Fig.1 A wire frame model
Surface and Solid Modellers are introduced and defined. I argue that developments in modelling software mean that there is no longer a clear distinction between the two types of software along application lines. We should think of models in terms of their applications rather than the software which generates them.
Computer Aided Design (CAD) systems were first developed at the end of the 1970s, and began to be widely used in the 1980s. This work was in two dimensions, and the advantages over paper drawings were largely practical. With the development of three-dimensional draughting packages in the second half of the 1980s, a conceptual advance became possible. It was possible, in principle, to link together plans and sections to produce a three-dimensional design or record. This reduced the conceptual limitations of the drawing, and facilitated interpretation.
Almost all the surface modellers used in archaeology are developments of CAD software, although Gill Chapman (1990, 49) preferred to use a surface modeller developed for computer animation (CGAL: Computer Graphics Animation Language) for her work on the Hoffman Limekiln at Langcliffe Quarry, near Settle. She felt it better developed for simple modelling than a CAD package. CAD packages are commonly used in archaeology, both for historical reasons and because they provide a robust, user-friendly general purpose graphical user interface (GUI) for working with complex data, which may subsequently need to be edited. Surface modelling work takes two forms: wire frame and surface.
It is simplest to produce a wire frame model (Fig. 1). This is simply the addition of a third dimension to a two-dimensional drawing essentially composed of points and lines. This is simple to do, and requires little more memory than two-dimensional work. However, because the model is dealing only with edges of entities rather than with surfaces, visualization is very difficult: it is impossible to remove detail hidden by an object nearer the viewer, or 'render' an object (to give its surfaces visual physical properties such as colour and shadow); it is impossible to detect interference between adjacent components. It is also impossible to recognise holes in objects (MacFarlane 1995, 1).
A more effective method is to work with surface models (Fig. 2). These are composed of points, lines and faces, and overcome the problems of wire frame models given above, other than interference. The limitations of surface models are that they are computationally more expensive, have no physical property other than surface area, and have no detail (i.e. they do not allow sections to be taken through objects). This third limitation makes such common archaeological needs as the visualization of a complex archaeological site by taking sections through it impossible (MacFarlane 1995,2)
Despite the initial expense, CAD systems had a number of advantages for designers:
It was this last feature that especially interested archaeologists (Chapman 1990, 4). The development of large-scale, open excavations of sites with deep stratigraphy in the 1970s and 1980s had produced a proliferation of spatial data in the form of context and section plans, only made worse in terms of quantity of drawing film by the popularity of single context planning. Using CAD layers, these different plans could be easily combined, and turned on and off on the screen at will. An essential difference between the use of CAD by archaeologists and its intended use in business and industry was that archaeologists were concerned with recording what was there, rather than designing what could be there. This seemingly trivial distinction has significant consequences when applied to three-dimensional modelling.
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Last updated: Thu May 1 1997
