The Invisible Man

The following London poster campaign is specifically in support of Troy Davis, a man described as having “been on death row for 19 years in the USA, despite serious doubts about his conviction.”
 
The posters, depicting a close-up Davis’s face, are mounted on fence railings that disguise the posters so that the face behind the bars is revealed only when viewed from an angle.

How they did it?  View the video.

Created by  Brothers & Sisters with the collaboration street art collective Mentalgassi in support of Amnesty International.