[vtkusers] How to rotate an object around its center?

[zfjgo]

Hi everybody,
   Usually we can rotate an object around its center by clicking mouse using vtkRenderWindowInteractor.Now I want rotate it at certain angle around its center.But RotateX,RotateY,RotateZ only rotate it around axes X,Y,Z,not its center.
   How to obtain the center of an object displayed in renderwindow. How to rotate it around its center?
  Help me!!
 Thanks 
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