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.
As with any production of this magnitude, there are also the inevitable glitches and hiccups. According to Reznor and Sheridan, many of those can be traced back to an archaic Windows machine known as the Hippotizer, as well as an antiquated lightning console that it interacts with called the Grand Ma.
At one point, during the band's recent Red Rocks, Colorado, performance the Hippotizer choked and spit out some text from the machine's video-labeling system. NIN fans immediately began dissecting still shots from a video someone had taken, and a three-page discussion ensued on NIN forums trying to decipher what the secret text meant.
"It was all just that stupid fucking Hippotizer getting the wrong trigger ... something from the lighting desk just misfired," Sheridan says.
But Reznor, who is an unabashed Mac fan, is also playful about having to partially rely on Windows boxes for some of the show's visuals.
"We purposefully put one frame of the Blue Screen of Death in this collage of static that comes up at the end of 'Great Destroyer,' and right away people caught it," he says.
For the next leg of the tour, Sheridan is working to permanently move the entire lighting and visual system over to a Mac rig running ArKaos VJ software.