Actually it was not a real hologram, just a 3D model of Tupac made by Digital Domain projected on a glass panel in a classical optical illusion technique called Peper's ghost. I think Hatsune Miku and X Japan's hide in Art of Life were real holograms(hide footage taken from old Art of Life concert).
I think they are walking on a very thin line here. While it's ok to make this a one time only thing as a tribute to dead legend, making regular concerts and tours with dead people's holograms is very unethical and disgusting even. It's just milking money from the fame of a dead person. I understand that show business was making this for decades(if not centuries) with selling after death albums and merchandise, but showing hologram mimicking dead person for entertainment of a crowd is taking it too far IMO.
