WPF enabling transparency - wpf

Question

Going off the how-to from MSDN here, I have the following code that displays a reflection right below an element.
How can I enable transparency on the reflection only, so what is behind the window shows through the reflection?
<Window x:Class="XAMLViewTests.AboutWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="AboutWindow" ResizeMode="CanResizeWithGrip" SizeToContent="WidthAndHeight" Width="400"
AllowsTransparency="True" WindowStyle="None" WindowStartupLocation="CenterOwner" Background="Transparent">
<Window.Resources>
<Style TargetType="TextBlock" x:Key="formattedText">
<Setter Property="FontFamily" Value="Calibri"></Setter>
<Setter Property="FontSize" Value="20"></Setter>
<Setter Property="Padding" Value="5,5,5,5"></Setter>
<Setter Property="TextWrapping" Value="Wrap"></Setter>
</Style>
<LinearGradientBrush x:Key="linearGradBrush">
<LinearGradientBrush.GradientStops>
<GradientStop Offset="0.0"
Color="White"></GradientStop>
<GradientStop Offset="1.0"
Color="LightSlateGray"></GradientStop>
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
</Window.Resources>
<StackPanel Orientation="Vertical" Background="{StaticResource linearGradBrush}">
<Button Width="50" HorizontalAlignment="Right" Click="ReturnToPreviousWindow" Background="White">Return</Button>
<TextBlock Style="{StaticResource formattedText}" x:Name="textBlock" HorizontalAlignment="Center">
Some text here.
</TextBlock>
<!-- Reflection visual courtesy of MS How-To at https://msdn.microsoft.com/en-us/library/aa970263(v=vs.110).aspx -->
<Rectangle Height="1" Fill="Gray" HorizontalAlignment="Stretch"></Rectangle>
<Rectangle Height="{Binding ElementName=textBlock, Path=ActualHeight}"
Width="{Binding ElementName=textBlock, Path=ActualWidth}"
HorizontalAlignment="{Binding ElementName=textBlock, Path=HorizontalAlignment}">
<Rectangle.Fill>
<VisualBrush Stretch="None" Visual="{Binding ElementName=textBlock}">
<VisualBrush.RelativeTransform>
<TransformGroup>
<ScaleTransform ScaleX="1" ScaleY="-1"></ScaleTransform>
<TranslateTransform Y="1"></TranslateTransform>
</TransformGroup>
</VisualBrush.RelativeTransform>
</VisualBrush>
</Rectangle.Fill>
<Rectangle.OpacityMask>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Color="#FF000000" Offset="0.0"></GradientStop>
<GradientStop Color="#33000000" Offset="0.5"></GradientStop>
<GradientStop Color="#00000000" Offset="0.9"></GradientStop>
</LinearGradientBrush>
</Rectangle.OpacityMask>
<Rectangle.Effect>
<BlurEffect Radius="2.5"></BlurEffect>
</Rectangle.Effect>
</Rectangle>
</StackPanel>
</Window>

Answer 1

The linear gradient brush has 100% opacity and is applied to the entire stackpanel which contains the element you want transparency on. I reformatted it a bit and split out the stackpanel. Probably needs more work, but this should demonstrate the concept. Note the second gradient brush with a 0.5 (50%) opacity applied to the second stackpanel.
<Window x:Class="XAMLViewTests.AboutWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="AboutWindow" ResizeMode="CanResizeWithGrip" SizeToContent="WidthAndHeight" Width="400"
AllowsTransparency="True" WindowStyle="None" WindowStartupLocation="CenterOwner" Background="Transparent">
<Window.Resources>
<Style TargetType="TextBlock" x:Key="formattedText">
<Setter Property="FontFamily" Value="Calibri"></Setter>
<Setter Property="FontSize" Value="20"></Setter>
<Setter Property="Padding" Value="5,5,5,5"></Setter>
<Setter Property="TextWrapping" Value="Wrap"></Setter>
</Style>
<LinearGradientBrush x:Key="linearGradBrush">
<LinearGradientBrush.GradientStops>
<GradientStop Offset="0.0"
Color="White"></GradientStop>
<GradientStop Offset="1.0"
Color="LightSlateGray"></GradientStop>
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="linearGradBrushWithTransparency" Opacity="0.5">
<LinearGradientBrush.GradientStops>
<GradientStop Offset="0.0"
Color="White"></GradientStop>
<GradientStop Offset="1.0"
Color="LightSlateGray"></GradientStop>
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
</Window.Resources>
<StackPanel Orientation="Vertical" Background="{StaticResource linearGradBrushWithTransparency}">
<StackPanel Orientation="Vertical" Background="{StaticResource linearGradBrush}">
<Button Width="50" HorizontalAlignment="Right" Click="ReturnToPreviousWindow" Background="White">Return</Button>
<TextBlock Style="{StaticResource formattedText}" x:Name="textBlock" HorizontalAlignment="Center">
Some text here.
</TextBlock>
<!-- Reflection visual courtesy of MS How-To at https://msdn.microsoft.com/en-us/library/aa970263(v=vs.110).aspx -->
<Rectangle Height="1" Fill="Gray" HorizontalAlignment="Stretch"></Rectangle>
</StackPanel>
<Rectangle Height="{Binding ElementName=textBlock, Path=ActualHeight}"
Width="{Binding ElementName=textBlock, Path=ActualWidth}"
HorizontalAlignment="{Binding ElementName=textBlock, Path=HorizontalAlignment}"
Opacity=".5">
<Rectangle.Fill>
<VisualBrush Stretch="None" Visual="{Binding ElementName=textBlock}">
<VisualBrush.RelativeTransform>
<TransformGroup>
<ScaleTransform ScaleX="1" ScaleY="-1"></ScaleTransform>
<TranslateTransform Y="1"></TranslateTransform>
</TransformGroup>
</VisualBrush.RelativeTransform>
</VisualBrush>
</Rectangle.Fill>
<Rectangle.OpacityMask>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Color="#FF000000" Offset="0.0"></GradientStop>
<GradientStop Color="#33000000" Offset="0.5"></GradientStop>
<GradientStop Color="#00000000" Offset="0.9"></GradientStop>
</LinearGradientBrush>
</Rectangle.OpacityMask>
<Rectangle.Effect>
<BlurEffect Radius="2.5"></BlurEffect>
</Rectangle.Effect>
</Rectangle>
</StackPanel>
</Window>