从HighGUI的一段代码中看OpenCV打开视频的方式

/**
 * Videoreader dispatching method: it tries to find the first
 * API that can access a given filename.
 */
CV_IMPL CvCapture * cvCreateFileCaptureWithPreference ( const char * filename, int apiPreference)
{
    CvCapture * result = 0;
    switch(apiPreference) {
    default :
        // user specified an API we do not know
        // bail out to let the user know that it is not available
        if (apiPreference) break;
# ifdef HAVE_FFMPEG
    case CV_CAP_FFMPEG :
        TRY_OPEN(result, cvCreateFileCapture_FFMPEG_proxy (filename))
        if (apiPreference) break;
# endif
    case CV_CAP_VFW :
# ifdef HAVE_VFW
        TRY_OPEN(result, cvCreateFileCapture_VFW (filename))
# endif
# if defined HAVE_LIBV4L || defined HAVE_CAMV4L || defined HAVE_CAMV4L2 || defined HAVE_VIDEOIO
        TRY_OPEN(result, cvCreateCameraCapture_V4L(filename))
# endif
        if (apiPreference) break;
    case CV_CAP_MSMF :
# ifdef HAVE_MSMF
        TRY_OPEN(result, cvCreateFileCapture_MSMF (filename))
# endif
# ifdef HAVE_XINE
        TRY_OPEN(result, cvCreateFileCapture_XINE (filename))
# endif
        if (apiPreference) break;
# ifdef HAVE_GSTREAMER
    case CV_CAP_GSTREAMER :
        TRY_OPEN(result, cvCreateCapture_GStreamer (CV_CAP_GSTREAMER_FILE, filename))
        if (apiPreference) break;
# endif
# if defined(HAVE_QUICKTIME) || defined(HAVE_QTKIT)
    case CV_CAP_QT :
        TRY_OPEN(result, cvCreateFileCapture_QT (filename))
        if (apiPreference) break;
# endif
# ifdef HAVE_AVFOUNDATION
    case CV_CAP_AVFOUNDATION :
        TRY_OPEN(result, cvCreateFileCapture_AVFoundation (filename))
        if (apiPreference) break;
# endif
# ifdef HAVE_OPENNI
    case CV_CAP_OPENNI :
        TRY_OPEN(result, cvCreateFileCapture_OpenNI (filename))
        if (apiPreference) break;
# endif
# ifdef HAVE_OPENNI2
    case CV_CAP_OPENNI2 :
        TRY_OPEN(result, cvCreateFileCapture_OpenNI2 (filename))
        if (apiPreference) break;
# endif
    case CV_CAP_IMAGES :
        TRY_OPEN(result, cvCreateFileCapture_Images (filename))
    }
    return result;
}