For the soundtrack Luftwerk returned to Condon, who this time wrote original music to sync up with the projection loops.

"I tried to think about ways to use percussion instruments that are all made of natural materials, much like the city is made of wood and metal, and trying to draw as many sounds as possible from those instruments," said Condon, 33.

"It's extremely important," Gallero said of the music. "It gives it a character. It gives it a soul, a voice."

In another occasional element, members of the local theater troupe Collaboraction will dance and otherwise interact with the projections at 7, 8 and 9 p.m. on the exhibition's two Friday nights. In a recent practice run, a Collaboraction performer wore all white, becoming a kind of curvy, kinetic video screen.

Such activity is intended to encourage onlookers to jump right in, as a family did a couple of weeks ago when Luftwerk projected some test patterns around Cloud Gate.

"These kids, they must have been 7 or 10 years old, they saw the light coming into the plaza, and it was like a magnet," Coyle said. "They just came and started jumping and chasing the light. At that moment we had a vision of what this could be like."

Gallero and Bachmaier are eager to see "that first pixel appear onto the ground" and expressed confidence in the results. Still, when it comes to artwork in public spaces, there are always variables. For instance, the plaza could get socked with a foot of snow.

"I actually would like that," Bachmaier said. "Everything would be even brighter."

Of course, Millennium Park workers would be plowing the pavement, but still, if it snows at night, the snowflakes would be illuminated as they fluttered to the ground.

"Which will be beautiful," Bachmaier said.

“Luminous Field” runs Feb. 10-20 from 6 p.m. to 9 p.m. Sunday-Thursday and 6 p.m. to 10 p.m. Friday and Saturday. Free family programming will take place in a heated tent to the north of Cloud Gate.

Twitter @MarkCaro