History Based Modeling
The parametric modeling tools record both input settings and geometry for each operation. These operations are referred to as features of the element. By retaining, rather than discarding this information, the parametric element captures not only the final geometric form, but also the complete design process. This set of features defining a parametric element is commonly referred to as its Feature Tree. A feature tree acts as the complete time-line of the modeling process with branching where boolean tools are used.
The true power of retaining a feature tree in parametric elements is in the ability to edit the element by modifying any of the features in the tree and regenerating the geometry. As each feature is typically defined by a set of parameters, varying these parameters can generate different versions of the element.
Associative Inputs
Parametric elements take the history modeling concept one step further by preserving inputs elements that are not parametric, as they typically originate from non-parametric inputs. This can occur in several scenarios. Examples include uniting two primitives to produce a single parametric element or using a complex shape element as a boundary for a parametric cut or protrusion. These elements will be referred to as input elements.
The parametric modeling tools do not alter or consume their input elements; therefore they can work on any type of element or application object that produces suitable geometry. They instead maintain a link or association to the input element and react automatically to changes in input element geometry. This associativity makes it possible to continue to edit the input elements with their native editing controls or by modifying the input element properties. This is how the parametric elements update to match the updated input geometry.