Window.Icon is an example project which shows how to convert the included Material Design Icons delivered via the PackIcon control into an ImageSource and bind it to the Window.Icon property.

First, a reference to the VisualToImageSourceConverter is added to the Window.Resources ResourceDictionary:

<Window.Resources>
    <local:VisualToImageSourceConverter x:Key="VisualToImageSourceConverter" />
</Window.Resources>

Then a Binding is specified, using the registered converter, between the Window.Icon property and an instance of the PackIcon control. The Kind property picks the Smiley glyph (from the PackIconKind enum), and the Height, Width and Foreground properties set the desired visual display properties for use on the window.

<Window.Icon>
    <Binding Converter="{StaticResource VisualToImageSourceConverter}">
        <Binding.Source>
            <materialDesign:PackIcon Kind="Smiley" Width="30" Height="30" Foreground="Red" />
        </Binding.Source>
    </Binding>
</Window.Icon>

The Convert method of the VisualToImageSourceConverter class first creates a Rect of the specified height and width. Then the VisualToImageSourceConverter uses the RenderTargetBitmap framework class to Render the glyph in the PixelFormats.Pbgra32 format into the Rect, while accounting for a display DPI of 96.

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    if (value is FrameworkElement element)
    {
        element.Measure(new Size(element.Width, element.Height));
        element.Arrange(new Rect(new Size(element.Width, element.Height)));
        var rtb = new RenderTargetBitmap((int)Math.Ceiling(element.ActualWidth), 
            (int)Math.Ceiling(element.ActualHeight), 96, 96, PixelFormats.Pbgra32);
        rtb.Render(element);

        return rtb;
    }

    return null;
}

Note: in a production application, the real client DPI should be taken into account.