With the song "Only," for instance, the front, convex screen starts out as solid static. On Reznor's side of the display, a laser above him detects whenever he crosses a vertical plane paralleling the screen. On the floor, a piece of tape and two tiny LED lights let him know exactly where that plane is.
As Reznor intersects that plane with his hand or body, the laser tracks his X and Y coordinates. The "brain" box then tells the particles to spread out to a predetermined dispersal pattern. Reznor says: "Then it follows me around. If I leave the plane, it fills back in. If I push through, it comes back out."
The band uses the same tech for another song later in the show called "Echoplex," from The Slip album. Like many other NIN songs, it's based around a drum machine beat. After rehearsing live a few times with real drums, Reznor realized it sounded better with a machine. "We recreated a grid drum sequencer," he says. "[Drummer Josh Freese] is actually touching and turning them on and off. But he's not really touching the screen. He's crossing the same laser on the back screen, which gets calibrated at sound check."
The end effect is so seamless, most people assume the band is simply pantomiming to a pre-rendered video, or has actually somehow installed a gigantic touchscreen sequencer on a backstage wall. "We went through so much effort to make this stuff interactive and people still think it's all staged," jokes Sheridan.
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