text: In graphics , superimposition is the placement of an image or video on top of an already-existing image or video , usually to add to the overall image effect , but also sometimes to conceal something ( such as when a different face is superimposed over the original face in a photograph ) .
simplified: In graphics , superimposition is when an image or video is put on top of an already-existing image or video . This is usually done to add to the overall image effect or to hide something ( such as when a different face is superimposed over the original face in a photograph ) .