<Window x:Class="OpacityMaskExample"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title=""
Height="430" Width="300">
<Grid>
<Button Height="60" Width="60"
RenderTransformOrigin="1,0.5">
<Button.OpacityMask>
<LinearGradientBrush StartPoint="0,0"
EndPoint="0,1">
<GradientStop Color="Transparent"
Offset="0" />
<GradientStop Color="#77000000" Offset="1" />
</LinearGradientBrush>
</Button.OpacityMask>
<Button.RenderTransform>
<ScaleTransform ScaleY="-1" />
</Button.RenderTransform>
</Button>
</Grid>
</Window>