Hip-hop fans are dropping their collective jaw as word of the Tupac "hologram" is ricocheting around the Internet. As seen in the five-minute video, a three-dimensional Shakur is seen, shirtless, moving across the stage, and even greeting the crowd at the beginning with a stunning voice that sounds an awful lot like Tupac himself: "What the [f] is up, Coachella?"

The virtual rendition of the late rapper then proceeds to do renditions of two classic ‘Pac tracks, "Hail Mary" and "2 of Amerikaz Most Wanted," while gesturing and walking back and forth across the stage in an extremely lifelike manner, replete with Thug Life tattoos and his characteristic necklace. Twitter, unsurprisingly, has been abuzz with chatter—spawning an admittedly hilarious new account: @HologramTupac: "Anybody got a spare 54 AA sized batteries? I think Snoop done smoked my charger. #PlugLife"

Only problem? This wasn’t a hologram at all. Rather, it was a clever optical illusion technique known as "Pepper’s Ghost," which dates back to a technique first described by an Italian scientist in the 16th century.

This combination of high-quality computer rendering and old-school optical trickery has impressed many experts.

"It is amazing, no question about it," David Brady, the head of the Duke Imaging and Spectroscopy Program at Duke University, told Ars on Monday. "The impressive thing here is how life-like and detailed and natural it seems, and that’s just an outcome of advances in computer rendering [rather] than display."

But of course, while Tupac may be the best (and possibly first posthumous) performance of a Pepper’s Ghost illusion, there have been plenty of others in recent years. Al Gore did it in 2007. Madonna and the Gorillaz did it in 2006. Richard Branson did it in 2005. Heck, even a Canadian backyard engineer built one in a few days for Halloween.

John Pepper, a British chemist, adapted a technique conceived by fellow Briton Henry Dircks, for use in the theater by the middle of the 19th century. As a result, the technique was named after him.

So here’s how it works: the audience needs to be able to see into the main room, but not into an adjacent hidden room. In the case of the Tupac "hologram," that’s the main stage where a real-life Snoop Dogg and Dr. Dre were sharing the limelight. However, hidden on stage is a piece of glass, where the images can be reflected from and pushed into a target area that makes it seem like a single room. But off to the side, behind the glass, there’s a hidden room that has the original object being projected.

These days, AV Concepts, the San Diego-based company behind the Tupac performance, uses a proprietary Mylar foil, known as Musion Eyeliner, rather than glass. The company said in a press release on Monday that its on-site server "delivered uncompressed media for 3 stacked 1920 x 1080 images, delivering 54,000 lumens of incredibly clear projected imagery."

"For a large piece of glass to support its own weight on the 45-degree angle, it has to be very thick," explained James Rock, a creative director at Musion, a British company that has pioneered this technique, in a 2010 interview with the UK newspaper The Independent. "And that means you get a double image. It was called a Pepper's 'ghost' as the image wasn't very bright when using pre-electric light sources."

However, in this case, a high-quality digital rendering of Tupac was simply projected onto that foil.

"It’s a beautiful piece of CGI," said Michael Bove, the head of the Object-Based Media group at the MIT Media Lab.

"This is digital substitution of a character who’s not here anymore—and that’s wonderful. And that’s the part we should be celebrating. The folks who made the content did a really lovely job. I’m surprised more people don’t know more about this because it’s been around since the middle of the 19th century," he said.

The company has been pretty quiet about the precise details of the Tupac performance, with company representatives being careful to say that it wasn’t a true hologram, but rather a "holographic illusion." This is despite the fact that the company’s own press release refers to it as a "hologram" and a "holographic projection." AV Concepts hasn’t said whether Tupac’s motion was taken from actual video footage of the rapper, or how the audio was created—Bove speculated that the audio may have been performed by a voice actor who sounds similar to Tupac.

"This was a beautiful piece of content, the stagecraft was very nice, but there were not a whole lot of breakthroughs in the display end," Bove said. "They did a nice job of generating the character—no question about that. But there’s less than meets the eye."