[Phil Lelyveld comment: this explains in simple detail how Tupac was brought back to life at Coachella. ]
Based on a Victorian-era light projection system called 'Pepper's Ghost' which was developed by two inventors -- Henry Dircks and John Pepper -- in the 1860s, Musion Eyeliner offers a range of spectacular stage effects that, in addition to bringing Frank Sinatra back to life, have been used to reanimate Tupac Shakur, Michael Jackson and Elvis Presley.
Uwe Maass, the inventor or Musion Eyeliner, explains that when it was first developed in the 19th century, Pepper's Ghost was quite straightforward: "The Pepper's Ghost system is relatively simple. It is a piece of foil in a 45 degree angle and it reflects the image from the floor onto the stage." From the audience's perspective the light projection can offer all kinds of effects including making realistic 3-D people appear on stage."
Maass -- an accomplished electronic engineer with 30 years experience in optics -- advanced the concept of Pepper's Ghost with modern video projection and animation techniques that serve to update the old technology.
Maass's main innovation is to replace the glass used in original systems with an invisible polymer foil to offer a more substantial image while also allowing the stage behind to remain visible. Maass's developments quickly attracted interest from senior figures around the entertainment industry.
See the full story here: http://www.cnn.com/2014/01/08/tech/innovation/optical-technology-tupac-back-to-life/index.html?curator=MediaREDEF