I need a Viewport3D for the sole purpose of doing geometric calculations using Petzold.Media3D.ViewportInfo. I do now want to place it in a Window.

I'm creating a Viewport3D using the following code:

private Viewport3D CreateViewport(MainSettings settings)
{
    var cameraPosition = new Point3D(0, 0, settings.CameraHeight);
    var cameraLookDirection = new Vector3D(0, 0, -1);
    var cameraUpDirection = new Vector3D(0, 1, 0);
    var camera = new PerspectiveCamera
    {
        Position = cameraPosition,
        LookDirection = cameraLookDirection,
        UpDirection = cameraUpDirection
    };
    var viewport = new Viewport3D
    {
        Camera = camera,
        Width = settings.ViewportWidth,
        Height = settings.ViewportHeight
    };
    return viewport;
}

Later, I'm attempting to use this viewport to convert the mouse location to a 3D location using this method:

public Point3D? Point2dToPoint3d(Point point)
{
    var range = new LineRange();
    var isValid = ViewportInfo.Point2DtoPoint3D(_viewport, point, out range);
    if (isValid)
        return range.PointFromZ(0);
    else
        return null;
}

Unfortunately, it's not working. I think the reason is that the ActualWidth and ActualHeight of the viewport and both zero (and these are read-only properties, so I can't set them manually). (I have tested the exact same with an actual rendered Viewport3D, so I know the issue is not with my converter method.)

Any idea how I can get WPF to assign the ActualWidth and ActualHeight based on my Width and Height settings?

I tried setting the HorizontalAlignment and VerticalAlignment to Left and Top, respectively, and I also messed with the MinWidth and MinHeight, but none of these properties had any effect on the ActualWidth or ActualHeight.