Object Hierarchy

In any modeling situation, the system contains several types of geometric information. This geometric information is organized as a four-level hierarchy, where each level may only contain certain types of objects. This four level hierarchy is shown in the diagram along with the Object window, which is used to interact with this structure. The diagram shows a model of a coffeepot. The coffee pot in it's entirety is the highest level of the structure called the root.

The model has to be broken down into manageable parts. In this example, the modeler has chosen to use four elements the Body, Spout, Handle and Lid.

The third level of the hierachy contains object to build surfaces. The Spout, for example, has three curve sets – X and Y projections and cross-sections. This level also includes the surface itself.

The lowest level of the object hierarchy contains the primitive objects. The Spout for example, has five curves – two in each of the X and Y projections and a single cross-section curve. Surfaces also have their basic primitive control curves. However, they are never manipulated directly from the object window. They are only ever manipulated with higher-level commands without directly dealing with the primitives themselves.