Scientists have unveiled new software that enables them to digitally alter people's facial expressions in existing videos.

The system, developed by researchers at Stanford University in the US and University of Erlangen-Nuremberg in Germany, tracks an actor's expressions and then maps them on to a subject's face, manipulating it to mirror whatever the actor is doing.

"The novelty of our approach," say researchers, "lies in the transfer and photorealistic re-rendering of facial deformations and detail into the target video in a way that the newly-synthesized expressions are virtually indistinguishable from a real video."

Essentially, anyone could be made to appear to say anything.