In a computer-implemented method of determining an outer shell of a composite three-dimensional model, an indication of a plurality of initial faces of the composite three-dimensional model is received. A plurality of intersections between the plurality of initial faces is determined. A plurality of split faces is generated by splitting each of the plurality of initial faces that intersects another one or more of the plurality of initial faces into two or more split faces along one or more lines of intersection. It is determined whether each of the plurality of split faces and each of the plurality of initial faces that does not intersect another one or more of the plurality of initial faces corresponds to the outer shell or to an interior portion of the composite three-dimensional model.