A topography simulator using a "Generalized Solid Modeling (GSM) method" to simulate isotropic or anisotropic deposition and etch process steps on a workpiece. A solids modeling system that utilizes a boundary representation model for representing material object solids provides a basis for the topography simulator. A workpiece in the model is comprised of a collection of material solids. The present invention provides for accurately representing the interfaces of different material solids. The airspace above the top surface material is defined as an air solid. Boolean set operations between the various material solids and the air solid are performed to deform the wafer topography. The present invention further provides means for simulating an etch process step at the interface of materials with different etch rates.