Abstract: A fly-through, three-dimensional computer aided design system is provided. Computational requirements are bounded, at least in part by defining a bounded volume or world defining the volume which may be displayed, in whole or in part. Preprocessing the data assists in organizing the data for high performance display. Full detail rendering is available. However in some situations, such as during fly-through, some or all portions of the image are rendered in a simplified fashion, e.g. to maintain a high frame rate. The volume-based simplification involves rendering faces of cells or collections of cells which are at least partly intersected by components of the item or system being displayed. A plurality of granularities, preferably organized as an octree, provide multiple levels of detail. Unnecessary rendering load is reduced by view frustum culling and considering certain cell face occlusion situations.