Graphics Reference
In-Depth Information
fully, we may create underconstrained situations where there is no unique solution,
or overconstrained situations where there is no solution.
For a more thorough discussion of physically based modeling see the references
in that section of the bibliography.
5.6
Parametric and Feature-Based Modeling
We have mostly talked about various technical aspects of modeling systems, but a
good modeler must also take the user's or designer's point of view into account. The
difference between a machine representation and a user representation was briefly
alluded to in Section 5.3. We also touched on this subject in our discussion of gener-
ative modeling in Section 5.3.5. Users should not have to be forced to adapt their way
of describing geometry to any low-level abstractions due to technical requirements of
a modeler. Defining nontrivial geometric models is usually a difficult task in the best
of circumstances. If possible, the process should require no more expertise than the
understanding of the final model. Of course, there are times when knowing the under-
lying mathematics is essential to building a model and so the option of taking advan-
tage of it should be there. Nevertheless, for those times when it is not, we would like
a modeler to have the ability to understand high-level, user-friendly concepts. Of
course, what is considered user friendly depends on the user. In this section we are
concerned with manufacturing environments, where, for example, designers often
think of geometric objects in terms of important features that are associated to them,
such as, “a block with a slot and rounded corners.” Today's modelers have a long way
to go in fully supporting that type of interface. This section will introduce the reader
to what is referred to as
feature-based modeling
. [ShaM95] is a good reference for this
subject. A brief survey can be found in [SodT94].
Systems using
parametric
or
variational models
were a first step toward feature-
based modeling and making life easier for the designer. As is pointed out in [ShaM95],
perhaps 80% of new designs are simply variations of old ones. Therefore, given the
effort that is sometimes needed to create an individual model, why should one have
to create each of these variants from scratch? As an oversimplified example of the type
of interface that would be found in a modeler using parametric models, the object in
Figure 5.31 might be defined by the following sorts of commands:
Figure 5.31.
A parametric model.
