Other Definitions
change state (dict)
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Change StateIn CAD or CSG, the change state or delta state 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 intermitant 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. As we see in the figure below, change state #1 has information representing the rectangular brick. Change state #2 has the informations 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 go back to state 1 and make a modification to the rectangular brick and force the CAD system to regenerate the BREP in state #3. This process is called feature based modeling, and the information in the state is often refered to as a feature. The change state makes up the feature history of the CAD model. However, it should be added that this term is not necessarily restricted to CSG, and arrived on the computer algorithms scene with the advent of a Concurrent Versioning Systems (CVS).
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