WPF get image under user control

Here is how I made.
If you run the app, try resizing the window so that the clipped area will change accordingly to the rectangle's margin.
The right-side image should be always the same as the reddish covered area on the left.

<Window x:Class="WpfApplication5.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication5"
        Title="MainWindow" Height="400" Width="800">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>
        
        <Grid x:Name="grid1">
            <Image
                Source="http://icdn4.digitaltrends.com/image/microsoft_xp_bliss_desktop_image-650x0.jpg" 
                Stretch="UniformToFill" 
                x:Name="image1"
                />
            
            <Rectangle 
                x:Name="control1" 
                Margin="50,10,100,80" 
                Fill="Red"
                Opacity="0.33"
                >
            </Rectangle>
        </Grid>

        <Rectangle 
            Width="{Binding Path=ActualWidth, ElementName=control1}" 
            Height="{Binding Path=ActualHeight, ElementName=control1}" 
            Grid.Column="1"
            >
            <Rectangle.Fill>
                <VisualBrush
                    Visual="{Binding ElementName=image1}"
                    ViewboxUnits="RelativeToBoundingBox"
                    >
                    <VisualBrush.Viewbox>
                        <MultiBinding>
                            <MultiBinding.Converter>
                                <local:MyConv />
                            </MultiBinding.Converter>
                            <Binding Path="ActualWidth" ElementName="grid1" />
                            <Binding Path="ActualHeight" ElementName="grid1" />
                            <Binding Path="Margin" ElementName="control1" />
                        </MultiBinding>
                    </VisualBrush.Viewbox>
                </VisualBrush>
            </Rectangle.Fill>
        </Rectangle>
    </Grid>
</Window>

You need also a converter:

public class MyConv : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var gw = (double)values[0];
        var gh = (double)values[1];
        var th = (Thickness)values[2];

        var clip = new Rect(
            th.Left / gw,
            th.Top / gh,
            (gw - th.Right - th.Left) / gw,
            (gh - th.Bottom - th.Top) / gh
            );

        return clip;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Hope it helps.
Mario