WPF渐变窗体的实现

Question

        想在WPF实现一个渐变窗体，从屏幕中间向四周扩展，直至布满整个屏幕。我用Animation来实现，但是达不到效果，运行结束后只在屏幕上方出现一条线。单独实现Width或者Height的渐变动画没问题，同时的话，就存在上面的现象。不知是哪里设置不对？求解，下面是我的代码：

        private double width = SystemParameters.PrimaryScreenWidth;
        private double height = SystemParameters.PrimaryScreenHeight;

        public NavigateWindow()
        {
            InitializeComponent();

            Left = width / 2;
            Top = height / 2;
            Width = 0;
            Height = 0;
            WindowStyle = System.Windows.WindowStyle.None;
            ResizeMode = System.Windows.ResizeMode.NoResize;
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            DoubleAnimation daWidth = new DoubleAnimation();
            daWidth.From = Width;
            daWidth.To = width;
            daWidth.Duration = new Duration(TimeSpan.FromSeconds(0.5));
            daWidth.AccelerationRatio = 0.5;

            DoubleAnimation daLeft = new DoubleAnimation();
            daLeft.From = Left;
            daLeft.To = 0;
            daLeft.Duration = new Duration(TimeSpan.FromSeconds(0.5));
            daLeft.AccelerationRatio = 0.5;

            DoubleAnimation daHeight = new DoubleAnimation();
            daHeight.From = Height;
            daHeight.To = height;
            daHeight.Duration = new Duration(TimeSpan.FromSeconds(0.5));
            daHeight.AccelerationRatio = 0.5;

            DoubleAnimation daTop = new DoubleAnimation();
            daTop.From = Top;
            daTop.To = 0;
            daTop.Duration = new Duration(TimeSpan.FromSeconds(0.5));
            daTop.AccelerationRatio = 0.5;

            BeginAnimation(LeftProperty, daLeft);
            BeginAnimation(TopProperty, daTop);
            BeginAnimation(WidthProperty, daWidth);
            BeginAnimation(HeightProperty, daHeight);
        }