Change resolution of extracted frame in Opencv

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I have an H.264 video stream and I need to extract frames from it. However when I extract the frames,the quality is really poor since I need to perform color segmentation!I want to know how can i extract the frame and convert it to B G R so as to have a better quality picture. Here is the code :

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

using namespace cv;
using namespace std;

int main(){
    VideoCapture capture("1.dv4");
    if(!capture.isOpened())
        return 1;

    double rate=capture.get(CV_CAP_PROP_FPS);

    bool stop(false);

    Mat frame;
    namedWindow("Extracted Frame",CV_WINDOW_NORMAL);

    cout <<"Rate is="<<rate;

    int delay=1000/rate;

    while(!stop){
        if(!capture.read(frame))
            break;

            imshow("Extracted Frame",frame);
            imwrite("C:/Users/DELL/Documents/Visual Studio 2010/Projects/VideoFrameCapture/VideoFrameCapture/frame.jpg",frame);

        if(waitKey(delay)>=0)
            stop=true;
    }
    capture.release();
    waitKey(0);
    return 1;
}