How to de-interlace a frame

edit

I was just wondering what would be the right way to deinterlace images coming from a video (interlace one of course). My code sort of looks like this:

VideoCapture cap("video.mp4",  cv::CAP_FFMPEG); // open the default camera
if(!cap.isOpened())  // check if we succeeded
    return -1;

for(;;)
{
    Mat frame;
    cap >> frame; // get a new frame from camera
    imshow("frame", frame);
    if(waitKey(30) >= 0) break;
}
// the camera will be deinitialized automatically in VideoCapture destructor
return 0;

but now I'm not entirely sure how to run the algorithm to de-interlace any given frame. Any ideas?