Dynamic Clipping

   
  
<StackPanel xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
  xmlns:d="http://schemas.microsoft.com/expression/interactivedesigner/2006"
  mc:Ignorable="d"
  x:Class="PaintDrawExamples.DynamicClipping" 
  Width="640" Height="480">

    <StackPanel.Resources>
        <Storyboard x:Key="OnLoaded"/>
    </StackPanel.Resources>
    <StackPanel.Triggers>
        <EventTrigger RoutedEvent="FrameworkElement.Loaded">
            <BeginStoryboard x:Name="OnLoaded_BeginStoryboard" Storyboard="{DynamicResource OnLoaded}"/>
        </EventTrigger>
    </StackPanel.Triggers>
    <Canvas Height="100" x:Name="Canvas" Width="436">
        <Canvas.Clip>
            <PathGeometry>
                <PathFigure StartPoint="1,5" IsClosed="True" IsFilled="True">
                    <BezierSegment IsSmoothJoin="True" Point1="2,2" Point2="26,1" Point3="24,127" IsStroked="True"/>
                    <BezierSegment IsSmoothJoin="True" Point1="1,1" Point2="14,9" Point3="19,5" IsStroked="True"/>
                    <BezierSegment IsSmoothJoin="True" Point1="14,11" Point2="18,-22.5" Point3="24,-2" IsStroked="True"/>
                    <BezierSegment IsSmoothJoin="True" Point1="26,-200" Point2="29,1" Point3="300,5" IsStroked="True"/>
                </PathFigure>
            </PathGeometry>
        </Canvas.Clip>
        <Rectangle d:LayoutOverrides="Height" Stroke="{x:Null}" Fill="Red" Width="436" Height="100" x:Name="Rectangle" Canvas.Left="0" Canvas.Top="0"/>
        <Label Background="Black" x:Name="Label" Content="This is my clipped space." Canvas.Left="46" Canvas.Top="26" d:IsHidden="True"/>
    </Canvas>
</StackPanel>
//File:Window.xaml.cs

using System;
using System.IO;
using System.Net;
using System.Windows;
using System.Windows.Input;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Navigation;

namespace PaintDrawExamples
{
    public partial class DynamicClipping
    {
        public DynamicClipping()
        {
            this.InitializeComponent();
            this.Canvas.VerticalAlignment = VerticalAlignment.Center;
            this.Canvas.HorizontalAlignment = HorizontalAlignment.Center;

            CompositionTarget.Rendering += CompositionTarget_Rendering;
        }

        private void CompositionTarget_Rendering(object sender, EventArgs e)
        {
            Point mousePos = Mouse.GetPosition(this.Canvas);
            Geometry clippingRegion = this.Canvas.Clip;

            TranslateTransform newPos = new TranslateTransform();
            newPos.X = mousePos.X - (this.Canvas.Width / 2);
            newPos.Y = mousePos.Y - (this.Canvas.Height / 2);

            clippingRegion.Transform = newPos;
        }
    }
}