private void PaintFingerPoint(FingerPoint point, DrawingContext drawingContext)
{
drawingContext.DrawEllipse(new SolidColorBrush(Color.FromArgb(Math.Min((byte)100, this.Opacity), 255, 255, 255)), null, new Point(point.X, point.Y), 10, 10);
drawingContext.DrawEllipse(new SolidColorBrush(Color.FromArgb(this.Opacity, 255, 255, 255)), null, new Point(point.X, point.Y), 5.5, 5.5);
}