[vtkusers] Question about vtkActor::Translate

[rchqh]

hi all,
I am using vtkActor to show images in 3D space.I want to rotate and tranlate 
an Actor by calling SetUserMatrix and AddPosition.
   actor->SetUserMatrix(mat);
   actor->AddPosition(x,y,z);
I found while rotating images this method worked well,but it didn't work while 
translating images.Can you tell me why I couldn't translate an Actor using 
Translate function and how to translate an Actor.
Any help is appreciated.