When modern-dance pioneer Merce Cunningham commissioned artist Daniel Arsham to design the set for a performance of eyeSpace in 2007, the only brief he gave the artist was not to injure the dancers. With his imaginative vertical proscenium, the 31-year-old Arsham joined the Merce Cunningham Dance Company’s illustrious collaborative pantheon, which has included John Cage, Robert Rauschenberg, Jasper Johns, and Andy Warhol. Now, three collaborations later, comes the conclusion of a two-year Legacy Tour following Cunningham’s passing, for which Arsham will create the sets for the troupe’s six site-specific “Events,” running December 29 through 31 at the Park Avenue Armory, before the ensemble disbands forever. The backdrops will consist of eight massive suspended “pixel clouds” composed of thousands of colored spheres, enlarged scale models of photos of clouds that Arsham took with his iPhone from airplane windows while on tour with the dancers in Paris, Melbourne, and Miami. He sourced grapefruit-size high-density polyethylene balls, commonly used in industrial processes, for the project. “One cloud is larger than my Greenpoint studio,” says Arsham.