In a three-dimensional (3-D) topography simulator, a method for removing sources of particle flux because of neighboring topology, for a point on a workpiece undergoing a deposition or etch process step. The method is practiced in a Generalized Solids Modeling system that utilizes a boundary representation model for representing a workpiece as one or more material object solids. For any given point on the 3-D structure, the neighboring topography forms a complex shadowing mask with respect to sources of particle flux, thus making analytical determination of visible sources of incoming particle flux difficult. The method is comprised generally of the steps of: defining a numerical mesh in a space over a surface of the workpiece; specifying an intensity of incident flux for each mesh point, identifying a set of mesh points defining a visible range of mesh points with respect to a particular target point and identifying mesh points in said set of mesh points that are obscured by neighboring topology.