How can I get WPF's ClipToBounds to work?

Question

I have an application that displays an image inside of an Image object in WPF. The image is contained in a control whose xaml looks like:

<UserControl x:Class="MyProgram.NativeImageDisplay"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Loaded="UserControl_Loaded">
    <Canvas Name="border" Background="Black" >
        <Image Name="image" StretchDirection="Both" Stretch="Uniform" ClipToBounds="True"
                SnapsToDevicePixels="True" RenderOptions.BitmapScalingMode="HighQuality"></Image>
    </Canvas>
</UserControl>

Two of these controls are contained in a grid in a window, like so:

    <Grid  Grid.Row="2" Name="DisplayCanvas">
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <l:NativeImageDisplay x:Name="imageDisplay2" Grid.Column="1" ClipToBounds="True"/>
        <l:NativeImageDisplay x:Name="imageDisplay" Grid.Column="0" ClipToBounds="True"/>           
    </Grid>

I'm calling clipping to be true all the way through here.

The user can zoom on the image by using the scroll button on the mouse, and that ultimately calls a ScaleTransform on the image:

    private void image_MouseWheel(object sender, MouseWheelEventArgs e)
    {
        if (!thumbnail)
        {
            TransformGroup transformGroup = (TransformGroup)border.RenderTransform;
            ScaleTransform transform = (ScaleTransform)transformGroup.Children[0];

            double oldScaleX = transform.ScaleX;
            double oldScaleY = transform.ScaleY;

            double zoom = e.Delta;
            transform.ScaleX += zoom / 10000;
            transform.ScaleY += zoom / 10000;
            if (transform.ScaleX > maxZoom || transform.ScaleY > maxZoom)
            {
                transform.ScaleX = maxZoom;
                transform.ScaleY = maxZoom;
            }
            if (transform.ScaleX < minZoom || transform.ScaleY < minZoom)
            {
                transform.ScaleX = minZoom;
                transform.ScaleY = minZoom;
            }
            Point thePoint = e.GetPosition(border);
            transform.CenterY = 0;
            transform.CenterX = 0;

            foreach (UIElement child in border.Children)
            {
                if (child is Anchor)
                {
                    TransformGroup group = (TransformGroup)child.RenderTransform;
                    ScaleTransform t = (ScaleTransform)group.Children[0];
                    t.ScaleX *= oldScaleX / transform.ScaleX;
                    t.ScaleY *= oldScaleY / transform.ScaleY;
                }
            }
        }
    }

Once that scale transform is called, the image is no longer contained in the boundary of its canvas or the grid selection. Essentially, ClipToBounds is being ignored. How can I make this transform pay attention to ClipToBounds?

Answer 1

Canvas is kind of unique in that it doesn't really participate in the layout system like other elements. It basically acts as an infinite size space with fixed position children so generally ignores clipping completely. I can't tell for sure without seeing more of the code but if you want to apply the clipping to the scaled object moving the scaling to a different element might do what you want. The simplest thing to do would be to wrap a Border around your Canvas and apply the ScaleTransform to that instead. The Border should give you better clipping behavior.

<Border x:Name="border" Background="Black" ClipToBounds="True">
    <Canvas x:Name="imageHost">
    ...
    </Canvas>
</Border>

Answer 2

The above comment help me out. Nest one canvas in another, add ClipToBounds="True" to the parent and bind the nested height and width to the parent properties respectively.
This way removes the need to perform transformations on the parent.

<Canvas ClipToBounds="True" Name="Outer">
     <Canvas x:Name="Inner" 
         Height="{Binding ActualHeight, ElementName=Outer, Mode=OneWay}"
         Width="{Binding ActualWidth, ElementName=Outer, Mode=OneWay}" />
</Canvas>