6.2.4.1  LINK ATTRIBUTES

While links primarily indicate connection of two feature nodes, they can also be enhanced to describe the nature of the connection. Of possible interest in connection are orientation and point of contact. Knowing the actual point of connection between features may be important for completeness. Without augmented links there may be occurrences of shape graph isomorphism: two different shapes may have the same graph pattern.

Consider the mounting boss of Figure 67, which can be described as a hole on top of a boss. This would be represented by the primitive feature graph of Figure 67 (a). Without further information, the primitive feature graph may represent the intended form of Figure 67 (b) or the equally valid figure of Figure 67 (c), illustrating a hole on the side of the boss.

Figure 67 Feature Graph Isomorphism

Although the shape of Figure 67 (c) would not be formable in one step, that does not preclude it from being designed. Isomorphism such as that in Figure 67 does not allow for unpredictable features that may need to be defined in the future. Such unpredictability is largely unacceptable in an attempt to provide a completely descriptive set of forms and attributes with which to work. As such, point-of-connection and/or orientation information is valuable in positioning the primitives relative to each other.

For representational purposes one may wish to consign orientation information back to the form object so that queries can be focussed upon the form objects alone. The link abstraction itself is made devoid of extraneous information content. With information focussed solely within the form, it may be acceptable to create geometric placement information to be able to place a primitive in 3D space. The orientation of connection can be derived from the orientation of the primitives themselves. Orientation and connection information of the link would be redundant because such information could be obtained from the form attributes.

In creating a flattened representation, removing information from the link and placing it within the features of interest would greatly expand the feature's storage requirements, especially if there are multiple links involved. However, it is argued here that connectivity information may be enough and the nature of the link can be interpolated from information within the objects themselves. This is a narrow approach and may prove to be too inflexible.