opencv library video play [closed]

I’m using the code from our previous conversation to build a minimal example that shows how to retrieve frames from the camera and display them inside a QLabel.

Notice that the value passed to cvCreateCameraCapture() can change on your system. On my Linux 0 is the magic number. On Windows and Mac I use -1.

For simplicity purposes, I’m just sharing the body of the main() function:

int main(int argc, char** argv)
    QApplication app(argc, argv);

    CvCapture* capture = cvCreateCameraCapture(0); // try -1 or values > 0 if your camera doesn't work
    if (!capture)
        std::cout << "Failed to create capture";
        return -1; 

    IplImage* frame = NULL;
    QLabel label;
    while (1) 
        frame = cvQueryFrame(capture);
        if( !frame ) break;

        QImage qt_img = IplImage2QImage(frame);

        // calling cvWaitKey() is essencial to create some delay between grabbing frames
        cvWaitKey(10); // wait 10ms

    return app.exec();


The procedure to play a video file or video from a camera are the same. The only thing that really changes is cvCreateCameraCapture() for cvCreateFileCapture().

If cvCreateFileCapture() is returning NULL, it means that it couldn’t open the video file. This could happen for 2 reasons: it doesn’t have the codec to deal with the video, or it couldn’t find the file.

A lot of people make the folowwing mistake on Windows when loading files:

capture = cvCreateFileCapture("C:\path\video.avi");

They use single slashes, when they should be using double:

capture = cvCreateFileCapture("C:\\path\\video.avi");

and if something can fail, you must ALWAYS check the return of the call:

capture = cvCreateFileCapture("C:\\path\\video.avi");
if (!capture)
   // print error message, then exit

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