stream a webcam image from a c++ opencv dll, through to a c# windows form?

edit

Hi, as above. I have an opencv dll application that i need to run through a windows forms application. I have this code compiling, but the resulting image is just black. Where am i going wrong here?

//c++ dll code:

    extern "C" {

       cv::Mat frame;
        __declspec(dllexport) void camera()
        {
            cv::VideoCapture cap;
            if (!cap.open(0))
                std::cout << "cam not up" << std::endl;

            while(true)
            {           
                cap >> frame;

                cv::imshow(":", frame);
                cv::waitKey(1);
            }

        }


        __declspec(dllexport)uchar*  img(void)
        {

        return frame.data;

        }

};

//c# form code:

     [DllImport("Cranium.dll", CallingConvention = CallingConvention.Cdecl)]
            public static extern void camera();

            [DllImport("Cranium.dll", CallingConvention = CallingConvention.Cdecl)]
            public static extern IntPtr img();

  private void stream()
        {
            Bitmap a = new Bitmap(640, 480, 3 * 640, PixelFormat.Format24bppRgb, img());
            pictureImg.Image = a;
        }

        private void buttonConnect_Click(object sender, EventArgs e)
        {
             Thread connectThread = new Thread(camera);
             connectThread.Start();

            Thread displayThread = new Thread(stream);
            displayThread.Start();

        }

When i run buttonConnect, the camera starts, and the c++ window runs fine. But the c# image in teh picturebox stays black.