[abenedik]

This way quite tricky :cool:

Sorry for waiting so long - I working on new version of Reader3ds.

So here is the solution:

The XAML part is the same as before:

Code:

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:zoomPanel="clr-namespace:Ab2d.Controls;assembly=Ab2d.Controls.ZoomPanel"
    Title="Window1" Height="300" Width="300"
    MouseWheel="Window_MouseWheel">
    <Grid>
        <zoomPanel:ZoomPanel Name="ZoomPanel1" IsMouseWheelZoomEnabled="False" ZoomMode="Move">
            <TextBlock Text="12345"/>
        </zoomPanel:ZoomPanel>
    </Grid>
</Window>

The tricky part is in the code:

Code:

private void Window_MouseWheel(object sender, MouseWheelEventArgs e)
{
    Point mousePosition;
    double zoomFactor;

    if (e.Delta > 0)
        zoomFactor = 1.3;
    else
        zoomFactor = 1 / 1.3;

    mousePosition = e.GetPosition(ZoomPanel1);

    ZoomPanel1.Viewbox = GetNewViewbox(zoomFactor, mousePosition);

    e.Handled = true;
}

private Rect GetNewViewbox(double zoomFactor, Point mousePosition)
{
    UIElement content;
    double zoomPanelAspectRatio, contentAspectRatio;
    double aspect;
    Rect newViewbox;
    Point relativeMousePosition;
    Point relativeViewboxPosition;
    Point relativeViewboxPosition2;
    Rect actualViewbox1, actualViewbox2;

    content = ZoomPanel1.Content as UIElement;
    
    if (content == null)
        return ZoomPanel1.Viewbox;

    zoomPanelAspectRatio = ZoomPanel1.ActualWidth / ZoomPanel1.ActualHeight;
    contentAspectRatio = content.DesiredSize.Width / content.DesiredSize.Height;

    // Apsect ration between ZoomPanel and its content
    aspect = contentAspectRatio / zoomPanelAspectRatio;


    relativeMousePosition = new Point(mousePosition.X / ZoomPanel1.ActualWidth,
                                      mousePosition.Y / ZoomPanel1.ActualHeight);


    // Calculate actual viewbox
    actualViewbox1 = AdjustRectForAspectRatio(ZoomPanel1.Viewbox, aspect);

    // mouse position on the original actual viewbox
    relativeViewboxPosition = new Point(actualViewbox1.X + actualViewbox1.Width * relativeMousePosition.X,
                                        actualViewbox1.Y + actualViewbox1.Height * relativeMousePosition.Y);





    // Do the normal zoom to the current center point for the zoomFactor
    double viewboxDx, viewboxDy;

    // viewboxDx and viewboxDy are the actual viewbox changes
    viewboxDx = ZoomPanel1.Viewbox.Width - (ZoomPanel1.Viewbox.Width / zoomFactor);
    viewboxDy = ZoomPanel1.Viewbox.Height - (ZoomPanel1.Viewbox.Height / zoomFactor);

    newViewbox = new Rect(ZoomPanel1.Viewbox.X + viewboxDx / 2,
                          ZoomPanel1.Viewbox.Y + viewboxDy / 2,
                          ZoomPanel1.Viewbox.Width - viewboxDx,
                          ZoomPanel1.Viewbox.Height - viewboxDy);



    // Now get where the mouse position would be in the newViewbox
    actualViewbox2 = AdjustRectForAspectRatio(newViewbox, aspect);


    relativeViewboxPosition2 = new Point(actualViewbox2.X + actualViewbox2.Width * relativeMousePosition.X,
                                         actualViewbox2.Y + actualViewbox2.Height * relativeMousePosition.Y);

    // Adjust the newViewbox so the objects under the mouse position stay on the same place
    newViewbox.X -= relativeViewboxPosition2.X - relativeViewboxPosition.X;
    newViewbox.Y -= relativeViewboxPosition2.Y - relativeViewboxPosition.Y;


    return newViewbox;
}

private Rect AdjustRectForAspectRatio(Rect originalRect, double aspect)
{
    Rect adjustedRect;

    adjustedRect = originalRect;

    if (aspect > 1) // content wider than ZoomPanel
    {
        //actualViewbox2.Y = actualViewbox1.Y * aspect - (actualViewbox1.Height * (aspect - 1) / 2);
        adjustedRect.Y *= aspect;
        adjustedRect.Height *= aspect;
    }
    else
    {
        adjustedRect.X /= aspect;
        adjustedRect.Width /= aspect;
    }

    return adjustedRect;
}

This should work.

The code will be probably included into next version of ZoomPanel (cannot say when it will be published).

Thank you for your recommendation.