How did hollywood manage to make Lindsay lohan apear to have a twin in the parent trap?

When annie and her twin sister hallie were hugging each other.


How could hollywood merge two films together to make Lindsay lohan hug a copy of herself|||There are a few tricks. First are stand-ins who are only filmed with their backs to the camera. Computer generated images are sometimes used.





The final trick involves exposing only part of the film at one time; then rewinding the film and changing costumes the actor performs a different role while another part of the film is exposed. This is how Micheal J. Fox appeared on screen in four different roles during "Back To The Future 2."|||They used a mirror|||Blue screen, like always