For more complex models, we need to construct them from several components, much like the real item. For example, a jug (Fig. 8) will be modelled in two parts – the body of the jug will be lathed, and the handle will be “extruded”. To extrude an item, we need the outline shape (as described for the lathing technique) and the longitudinal “path”. The model is created from extruding the outline along the longitudinal path.

Fig. 7	Fig. 8

The process of joining the two items involves connecting the ends of the handle to the body using a process known as “vertex welding”. Each model is composed of vertices and splines. As you can see from the pot above (Fig. 7) the model is made up of lines. These are known as splines, and where they meet they make a vertex. Splines can be removed, and vertices deleted, which open up the model. Splines and vertices can also be “welded” together, so elements of the model can be joined.

More complex models are created from a series of components. Some components are created simply from “primitives” – these are cubes, spheres, cylinders, tubes, cones and pyramids…  (although some software allows a “teapot” as a primitive – don’t ask me why!!) If you look at the Gribeauval 12 pounder cannon (Fig. 9), you should be able to see the use of primitives. The barrel is a modified tube, the wheel rims are thin tubes and the spokes are cylinders. By modifying the primitives, we can create extremely complex shapes.

Fig. 9