RenderTargetBitmap rtb = new
 RenderTargetBitmap((int)canvas.Width, (int)canvas.Height, 96d,
 96d,PixelFormats.Default);
 
rtb.Render(canvas);

System.Windows.Media.Imaging.JpegBitmapEncoder encoder1 =
 new JpegBitmapEncoder();
 encoder1.Frames.Add(BitmapFrame.Create(rtb));
 encoder1.Save(fileStream);