

     ffmpeg -f v4l2 -i /dev/video0 -vf scale=640:480 -r  20 -t 00:00:10 output.mp4

and it will tell you if the source is busy or why the camera can't be opened, while OpenCV just returns false.