[vtkusers] how to get 3D image\'s barycenter and how to shoot rays

[xujf]

Hi,
  I am new of VTK. I want to get the barycenter of a 3D image,and I want to shoot some rays to the surface of the 3D image from this barycenter.I don\'t know how to get the barycenter of a 3D image and I also don\'t know how to shoot rays form certain point.which classes  should I use? Give me some examples if it is possible.thanks!

Best Regards!
xujf