Access UI controls from another thread in WPF

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WPF

webcam

Task

Dispatcher

Hi, i am capturing image with Webcam. based on the device(laptop) used, the image capture either freezes at some point or not. What i want to achieve is to ensure that the capture does not freeze.

Here is the function that captures the image. it uses Direct Show Library

private void captureImage()
       {
           // Release any previous buffer
           if (m_ip != IntPtr.Zero)
           {
               Marshal.FreeCoTaskMem(m_ip);
               m_ip = IntPtr.Zero;
           }

           // capture image
           m_ip = cam.Click();
           b = new Bitmap(cam.Width, cam.Height, cam.Stride, System.Drawing.Imaging.PixelFormat.Format24bppRgb, m_ip);

           // If the image is upsidedown
           b.RotateFlip(RotateFlipType.RotateNoneFlipY);

           System.Windows.Controls.Image win = UIHelper.FindChild<System.Windows.Controls.Image>(System.Windows.Application.Current.MainWindow, "photo");

           win.Source = BitmapToImageSource(b);
           cam.Dispose();
           this.DialogResult = true;
           this.Close();
       }

What I have tried:

i'm trying to implement the asynchronous operation in a the Tick event handler of a dispatcher timer

void timer_Tick(object sender, EventArgs e)
        {
            i = --i;
            count.Content = i;
            if (i == 2)
            {
                prog.IsActive = true;
                new Task(captureImage).Start();
                
            }
            if (i == 0)
            {
                timer.Stop();
            }
        }

when i do that, i get this error "The calling thread cannot access this object because a different thread owns it" on this line "

System.Windows.Controls.Image win = UIHelper.FindChild<System.Windows.Controls.Image>(System.Windows.Application.Current.MainWindow, "photo");

" in the capture function

but when i use a dispatcher.Invoke like this

this.Dispatcher.Invoke((Action)(() =>
                {
                    captureImage();
                }));

the frame still freezes, any idea how to fix this?