canvas 超出部分自动隐藏

1.   <Canvas x:Name="a" BACkground="White" Loaded="LayoutRoot_Loaded">
               <Canvas.Clip>
                   <RectangleGeometry RadiusX="5" RadiusY="5" Rect="0,300,50" />
               </Canvas.Clip>
               <Canvas.triggers>
                   <Eventtrigger RoutedEvent="Canvas.Loaded" >
                       <BeginStoryboard>
                           <Storyboard x:Name="animation" Storyboard.TargetProperty="(Canvas.Left)" RepeatBehavior="Forever" >
                               <DoubleAnimation Storyboard.TargetName="ScrollingText" From="0" To="-820" Duration="0:0:20"  />
                               <DoubleAnimation Storyboard.TargetName="ScrollingText2" From="820" To="0" Duration="0:0:20"  />
                           </Storyboard>
                       </BeginStoryboard>
                   </Eventtrigger>
               </Canvas.triggers>
               <TextBlock x:Name="ScrollingText" FontFamily="Comic Sans MS" FontSize="36" Foreground="Green"  TEXT="I have downloaded the moded bios for the x200 which is revision 3.03. The new 3.05 Bios for x200 add some support for Windows 7. " />
               <TextBlock x:Name="ScrollingText2" FontFamily="Comic Sans MS" FontSize="36" Foreground="Green" Text="I have downloaded the moded bios for the x200 which is revision 3.03. The new 3.05 Bios for x200 add some support for Windows 7. " />
           </Canvas>

启到作用的是canvas.clip