Concert promoters hoping to bring out legends such as Whitney Houston, Billie Holiday and Elvis Presley used to face an obvious problem — the singers are dead.
But with rapid advances in technology, those stars and many more are returning to life through holograms, the three-dimensional light projections that have opened new frontiers for the live music and other industries.
The hologram boom began in April 2012 when Coachella, one of the most influential music festivals, resurrected slain rapper Tupac Shakur in a headlining set by his former collaborators Dr. Dre and Snoop Dogg.
Two years later, the King of Pop Michael Jackson came back from the dead at the Billboard Music Awards, the annual event hosted by the US music industry magazine.
In recent months, hologram announcements have become so regular they are almost mainstream.
Hologram USA has announced tours by Whitney Houston, who died in 2012.
The firm also plans to bring Billie Holiday, who died in 1959, back to Harlem's Apollo Theater in what would be the first regularly scheduled hologram show.
Holograms record light fields, rather than standard camera images, thereby allowing a three-dimensional presentation.
For Tupac and Michael Jackson, the holograms, however realistic looking, still used a 150-year-old technology — the image was projected onto a transparent screen.
For Whitney Houston's tour, which four venues have so far agreed to welcome, the late singer's image will be projected onto a live artist on stage.
"Whitney will be able to interact live in a real performance. It won't be scripted," said Alki David, the founder of Hologram USA.
"There is a performer — an actor whose body is like Whitney," he told AFP.
Another US company, Pulse Evolution, is preparing a musical based on Elvis that will take place on stage through virtual reality.
What happens when the novelty fades?
Concert promoters are avidly seeking unique headliners as the number of summer festivals has grown rapidly in the past years, in one of the music industry's few areas of substantial growth.
But will the public keep shelling out money to see dead entertainers once the novelty fades?
Reid Genauer, a member of the band Strangefolk and chief marketing officer of the movie app company Magisto, said he struggled to see how holograms would work once the technology becomes commonplace.
"Meeting dead celebrities, it seems reasonable to me. Is there business there? Probably. The difficulty I have — and it's a lack of imagination, not a lack of possibility — is envisioning how holograms scale," he said.
Jason Ross, a veteran producer of multi-media music projects, said that holograms may have more practical uses in classrooms — for example, for dissections in biology courses.
"I think there is a market for it, but I think from an instructional video standpoint it's going to be much stronger," he said.
Some promoters already shy away from hologram concerts, finding them tacky.
When rock legends the Grateful Dead threw five self-declared final concerts in June and July, the band considered — but decided against — creating a hologram of Jerry Garcia, the hippie-era group's most recognizable member who died in 1995.
Crossing threshold on believability
John Textor, the chairman of Pulse Evolution which spearheaded the Tupac and Michael Jackson holograms, said it was critical to design real shows, not just to rely on holograms.
He expected the Elvis musical to run 90 minutes and feature live dancers and actors, in the fashion of a Broadway show.
"I don't believe in digital performance concerts. By the time you get to the third song, you're done. The novelty is gone," Textor said in an interview.
Textor sees concerts not as an end in themselves but as one application for virtual reality.
Digital Domain, the company he ran from 2006 to 2012, made its name in cinema and won several Oscars, most notably for 2008's "The Curious Case of Benjamin Button" in which the hero, played by Brad Pitt, ages backwards.
Virtual reality technologists have long warned of the dangers of the "uncanny valley" — a theory that, the more perfectly an image resembles a real being, the more the audience will notice the imperfections.
But for Textor, that bridge has been crossed. In the first part of "Benjamin Button," he recalled that audiences believed they were watching Pitt, when in fact it was a virtual image.
University of Southern California professor Paul Debevec, a pioneer in the field, said that creating holograms of dead people was much more challenging than the actual concert design.
"The technology that is new and interesting is the ability to create a human likeness to someone that is dead — Tupac, Michael Jackson," he said.
"The technology of putting it on stage is completely uninteresting."
Beyond concerts, Textor has plenty of other ideas in store for the technology.
He hopes to "reinvent karaoke" by letting amateur crooners perform with their favorite singers.
Textor also sees great possibilities for video games by allowing players to create virtual doubles.
"We're working with gaming companies to bring them out of the stone age," he said.