internal static void OnGeometryChanged(DependencyObject d)
{
Primitive3D p = ((Primitive3D)d);
p._content.Geometry = p.Tessellate();
}
internal static void OnMaterialChanged(Object sender, DependencyPropertyChangedEventArgs e)
{
Primitive3D p = ((Primitive3D)sender);
p._content.Material = p.Material;
}