1. A computer-implemented method for reducing a size of a data structure of a computer-aided design model, the method comprising: given a computer-aided design model comprised of a plurality of modeling elements, providing one or more geometric entities defining each of the plurality of modeling elements of the computer-aided design model, the geometric entities having corresponding graphical entities configured to display the computer-aided design model; for each modeling element, determining visibility of the modeling element; and producing a reduced data structure in a computer memory by storing in the reduced data structure: i) graphic data representing the graphical entities corresponding to the geometric entities defining each of the plurality of modeling elements of the computer-aided design model; and ii) geometric data of only certain ones of the geometric entities, each of the certain geometric entities being determined by at least one of: (a) a function of visibility of the corresponding modeling elements defined by said certain geometric entity, and (b) the certain geometric entity supporting a set of computer-aided design operations, after said storing of the graphic data of i) and the geometric data of ii), the reduced data structure resulting from said producing, and wherein: the resulting reduced data structure excludes geometric data of other ones of the geometric entities corresponding to modeling elements that are unnecessary to support the set of computer-aided design operations; and the computer-aided design model is efficiently updated by rendering entire said computer-aided design model while storing only the certain ones of the geometric entities in the resulting reduced data structure, and wherein determining the visibility includes determining a respective visibility factor for the modeling element including: generating viewing vectors that emanate from points that form a bounding enclosure around the computer-aided design model toward a center of the bounding enclosure; and for each viewing vector, determining a number of pixels corresponding to the modeling elements that are visible when the modeling element is viewed in a direction of the viewing vector from a point along the viewing vector, the visibility of the modeling element being a sum of the number of pixels determined for each viewing vector.