Silverlight - polygon scaling and clipping

Question

I have a Canvas, to which I've added several thousand polygons.

I would like to be able to zoom in (which I'm doing via a ScaleTransform.

However I've been trying to use a Canvas.Clip as well to only draw a portion of the Canvas, but as soon as the ScaleTransform values are changed, the clipping stops working...

    <Canvas Grid.Row="1" Margin="10" x:Name="cnvMain" Background="Transparent" >
        <Canvas.Clip>
            <RectangleGeometry x:Name="CanvasClip"  Rect="0, 0, 300, 300"/>
        </Canvas.Clip>
        <Canvas.RenderTransform>
            <ScaleTransform x:Name="CanvasScaleTransform" ScaleX="1" ScaleY="1"></ScaleTransform>
        </Canvas.RenderTransform>
     </Canvas>

And in my codebehind,

    private void slScale_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
    {
        CanvasScaleTransform.ScaleX = slScale.Value;
        CanvasScaleTransform.ScaleY = slScale.Value;
    }

Am I doing anything obviously wrong?

Answer 1

Put a border around your canvas and attach the clip region to the border rather than the canvas.

Answer 2

The ScaleTransform (as all other transformations) is applied AFTER every other rendering. That means, first the cliprect is applied, then the scale transform. A solution would be to do the clipping one level higher, by putting another canvas around this one.