How do they make those projections of dead celebrities and politicians?

Snoop Dogg shares the stage with deceased rapper Tupac Shakur at the 2012 Coachella Valley Music & Arts Festival.
Snoop Dogg shares the stage with deceased rapper Tupac Shakur at the 2012 Coachella Valley Music & Arts Festival.
©Christopher Polk/Getty Images

The big news at the 2012 Coachella music festival was the live-action Princess Leia-style "hologram" of Tupac Shakur -- who was shot to death in 1996 -- that took the stage to duet with Snoop Dogg on a couple of songs. Now, of course, the discussion has moved on to possible concert tours with other dead people, reproductions of the technology using Marilyn Monroe, Ronald Reagan and host of other celebrities. But most people didn't understand what they were really looking at.

It makes sense that people would call it a hologram, and there's a case to be made here for linguistic drift -- if it looks like the holograms we know from media and childhood, and works just like them, then what's the point in splitting hairs? -- but the real story is more complicated and, to my mind, more interesting. A true hologram is a system of projected images that changes with the angle from which you look at it, just like a real object would. While Snoop interacted with the Tupac avatar onstage in such a way as to give the illusion of three dimensions, it was really just a two-dimensional optical illusion -- and not only that, but one first invented in the 1860's!

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There are two parts to the illusion: first, the computer program created by the Oscar-winning Digital Domain Media Group, which worked with Dr. Dre to create a perfect simulacrum, or likeness,of the deceased rap star. Digital Domain is the leader in the field of this sort of computer-generated imagery, and has done similar posthumous reconstructions for several unnamed corporate clients.

The company took physical images and video movement from footage of the rapper when he was still alive, got input from Tupac's friends and colleagues on the realism of the animation, and finally created an all-new video recording of a performance that never happened. In fact, when the ghost first appeared, he saluted the Coachella crowd by name, despite having died three years before the annual festival even started.

And the second part of the illusion? Read on to find out how the Coachella audience was artfully deceived by a projector and a reflection.

The Projection, the Illusion and Pepper's Ghost

This illustration, published in "The World of Wonders" circa 1865, shows how the Pepper's Ghost illusion was employed in a theatrical performance using a projection reflected off of a large sheet of glass.
This illustration, published in "The World of Wonders" circa 1865, shows how the Pepper's Ghost illusion was employed in a theatrical performance using a projection reflected off of a large sheet of glass.
©Archive Photos/Getty Images

For the performance itself, the San Diego-based company AV Concepts used an updated version of the stage technique "Pepper's Ghost," which was first invented by John Henry Pepper for an 1862 Charles Dickens theatrical show. In essence, the illusion uses the fact that glass can be both transparent and reflective to create effects that confuse the two. Perhaps the simplest way to understand it is to think about your living room windows at night. If you have your indoor lights on, people can see in but you can't really see out -- the light reflects back at you. If you turn off the lights inside, the opposite happens and people can't see you from the street.

The same effect is in play with Pepper's Ghost: Trickery with your light source makes objects reflected in a glass look like they're on the other side of it. The area where the image appears is built to be identical to the hidden area where the object actually is, so the glass we're looking at is actually doing double-duty: Both reflecting the hidden area (the "blue room," in theatrical parlance) and transparently revealing the area we're supposed to be looking at. The illuminated objects in the hidden area, then, appear to hang in the room we're actually looking at.

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For the Tupac illusion, for example, the video was actually projected from above the stage, straight down onto a reflective surface that bounced the image up onto a Mylar screen for Snoop to sing along with. Because the only light source being reflected onto the screen was Tupac himself, the stage area and the mirror that bounced the image onto the stage foil looked exactly the same, letting Tupac's image hang in the air ... just like a ghost.

Author's Note

Birthdays, accomplishments, any special occasions were, for me, chances to ask for a trip to the science museum. From Phoenix to Albuquerque to Houston and San Antonio, my trips to health and science museums across the southwest are some of the best memories of my childhood. To this day, I still love animatronic dinosaurs and hope one day to own an army of them -- but my favorite up-close exhibits were always the ones that used the Pepper's Ghost illusion to transform my face into something else (an ape's face, usually) with hand-operated lights and mirrored glass. They're scary and funny, and because the physics behind the illusion are so strong, just as visceral and fascinating 20 years later as they were when I was a kid.

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