[vtkusers] Saving PNG file without rendering window

[So Drafts]

Guys,

I am trying to render images of just simple shapes and
save the png files of them.
In that process, I do not want to open any rendering windows
on screen.

The python script I am trying is as follows.
by using this script, I found a png file is successfuly created,
but a rendering window appears for only a moment.
Does anyone know how to modify this script to hide the rendering windows?

thank you in advance,
So

------------(from here)-----------------------
import vtk

cone = vtk.vtkConeSource()

mapper = vtk.vtkPolyDataMapper()
mapper.SetInput(cone.GetOutput())

actor = vtk.vtkActor()
actor.SetMapper(mapper)

ren = vtk.vtkRenderer()
ren.AddActor(actor)

renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(ren)
renWin.Render()

w2if = vtk.vtkWindowToImageFilter()
w2if.SetInput(renWin)

writer = vtk.vtkPNGWriter()
writer.SetInputConnection(w2if.GetOutputPort())
writer.SetFileName('output.png')
writer.Write()
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