[vtkusers] map image onto surface

[Martin Genet]

Dear VTK users:

I have a (3D) vtkImage that has a black region and a non black one. I 
also have a surface (vtkPolyData) that roughly represent the limit 
between those two regions. I need to assign to every point of the 
surface the value of the closest point of the image that's non zero.

I have been trying to use a vtkProbeFilter, but unfortunately I can't 
seem to figure out how to tell him not to use black pixels. Is there a 
way to do so, maybe using another filter? More generally, is there a 
better way to perform such mapping?

Thanks!

Martin