Tupac, in fact, appeared courtesy of a very old stage-craft technique known as "Pepper’s Ghost". A thin, transparent plastic sheet, ten metres across and four metres high, was lowered across the stage, slanting from the stage up towards the audience. While Snoop Dogg and Dr Dre rapped behind the mylar film, Tupac was projected on it using high-definition video projectors reflecting off mirrors below the stage. By carefully avoiding any stage lights glinting on the plastic, technicians kept the audience unaware they were looking at the (living) performers through the screen. Most of the light from the projector passed through the screen, but a few percent reflected from the front and back surface of the film; the same partial reflection that lets you see yourself in a shop window.
Because the projectors were very bright, and the spotlights on Snoop and Dr Dre well-controlled, everyone appeared with the same brightness, adding to the realism of the illusion. But Tupac had as much depth as any other 2D projection – none – and the illusion only worked because the audience was too far back to see this.
(Poster via John Coulthart)