C# WPF 擦出效果，刮图效果

找了很久

<Window x:Class="TestWebbowser.TestMaskWind"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Background="WhiteSmoke"
      Title="MainWindow" Height="1277" Width="947" MouseMove="canvas_MouseMove" MouseDown="Window_MouseDown" MouseUp="Window_MouseUp">
   
     <Grid Height="1024" Width="800">
        <Grid.Background>
            <ImageBrush ImageSource="s.jpg" />
        </Grid.Background>
        <Rectangle Height="100" HorizontalAlignment="Left" Margin="164,22,0,0" Name="rectangle1" Stroke="Black" VerticalAlignment="Top" Width="200" Fill="Lime" />
        <Button Content="Button" Height="83" HorizontalAlignment="Left" Margin="526,109,0,0" Name="button1" VerticalAlignment="Top" Width="115" Click="button1_Click" />
        <Grid x:Name="grid" Background="#E54E4E4E" Opacity="1" ></Grid>
    </Grid>
</Window>

　　后台代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

namespace TestWebbowser
{
    /// <summary>
    /// Interaction logic for TestMaskWind.xaml
    /// </summary>
    public partial class TestMaskWind : Window
    {
        public TestMaskWind()
        {

            InitializeComponent();
            RectangleGeometry rg = new RectangleGeometry();
            rg.Rect = new Rect(0, 0, this.Width, this.Height);
            gridGeometry = Geometry.Combine(gridGeometry, rg, GeometryCombineMode.Union, null);
            grid.Clip = gridGeometry;
        }
        private bool isDown = false;
        PathGeometry gridGeometry = new PathGeometry();
        private void canvas_MouseMove(object sender, MouseEventArgs e)
        {
            if (isDown)
            {
                EllipseGeometry rg = new EllipseGeometry();
                rg.Center = e.GetPosition(this);
                rg.RadiusX = 50;
                rg.RadiusY = 50;
                //排除几何图形
                gridGeometry = Geometry.Combine(gridGeometry, rg, GeometryCombineMode.Exclude, null);
                grid.Clip = gridGeometry;
            }
        }


        private void button1_Click(object sender, RoutedEventArgs e)
        {
            MessageBox.Show("OK!");
        }


        private void Window_MouseDown(object sender, MouseButtonEventArgs e)
        {
           // if (e.LeftButton == MouseButtonState.Pressed)
           // {
                isDown = true;
           // }
        }


        private void Window_MouseUp(object sender, MouseButtonEventArgs e)
        {
            isDown = false;
        }
    }
}