OpenCV VideoCapture :: get提供错误的输出

Question

我正在使用OpenCV 2.3.1版本。当我尝试获得各种Videocapture属性时，opencv会提供错误的输出，除了高度和宽度：

我的代码：

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

using namespace cv;
using namespace std;

int main(int argc, char* argv[])
{
    VideoCapture cap(0); // open the video file for reading

    if ( !cap.isOpened() )  // if not success, exit program
    {
         cout << "Cannot open the video file" << endl;
         return -1;
    }

    //cap.set(CV_CAP_PROP_POS_MSEC, 300); //start the video at 300ms

        double msec = cap.get(CV_CAP_PROP_POS_MSEC ); //Current position of the video file in milliseconds or video capture timestamp
        cout << "current position : " << msec << endl;
        double fra = cap.get(CV_CAP_PROP_POS_FRAMES); //0-based index of the frame to be decoded/captured next
        cout << "fra: " << fra << endl;
    double width = cap.get(CV_CAP_PROP_FRAME_WIDTH); //width of the frames in the video stream.
        cout << "width: " << width << endl;
    double height = cap.get(CV_CAP_PROP_FRAME_HEIGHT); //Height of the frames in the video stream.
        cout << "height: " << height << endl;
    double fps = cap.get(CV_CAP_PROP_FPS); //frame rate
        cout << "fps: " << fps << endl;
    double fourcc = cap.get(CV_CAP_PROP_FOURCC); //4-character code of codec
        cout << "fourcc: " << fourcc << endl;
    double frame_count = cap.get(CV_CAP_PROP_FRAME_COUNT ); //Frame count in the video
        cout << "frame count: " << frame_count << endl;

    namedWindow("MyVideo",CV_WINDOW_AUTOSIZE); //create a window called "MyVideo"

    while(1)
    {
        Mat frame;

        bool bSuccess = cap.read(frame); // read a new frame from video

         if (!bSuccess) //if not success, break loop
        {
                        cout << "Cannot read the frame from video file" << endl;
                       break;
        }

        imshow("MyVideo", frame); //show the frame in "MyVideo" window

        if(waitKey(30) == 27) //wait for 'esc' key press for 30 ms. If 'esc' key is pressed, break loop
       {
                cout << "esc key is pressed by user" << endl; 
                break; 
       }
    }

    return 0;

}

输出类似于除高度和宽度之外的任何属性：

HIGHGUI ERROR: V4L2: Unable to get property <unknown property string>(0) - Invalid argument
current position : -1
HIGHGUI ERROR: V4L2: Unable to get property <unknown property string>(1) - Invalid argument
fra: -1
width: 640
height: 480
HIGHGUI ERROR: V4L2: Unable to get property <unknown property string>(5) - Invalid argument
fps: -1
HIGHGUI ERROR: V4L2: Unable to get property <unknown property string>(6) - Invalid argument
fourcc: -1
HIGHGUI ERROR: V4L2: Unable to get property <unknown property string>(7) - Invalid argument
frame count: -1