[vtkusers] how to get a vtkvolume\'s normal and what is the encoded normal!

[xujf]

hello,all:
 I want volume\'s normal to do shading !in my mind the normal should be the opposite direction of gradient!
is it right?
    i find encoded normal is just a value per vertex, what is this encoed normal, it is not a direction?

Thanks in advance!
xujf