Yeah I suppose it is used for films like 3D objects in real backgrounds. But I have never seen how it is done. I was always amazed how it is possible. Or when I saw shooting of Walking With Dinosaurs, I saw how they are making envoiroment movement (splashing water by foot) by actors and then they SOMEHOW remove whole person and suddenly only the splashes are visible, then they add there the 3D dinosaur and it seems like the dinosaur was making the splashes by his feet. This is so freaky awesome!!