Did New York city have to be demolished to get that shot? Can we really conjure mystical clouds from the end of a stick? Do we have ten foot tall blue people on our neighboring planets? Since when did we have to share the pavement with dinosaurs? Film has the power to make us believe that the answer to all the questions is 'yes'.
These visual spectacles are possible through a cinematic effects technique called compositing.
So what is compositing?
Compositing is combining visual elements from different sources into one single image, making it appear as if all of them are part of the same scene. The trick is to do this so seamlessly that the viewer will not be able to tell that the image is made up of several juxtaposed or overlaid elements.
Compositing is extensively used in filmmaking to generate imagery or footage that is physically not possible to shoot all at once. It has broken down all boundaries to what can be shown, surrendering to the imagination of the creative filmmakers and artists. Today, with digital compositing, there is nothing that cannot be created. Live action people can be placed in virtual worlds, animated characters can be put into real world, or both the characters and the world can be generated on the computer. It’s unbelievably awesome!
Here is an explicit clip about the composition work done for Star Wars : Threads of Destiny that was nearly 80% composited.