Save render window to an image.

        Arguments:
            filename:
                filename to save to. Defaults to image.png.
            magnification:
                magnification. Use it to render high res images.
            image_format:
                choose between jpeg, png.  Png is the default.

def write_image(self, filename="image.png", magnification=1,
                    image_format="png"):
        """
        Save render window to an image.

        Arguments:
            filename:
                filename to save to. Defaults to image.png.
            magnification:
                magnification. Use it to render high res images.
            image_format:
                choose between jpeg, png.  Png is the default.
        """
        render_large = vtk.vtkRenderLargeImage()
        render_large.SetInput(self.ren)
        if image_format == "jpeg":
            writer = vtk.vtkJPEGWriter()
            writer.SetQuality(80)
        else:
            writer = vtk.vtkPNGWriter()

        render_large.SetMagnification(magnification)
        writer.SetFileName(filename)

        writer.SetInputConnection(render_large.GetOutputPort())
        self.ren_win.Render()
        writer.Write()
        del render_large