Modelling is currently one of the most important areas in virtual environments research (Bishop et al., 1992). Of all the software areas, this is the area that we know the least about. Modelling has been an active research area in computer graphics for many decades, and is still a major research area. Many of the modelling issues addressed in computer graphics and virtual environments are also of concern to researchers in robotics, mechanical engineering and biomechanics, so progress in modelling can have an impact on many fields. Modelling is difficult since most of the objects that we would like to model, such as people, animals, and airplanes, are quite complex. They have a large amount of geometrical detail and move in complex ways. This difficulty is compounded by the different fields that use modelling techniques, since each field has its own requirements and priorities. For example, it is highly unlikely that the same model of a human figure would be optimal for applications in both virtual environments and biomechanics. The following criteria can be used as a basis for evaluating different modelling techniques. Accuracy. The model should be an accurate representation of the real-world object. Ideally, we would like all of our models to be precise representations of the real-world objects, and not simply approximations to them. But, in reality accuracy comes with a price, usually increased display time or memory usage. The amount of accuracy required often depends upon the application. For example, in some applications it is acceptable to approximate a sphere with a large number of polygons, but for a large number of computer-aided design (CAD) applications the precise mathematical representation of the sphere is required. Since the polygonal representation can be drawn faster, it is often used in applications where display speed is more important than accuracy. Display speed. Many applications place restrictions on the time available to display individual objects. In the case of interactive applications, short display times increase the level of interaction between the user and the model. In large CAD applications there may be many objects, therefore, the time required to display individual objects becomes an important consideration in the usability of the application even if a high level of interactivity is not required.
Virtual Object Replacement Based on Real Environments: Potential Application in Augmented Reality Systems
