[vtkusers] Draw one image on top of another

[Artem Babayan]

Hello,

I draw concentration distribution field over the map -- images for map
and concentration fields are in the separate actors (vtkActor). I add
these actors to the render window:

         window->GetRenderers()->GetFirstRenderer()->AddActor(actor_map);
         window->GetRenderers()->GetFirstRenderer()->AddActor(actor_field);

However, two images gets mixed together -- instead of non-transparent
parts of concentration field overwriting the map image they are shown
together with pixels from the map going through the fields' pixels in
some places. It happens regardless of the order I add actors to
rendered. Is it a way to ensure, that image in actor_field is always
on the top of actor_map?

Best wishes
Artem