[vtkusers] how to get pixel coordinates of iso-surface

[diaoxianfen]

I use the ray casting volume rendering to visualize image datas.The procedure as follows,
vtkVolumeRayCastIsosurfaceFunction *isosurfaceFunction = 
    vtkVolumeRayCastIsosurfaceFunction::New();
  isosurfaceFunction->SetIsoValue(800);
 raycastMapper->SetVolumeRayCastFunction( isosurfaceFunction ); 
Every thing is Ok.
Now I want to get the pixel coordinates of the isosurface of 800 gray value.I know that the isosurface
is connected by triangles,how can I get the pixel coordinates or nearby coordinates corresponding to 
the isosurface?
thanks a lot!
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