private void RefreshBackground(double rate)
{
DrawingBrush drawingBrush = ((DrawingBrush)this.Background).Clone();
Rect rect = (Rect)drawingBrush.GetValue(DrawingBrush.ViewportProperty);
drawingBrush.SetValue(DrawingBrush.ViewportProperty, new Rect(0, 0, rect.Width * rate, rect.Height * rate));
this.Background = drawingBrush;
}