[WPF] Draw a hole in a semi-transparant canvas

Hi,

I am creating a small application for my own use which overlays a semi-transparant black window on top of my whole screen. I can then draw a 'selection rectangle' on top of that window, and when I release the mouse, a screenshot is made from the region underneath the rectangle and immediately uploaded to my webserver and the link is copied to my clipboard.

Now instead of just having a white line that defines the selection I'd like to actually 'cut a hole' through the canvas, so that the screen is (semi-transparent) black except for my selection, which should be completely transparant and show the desktop underneath.

How can I achieve this?

At the moment I merely have a Canvas that handles some mouse properties:

csharp Code:

public partial class MainWindow : Window
    {
        private Rectangle rectangle;
        private Point clickLocation;
 
        public MainWindow()
        {
            InitializeComponent();
        }
 
        private void Window_KeyUp(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.Escape)
            {
                this.Close();
            }
        }
 
        private Rectangle CreateRectangle()
        {
            var rect = new Rectangle();
            rect.Stroke = Brushes.White;
            rect.StrokeThickness = 2;
            return rect;
        }
 
        private void canvas_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            rectangle = this.CreateRectangle();
 
            clickLocation = e.GetPosition(canvas);
            Canvas.SetLeft(rectangle, clickLocation.X);
            Canvas.SetTop(rectangle, clickLocation.Y);
            rectangle.Width = 0;
            rectangle.Height = 0;
 
            canvas.Children.Add(rectangle);
        }
 
        private void canvas_MouseMove(object sender, MouseEventArgs e)
        {
            if (rectangle == null) return;
 
            var location = e.GetPosition(canvas);
            var width = location.X - clickLocation.X;
            var height = location.Y - clickLocation.Y;
 
            if (width < 0)
            {
                Canvas.SetLeft(rectangle, location.X);
                width = -width;
            }
            if (height < 0)
            {
                Canvas.SetTop(rectangle, location.Y);
                height = -height;
            }
 
            rectangle.Width = width;
            rectangle.Height = height;
        }
 
        private void canvas_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
        {
            var rect = new Rect();
            rect.X = Canvas.GetLeft(rectangle);
            rect.Y = Canvas.GetTop(rectangle);
            rect.Width = rectangle.Width;
            rect.Height = rectangle.Height;
 
            this.HandleScreenGrab(rect);
 
            canvas.Children.Clear();
            rectangle = null;
        }
 
        private void HandleScreenGrab(Rect region)
        {
            //...
        }
    }

I have read in the past about being able to 'cut' geometries by combining them in an exclude combination, or something, but I cannot figure it out... This seems to be close:
http://stackoverflow.com/questions/7...background-wpf
But I cannot figure out how to apply it in my case. I've tried creating a RectangleGeometry from the canvas, as well as from the 'rectangle' (which is the selection) and combining them, but the results varied from nothing at all (no change) to a completely transparant window everywhere. Basically everthing except what I want: a rectangular 'hole' (completely transparent) through an otherwise semi-transparent canvas.

Thanks!