wpf inkcanvas strokes to image

See more:

C#

hi
I am saving the ink canvas strokes into image using the following code
RenderTargetBitmap rtb = new RenderTargetBitmap((int)inkSign.Width, (int)inkSign.Height, 96d, 96d, PixelFormats.Default);
rtb.Render(inkSign);
BmpBitmapEncoder encoder = new BmpBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(rtb));
FileStream fs = File.Open(Environment.CurrentDirectory + "/Photos/Sign"+NextId+".jpg", FileMode.Create);
encoder.Save(fs);
fs.Close();
but it saves the image as compete black rectangle how solve this in wpf