RippleEffect is not updated on changing Center property

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WPF

Sorry,in fact,i use a Rectangle's RippleEffect to Render a water reflection.

<Rectangle x:Name="imgShader" RenderTransformOrigin="1,0.5" Height="256" Width="1024" MouseMove="Rectangle_MouseMove" MouseLeftButtonDown="Rectangle_MouseLeftButtonDown">
    <Rectangle.Fill>
        <VisualBrush Visual="{Binding ElementName=gdBackground}" />
    </Rectangle.Fill>
    <Rectangle.RenderTransform>
        <ScaleTransform ScaleY="-1" />
    </Rectangle.RenderTransform>
    <Rectangle.OpacityMask>
        <LinearGradientBrush StartPoint="0,0"
                             EndPoint="0,1">
            <GradientStop Offset="0.3"
                          Color="Transparent" />
            <GradientStop Offset="1"
                          Color="#44000000" />
        </LinearGradientBrush>
    </Rectangle.OpacityMask>
    <Rectangle.Effect>
        <ee:RippleEffect x:Name="effectImgShader" Frequency="0" Center="0.5,0.81" Phase="0" Magnitude="0.03"/>
    </Rectangle.Effect>
</Rectangle>

and when mousemove on the Rectangle, I change the RippleEffect's Center to make it likes something in the water;

private Point effectCenter = default(Point);
private void Rectangle_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
        {
            Point position = e.GetPosition(this.imgShader);
            this.effectCenter.X = position.X / this.imgShader.ActualWidth;
            this.effectCenter.Y = position.Y / this.imgShader.ActualHeight;
            this.effectImgShader.Center = this.effectCenter;
        }

but when I move mouse, it doesn't show effect because the Rectangle didn't refresh, so I use a WriteableBitmap in it to make it can use Invalidate() to repaint.

finally, I just want to refresh Rectangle when mouse moves.