Change state
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In CAD or CSG, the change state or delta state (as defined by ACIS) refers to the intermediate modifications made to a document before it is saved. Hence, if one has a history of changes, it is the collective set of intermittent modifications, whose final outcome is one which satisfies certain properties.
In CAD, this state is the set of modifications made to a boundary representation in-between the two state in which it is Euler consistent.
With this newly found information, it is possible to design a parametric CAD system capable of storing and saving the dual CSG and BREP data. The constructive solid geometry data as well as parametric associations (LISP-like expressions) are used to generated the boundary representation.
As we see in the figure below, change state #1 has information representing the rectangular brick. Change state #2 has the information about a cylinder. Change state #3 has information that it is derived by taking the rectangular brick in state #1 and subtracing the cylinder in state #2 as well as information about the relative offsets of the 2 bodies.
The advantage of this approach in a parametric CAD modeler is that one can go back to state 1 and make a modification to the rectangular brick and force the CAD system to regenerate (as exhibited by Parametric Technology Corporations's Pro/Engineer) the BREP in state #3. This process is called feature based modeling, and the information in the state is often referred to as a feature. The change state makes up the feature history of the CAD model.
This term is not restricted to CSG, and became common in the computer algorithms community with the advent of Concurrent Versioning Systems (CVS).
