Game Development Reference

In-Depth Information

Consequently, modelling the road network becomes geometrical as well as topological

(orientation in a network) and semantic (the meaning of the red light) (Thomas, 1999).

Though we have quickly shown that modelling a virtual environment is not purely

geometric, it is nonetheless an important step of the process of constructing a virtual

environment. We will talk about this subject in this chapter and the next. Firstly, we

will have a look at some theoretical models related to the geometric description and

then we will see the modelling tools. The non-geometric aspects will be covered later

in part II “Models for sensorimotor rendering'' and part III “Models for behavioural

rendering''.

14.1.1 Types of objects

Before modelling geometric objects, it is necessary to answer one question: what are we

trying to model? It is obvious that the model of a non-deformable solid is different than

that of a compressible fluid. The simplest object is of course the non-deformable rigid

solid. This object can only undergo solid transformations
T

(R, t), where R is rotation

in space and t is translation. The cube in figure 14.2 is an example of non-deformable

rigid solid. The main advantage of rigid solids is that all we need to calculate is their

solid position in the space before rendering them. The non-rigid models, on the other

hand, can be divided into various categories:

=

•

Articulated solids: These are non-deformable rigid solids connected to each other

by mechanical connections (for example a pivot). They are used to represent for

example, of course as an initial approximation, the movement of an arm connected

to a trunk or the wheels of a car that are connected to the chassis. The pendulum in

figure 14.3 is a similar example. The connections between the solids are represented

by their parameters (here, angles
θ
1
and
θ
2
);

•

The deformable solids whose deformations follow the laws of mechanics (or do

not follow them, in case of an imaginary world). For example, the skin of a living

being or a ball that changes its shape when it touches the ground. These models

are controlled by their mechanical parameters. It is thus necessary to calculate the

changes in their shape before carrying out their visual rendering. The images in

figure 14.4 show the changes in the shape of the Luxo lamp under the effect of its

own weight (Cozot, 1996);

•

Lastly, the non-solid models which represent everything that cannot be associated

with the notion of an object. These include fluids, the special models that connect

multiple sub-models connected mechanically to each other. For example, special

models are used for creating a lava fountain.

There are also some other models which do not come within the scope of virtual reality

for professional use; they are related to the techniques called
non-photorealistic ren-

dering
(Gooch & Gooch, 2001). These techniques often produce spectacular results,

but they cannot be classified into simple groups like the groups we just mentioned.

Besides, their use in virtual reality is negligible these days. Hence, we will not discuss

them in this topic.

The major constraint of virtual reality in terms of graphic rendering is obviously

the absolute necessity of creating a real time rendering of the 3D model. It is thus

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