If you want to add an oval clip to your control, you can do it the following way:
Xaml
<Button x:Name="MyButton" Content="Click me!" Width="80" Height="30">
<Button.Clip>
<EllipseGeometry RadiusX="35" RadiusY="13" Center="40,15"></EllipseGeometry>
</Button.Clip>
</Button>
C#
EllipseGeometry myClip = new EllipseGeometry();
myClip.Center = new Point( MyButton.Width / 2, MyButton.Height / 2 );
myClip.RadiusX = MyButton.Width / 2;
myClip.RadiusY = MyButton.Height / 2;
MyButton.Clip = myClip;
For the example we use a button. We set the clip using the System.Windows.Media.EllipseGeometry object. The center is in relation to the size of the button. In this case we have a button with height 30 and width 80. The center of the button will be with coordinates (40,15). We set the center of the clip to the center of the rectangle. Of course we can set it anywhere we want. We use the RadiusX and RadiusY properties to define the size.
Without clip
With clip
That's it!