opencv摄像头捕获图像

#include <iostream>
#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;
int main()
{
        Mat input_image;
        VideoCapture cam(0);
        if (!cam.isOpened()) exit(0);
        Size s(320, 240);
        VideoWriter writer = VideoWriter("myvideo.avi", CV_FOURCC('M', 'J', 'P', 'G'), 25, s);
        if (!writer.isOpened())
        {
                cout << "cannot create a file. " << endl;
                return -1;
        }

        namedWindow("input image");
        while (true) {
                cam >> input_image;
                //WriteFrame(writer, outimage)
//                cvWriteFrame(writer, input_image);
                imshow("input image", input_image);
//                writer << input_image;
                if (27 == waitKey(30))
                        break;
        }
       
}=====3.0里面是这个 函数 视频帧  cam >> input_image;