NPSNET: Constructing a 3D Virtual World. - Michael Zyda; David R. Pratt; James G. Monahan; Kalin P. Wilson (SI3D, 1992)

NPSOFF: An object description language for supporting virtual world construction. - Michael Zyda; Kalin P. Wilson; David R. Pratt; James G. Monahan; John Falby (Comput. Graph., 1993)