[vtkusers] help:How can I convert polydata to imagedata!

[susan]

hi,all
   I used vtkSweptSurface to generate a swept sueface.I must read data from VRML file with vtkVRMLImporter,but vtkSweptSurface input type is vtkImageData, through conversion,I can get polydata from vtkVRMLImporter,I wonderer how to convert polydata to imagedata.
basic steps of swept surface program lists as follows:
vtkVRMLImporter-->vtkSweptSurface-->vtkContourFilter