VideoCapture won't open camera on osx [closed]

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I have a MacBook with a built in camera. When I try to open device 0 with VideoCapture, isOpen() always returns false.

My OpenCV lib is compiled from source. Build options are largely default, with the exception that I enabled a few extra WITH_* flags (qt, for example). I am compiling on the command line, not with XCode.

• How do I troubleshoot why VideoCapture fails to open?
• How do I inspect available devices and their status?
• What is necessary in building opencv (or my own project) to ensure that video capture support is enabled?
• What might cause VideoCapture to fail?

I'm using the video capture example verbatim:

#include "opencv2/opencv.hpp"

using namespace cv;

int main(int, char**)
{

    VideoCapture cap(0); // open the default camera
    if(!cap.isOpened()) { // check if we succeeded
        std::cout << "no capture device :(\n";
        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;
    }
    // the camera will be deinitialized automatically in VideoCapture destructor
    return 0;
}