[vtkusers] writing images from window to file in vtk4

[Michael Halle]

I feel this must be an embarrassingly simple question, but...

I'm using vtk4, and I want to write an image out to a file.  That means using
vtkWindowToImageFilter and connecting it to an ImageWriter (the PNG one, for
instance).  

I write out the file once, fine.  I change the scene, write the file again,
I get the same image.  The file is getting written, just with the old image.

I've played with adding various calls to "Update","UpdateInformation", and 
"ReleaseDataFlagOn/Off", (heck, even "Render"), and I have been unable to get 
the image to change after the first render.

Am I missing something obvious here?  I can post code if it's non-trivial....

Thanks.

								--Mike