VisualBrush with fading margins in WPF

I'm trying to create a VisualBrush with a hatched pattern that would fade at the top and bottom margins. While I had no problem with hatch itself:

<VisualBrush x:Key="b1" TileMode="Tile" Viewport="0,0,15,15" ViewportUnits="Absolute" Viewbox="0,0,15,15" ViewboxUnits="Absolute">
    <VisualBrush.Visual>
        <Canvas>
            <Path Data="M 0 15 L 15 0" Stroke="Gray"></Path>
            <Path Data="M 0 0 L 15 15" Stroke="Gray"></Path>
        </Canvas>
    </VisualBrush.Visual>    
</VisualBrush>

I don't really see a way to make it fade. Any idea how to do this?

Trying to get a background as per image below.

As I understood from your example image, you need to specify a general Background. For this I setting Grid instead Canvas and set for him Background color.

For panel which will display the content need to set OpasityMask like this:

<Window.Resources>
    <VisualBrush x:Key="b1" TileMode="Tile" Viewport="0,0,15,15" ViewportUnits="Absolute" Viewbox="0,0,15,15" ViewboxUnits="Absolute">
        <VisualBrush.Visual>
            <Grid Background="Black">
                <Path Data="M 0 15 L 15 0" Stroke="Gray" />
                <Path Data="M 0 0 L 15 15" Stroke="Gray" />
            </Grid>
        </VisualBrush.Visual>
    </VisualBrush>
</Window.Resources>

<Grid Background="{StaticResource b1}">
    <Grid.OpacityMask>
        <LinearGradientBrush StartPoint="0,0" EndPoint="1,0">
            <GradientStop Offset="0" Color="Black" />
            <GradientStop Offset="1" Color="Transparent" />
        </LinearGradientBrush>
    </Grid.OpacityMask>

    <Label Content="TEST" Foreground="White" HorizontalAlignment="Center" VerticalAlignment="Center" />
</Grid>

Result