There are a number of issues that must be considered if one is to import a pre-designed model for use in a virtual environment.
Currently, with the distinction between development packages and modelling packages, conversion needs to occur across both standards and platforms. Currently, conversion between established modelling standards and from modelling packages to different development languages is relatively straight forward. However, there are more problems involved in converting back to the modelling package standard from the development language. This is typically due to the added functionality introduced by the development language. Furthermore, it is important to establish conversion between development languages. Due to the disparate manner in which development languages have evolved there is currently very little conversion at this level.
Figure 3. Conversion between standards.
The resolution of the model can be addressed in terms of behaviour and solid model complexity. Essentially the resolution of a model is linked to the fidelity of the representation. For the solid model this may be how accurately the surfaces are modelled or how many of the components of the actual object are included in the model (e.g. is the engine included in a model of the car?). The behaviours can be similarly addressed in terms of complexity. For example, when considering the vehicle dynamics model of a car for a driving simulation the designer must consider a number of different aspects of its behaviour. Namely ride, handling, steering and braking. Models may be developed with different resolutions for each of these components. Obviously, the higher the resolution the more realistic the model. Resolution across models is tied largely to application requirements and hardware performance.
Many of the current graphics workstations have a high performance graphics pipeline built into the hardware architecture of the system. Typically a graphics pipeline is a 3 stage process: Application, Cull, Draw. This is carried out once per frame. The Application process sets up the scene graph from the current state of the model database. The Cull process traverses the scene graph to determine which elements of the scene graph lie within the viewing frustum. Finally, the Draw process draws the scene to the current viewpoint. Many systems have multiple pipelines for multiple displays (e.g. stereo headsets, multiple projector systems). By examining the model database with respect to the graphics pipeline the designer can appreciate where performance trade offs can be made.
Some issues in terms of real-time performance are:
The copyright issues associated with the production of software such as 3D models is, in this sense , identical to that of producing documents. The producer of the model is, therefore, the sole owner of the copyright and can limit the distribution of the model as they wish. Several sites investigated in the course of this report place very specific limitations on the use of their models. Some indicate that models may only be used for personal work and cannot be passed on freely, sold or incorporated in other work that may be sold. The sites indicate that any other usage other than personal must be approved by the owners. What is less clear is the situation on sites where stipulations are not laid down. In these cases, it is not clear whether the authors are placing any limitations on the use of their objects, and as such, whether any copyright limitations are then present.
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