OpenCV的imshow无法正常显示视频

OpenCV2.2,vs2010,win7

程序读一段视频，然后进行分析处理，在显示视频的时候，却发现imshow无法显示读取的帧。代码如下：

VideoCapture cap;
cap.open("test.avi");
namedWindow("video",1);
for(;;){
 Mat frame;
 cap<<frame;
 if(frame.empty())
	break;
 imshow("video",frame);
 //waitKey(30)
}

跟踪了一下，发现确实是读入了帧，只是在显示的时候没有出来。
在网上查了下文档，原来是循环中，highgui没有给予imshow绘制处理的时间。需要在imshow添加waitKey(30)

引用

A common mistake for opencv newcomers is to call cv::imshow() in a loop through video frames, without following up each draw with cv::waitKey(30). In this case, nothing appears on screen, because highgui is never given time to process the draw requests from cv::imshow().

来自： http://stackoverflow.com/questions/5217519/opencv-cvwaitkey