opencv cv::VideoCapture 任何时候设备可用，都可以正常工作

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void qr_frame::loop(Mat &raw, Mat &out)
{
	if (boxload_requested || show_raw_img)
	{
		//printf("loop\n");
		try
		{
			if (false == inputVideo.isOpened())
			{
				//std::cout << "try open \n";
				inputVideo.open(camera_id);
			}

			if (inputVideo.isOpened() && inputVideo.grab())
			{
				//std::cout << "open ennn \n";

				inputVideo.retrieve(raw);
				//std::cout<<raw.size()<<endl;
				if (raw.size().height > 1)
				{
					if (raw.channels() > 1)
						cvtColor(raw, raw, CV_BGR2GRAY);
					raw.copyTo(out);
					seeMarker(out);
				}
			}
			else
			{
				//printf("releas\n");
				inputVideo.release();
			}
		} catch (cv::Exception& e)
		{
			printf("catch %s\n", e.what());
		}

	}
	else
	{
		if (inputVideo.isOpened())
		{
			//std::cout << "release \n";
			inputVideo.release();
		}
	}
}