How to clip rendering to be within a rounded border?

I'm trying to create a border with rounded corners and a highlight on its top half. I'm using an ellipse with a radial gradient, overlapping the top half of the border, to give the highlight, but I can't figure out how to prevent the ellipse coloring the corners of the border. Here's a screenshot from Kaxaml:

And here's the XAML code:

<Page
  xmlns="ht开发者_JAVA百科tp://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Background="DarkGray">

  <Grid Width="256" Height="256">
    <Border CornerRadius="16" Background="Black" Margin="4">
      <Border Background="Gray" Margin="32">
        <TextBlock Foreground="Black" Text="1" FontFamily="Trebuchet MS" FontSize="96pt"
                   HorizontalAlignment="Center" VerticalAlignment="Center"/>
      </Border>
    </Border>
    <Border CornerRadius="16" ClipToBounds="True">
      <Ellipse>
        <Ellipse.Fill>
          <RadialGradientBrush>
            <GradientStop Color="White" Offset="0"/>
            <GradientStop Color="Transparent" Offset="1"/>
          </RadialGradientBrush>
        </Ellipse.Fill>
        <Ellipse.RenderTransform>
          <TransformGroup>
            <ScaleTransform ScaleX="3" ScaleY="2" CenterX="128" CenterY="128"/>
            <TranslateTransform Y="-235"/>
          </TransformGroup>
        </Ellipse.RenderTransform>
      </Ellipse>
    </Border>
    <Border CornerRadius="16" BorderThickness="8" BorderBrush="Black"/>
  </Grid>
</Page>

How can I stop the top corner areas being affected by the ellipse's shading?

I've tried playing around with OpacityMask, but I have to confess I don't really understand how to use it, especially with the ellipse being transformed for rendering. Whatever I try, the ellipse either disappears entirely or is completely unaffected.

Any help would be greatly appreciated.

Thanks in advance.

---

Instead of using ClipToBounds or OpacityMask, I ended up using the radial gradient as the background to an additional Border element. Thanks to alpha-mouse for the tip.

<Page
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Background="DarkGray">

  <Grid Width="256" Height="256">
    <Border CornerRadius="16" Background="Black" Margin="4">
      <Border Background="Gray" Margin="32">
        <TextBlock Foreground="Black" Text="1" FontFamily="Trebuchet MS" FontSize="96pt"
                   HorizontalAlignment="Center" VerticalAlignment="Center"/>
      </Border>
    </Border>
    <Border CornerRadius="16" Margin="4">
      <Border.Background>
        <RadialGradientBrush>
          <RadialGradientBrush.Transform>
            <TransformGroup>
              <ScaleTransform ScaleX="3" ScaleY="2" CenterX="128" CenterY="128"/>
              <TranslateTransform Y="-235"/>
            </TransformGroup>
          </RadialGradientBrush.Transform>
          <GradientStop Color="White" Offset="0"/>
          <GradientStop Color="Transparent" Offset="1"/>
        </RadialGradientBrush>
      </Border.Background>
    </Border>
    <Border CornerRadius="16" BorderThickness="8" BorderBrush="Black"/>
  </Grid>
</Page>

And here's the final appearance:

Thanks.

---

First, it looks fine when I compiled and run you code in Visual Studio. Second, why do not you use this RadialGradientBrush as Background of the first Border? Something like this:

<Border CornerRadius="16" >
    <Border.Background>
        <RadialGradientBrush Center=".5 0" GradientOrigin=".5 0" RadiusX="1" >
            <GradientStop Color="White" Offset="0"/>
            <GradientStop Color="Black" Offset="1"/>
        </RadialGradientBrush>
    </Border.Background>
</Border>