<Window
    x:Class="WpfApp2.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="clr-namespace:WpfApp2"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Title="MainWindow"
    Width="329"
    Height="450"
    MouseDown="UserControl_MouseDown"
    MouseMove="UserControl_MouseMove"
    MouseUp="UserControl_MouseUp"
    mc:Ignorable="d">
    <Grid>
        <Grid.Background>
            <ImageBrush ImageSource="/220810025-677037712005f5749b8c6481aa1b6c9a507047e97479d68af717064904b1db2d (1).png" Stretch="Uniform" />
        </Grid.Background>
        <Grid
            x:Name="gridShadow">
            <Grid.Background>
                <ImageBrush ImageSource="/325e99d639690e7e8a30d908d6c8b1cd399f0ac611a49e30a848ccf2eaae4ccf.png"/>
            </Grid.Background>
            <TextBlock
                HorizontalAlignment="Center"
                VerticalAlignment="Center"
                FontSize="30"
                Foreground="White">
                刮奖区
            </TextBlock>
        </Grid>
    </Grid>
</Window>

using System;
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.Navigation;
using System.Windows.Shapes;

namespace WpfApp2
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
        {
            PathGeometry gridGeometry = new PathGeometry();
            private bool isDown = false;

            double originArea;
            bool hasShowResult = false;
            public MainWindow()
            {
                InitializeComponent();
                RectangleGeometry rg = new RectangleGeometry();
                rg.Rect = new Rect(0, 0, this.Width, this.Height);
                gridGeometry = Geometry.Combine(gridGeometry, rg, GeometryCombineMode.Union, null);
                gridShadow.Clip = gridGeometry;

                originArea = gridGeometry.GetArea();
            }


            private void UserControl_MouseMove(object sender, MouseEventArgs e)
            {
                if (isDown)
                {
                    EllipseGeometry rg = new EllipseGeometry();
                    rg.Center = e.GetPosition(gridShadow);
                    rg.RadiusX = 20;
                    rg.RadiusY = 20;
                    //排除几何图形
                    gridGeometry = Geometry.Combine(gridGeometry, rg, GeometryCombineMode.Exclude, null);
                    gridShadow.Clip = gridGeometry;

                    var currentArea = gridGeometry.GetArea();
                    if ((currentArea * 100 / originArea) < 50 && !hasShowResult)
                    {
                        hasShowResult = true;
                        MessageBox.Show("恭喜中奖1个亿!");
                    }
                }
            }

            private void UserControl_MouseDown(object sender, MouseButtonEventArgs e)
            {
                isDown = true;
            }

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