Before I woke up this morning, it would have never occurred to me to type the phrase "Holographic Tupac" into Google. Now, not only have I done just that, but that search yields this video:
First response from the people I've shown it to has ranged from "whoaaaaaa" to "SHUT DOWN EVERYTHING, THIS IS TOO MUCH."
Second response: "I guess you can kinda see through him, down there by his shoes." and "I HOPE HIS ESTATE IS GETTING PAID FOR THIS."
My response: What is it with giant leaps forward in holographic technology being unveiled for the masses via rapper? Remember when a blue glowy Will.I.Am was discussing politics with Anderson Cooper back in 2008?
This most recent attempt at holographic rapper technology, aside from being way more impressive, is also way more creepy/disturbing, because a) Tupac is dead, and b) the song he's performing is the last single from his last album, which featured the prevailing theme "Death and how to cheat it."
How this got done isn't being revealed, but (and this is just my guess) it looks like they took some footage of Tupac at a previous Coachella, keyframed him out of the background, recomped/reprocessed the footage, and then projected it holographically as Dr. Dre & Snoop Dogg interacted with him. A more outlandish theory involves a wicked mimic, and the Tupac look-alike that seems to have made a decent career of just appearing at sporting events waiting for people to notice him and then disappearing.
(edit: According to this io9 article - Coachella didn't even exist when Tupac died. So maybe soundalikes were involved - he's obscuring his mouth when he welcomes the crowd to Coachella, provides a pretty decent opportunity to insert a well placed overdub. Or it could just be entirely computer generated.)
I don't know exactly how this was made, but I do know this: Within the next 3 to 4 years, I'm betting you'll hear an announcement from Michael Jackson's estate that he's going back out on tour via this technology, and people will pay 40-50 bucks a ticket to witness it. And once that happens, the estates of other beloved dead artists will follow suit, and we'll just slowly become acclimated to the reality that just because you're dead doesn't mean you can't still get up and shake that moneymaker for our enjoyment.
Also, if you thought post-processing classic movies was groan-inducing already, wait the movie industry gets a hold of this.