wpf 生成一张叉号图片

 DrawingVisual drawingVisual = new DrawingVisual();
            DrawingContext dc = drawingVisual.RenderOpen();
            dc.DrawLine(new Pen(Brushes.Red,2),new Point(0,0),new Point(100,100));
            dc.DrawLine(new Pen(Brushes.Red,2),new Point(100,0),new Point(0,100));
            dc.Close();

            RenderTargetBitmap bmp = new RenderTargetBitmap((int)100, (int)100, 96, 96, PixelFormats.Pbgra32);
            bmp.Render(drawingVisual);
            BitmapEncoder encoder = new JpegBitmapEncoder();
            encoder.Frames.Add(BitmapFrame.Create(bmp));
            using (Stream stm = File.Create("d:\\aaa.png"))
            {
                encoder.Save(stm);
            }
           bmp.Clear();