I'm trying to achieve this:

[![enter image description here](https://i.stack.imgur.com/cBkPf.png)](https://i.stack.imgur.com/cBkPf.png)

The gradient proportion will grow so I did a circle as opacityMask then a recangle using the circle as mask, like that:

```
<Ellipse Name="Mask" 
    Fill="Red" 
    Width="{Binding ActualWidth, ElementName=mainGrid}" Grid.ColumnSpan="3" 
    Visibility="Visible"/>

<Rectangle Name="Gradient" Fill="Azure" 
   Grid.Column="2" Width="100" 
   HorizontalAlignment="Left" 
   VerticalAlignment="Stretch">
   <Rectangle.OpacityMask>
        <VisualBrush Visual="{Binding ElementName=Mask}" />
   </Rectangle.OpacityMask>
</Rectangle>

```

and I get this:
[![enter image description here](https://i.stack.imgur.com/7u69x.png)](https://i.stack.imgur.com/7u69x.png)

How do I keep the rectangle's form and mask it using the circle ??