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(1) Interactive feature definition:
A human determines the features either at
model creation time or later.
(2) Automatic feature recognition:
Here one extracts feature information à
posteriori if the geometric models are
already defined.
(3) Design by feature:
Here one designs with features in the first
place.
Approach (1) is easiest to implement, but it is up to the user to check for validity and
it may be a tedious job if there are lots of features. Approach (2) is much more com-
plicated than (1), but has been incorporated in some modelers (for example, BUILD).
A number of different algorithms have been developed to get a handle on the recog-
nition problem. One basically needs a program that looks for patterns. For example,
to look for a pocket in a face one could look for a cycle of edges at which the solid
has a convex corner. Difficult as automatic feature detection may be, one may need it
if different features are used at different stages in the manufacture of an object. With
regard to approach (2), some features could be defined at model creation time, as
when one creates a slot by sweeping. However, these would by in large be purely geo-
metric features and not all features are that. Furthermore, the primitive operations
of a modeler may not directly correspond to the features that are of interest to
someone and some may lead to ambiguous features (see Figure 5.33). A good overview
of feature recognition techniques can be found in [JiMa97].
Approach (3) is probably the most attractive. A modeler might have a menu allow-
ing a designer to create an object in terms of its features. One would be able to create
an object with a slot or hole in essentially one step, or at least in a number of steps
that depended on the number of varying parameters for that particular shape. Figure
5.34 shows 10 of the 13 steps needed to create the ANC101 part in Figure 5.32.
Although this might make life easier for the designer, it would certainly make life
much harder for the implementer of this modeler. The problem is validity. The
modeler would have to make sure that the chosen features were consistent, a difficult
task in general. For example, if someone defined a block with a hole, the modeler
would have to make sure that the hole was not too close to the side so that it would
break through. The bigger the collection of features, the more checking that would
have to be done. Roller ([Roll95]) discusses designing with constrained parameterized
features.
There are two ways to deal with feature definitions in a design-by-feature sys-
tem, procedural or declarative, although these can be combined. In the procedural
approach, features are defined by a collection of procedures in the programming lan-
guage of the system. In the declarative approach a feature definition consists of a col-
lection of constraint specifications, rules, or predicate logic statements. Satisfaction
Figure 5.33.
Ambiguous features: boss on disk or flange on
cylinder?
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