Viewport2DVisual3D.Geometry Property

<Viewport3D>
    <Viewport3D.Camera>
        <PerspectiveCamera Position="0, 0, 4"/>
    </Viewport3D.Camera>

    <!-- Button on 3D -->
    <Viewport2DVisual3D>
        <!-- Give the plane a slight rotation -->
        <Viewport2DVisual3D.Transform>
            <RotateTransform3D>
                <RotateTransform3D.Rotation>
                    <AxisAngleRotation3D Angle="40" Axis="0, 1, 0" />
                </RotateTransform3D.Rotation>
            </RotateTransform3D>
        </Viewport2DVisual3D.Transform>

        <!-- The Geometry, Material, and Visual for the Viewport2DVisual3D -->
        <Viewport2DVisual3D.Geometry>
            <MeshGeometry3D Positions="-1,1,0 -1,-1,0 1,-1,0 1,1,0"
                            TextureCoordinates="0,0 0,1 1,1 1,0" TriangleIndices="0 1 2 0 2 3"/>
        </Viewport2DVisual3D.Geometry>

        <Viewport2DVisual3D.Material>
            <DiffuseMaterial Viewport2DVisual3D.IsVisualHostMaterial="True" Brush="White"/>
        </Viewport2DVisual3D.Material>                               

        <Button>Hello, 3D</Button>
    </Viewport2DVisual3D>

    <!-- Lights -->
    <ModelVisual3D>
        <ModelVisual3D.Content>
            <DirectionalLight Color="#FFFFFFFF" Direction="0,0,-1"/>
        </ModelVisual3D.Content>
    </ModelVisual3D>
</Viewport3D>