Next: Typical Applications Up: Constructional Relations Previous: Constructional Relations

## Meaning of Constructional Relations

The essential idea behind the introduction of constructional relations is that the user builds up a relation between existent geometrical objects and objects to create.
These relations are lost in most of the usual CAD systems whereas in SISCAD-P they can be saved under the CAD model, if necessary.
If constructional relations are saved for a geometrical object, the system knows the way of construction (i.e. the corresponding SISCAD-P command) and all objects and parameters involved in this construction at any later time. If only one of these objects is manipulated (e.g. moved), the entire construction is re-constructed in accordance with the new conditions.
Example (tangential construction): Start objects are 2 circles k1 and k2 as well as a point pp.
With the command CREATE LINE TGTE 2 tangents to the circles are created, whereas the optional parameter SAVREL=1 (save relation) is activated.

Figure: Tangential construction with SAVREL=1 (original construction)

If circle k2 is moved so that its center is identical with point pp, the tangential lines are not simply manipulated (i.e. they would not tangential to the 2 circles any longer), but the original tangential construction is done anew.
The result is the modified circle k2 with newly constructed tangents to both circles.

Figure: Tangential construction with SAVREL=1 (after modification)

This adaption is done automatically for each manipulation of one of the two circles.
This example shows that objects tied together with constructional relations can only be manipulated entirely.
Please note:

> Constructed objects without their constructing objects will have no right to exist, because the saved construction rule cannot be understood anymore.
> Therefore the deletion of constructing objects causes the deletion of their constructed objects too.

Due to these reasons you should think of the consequences before saving the constructional relations.
The practical use of constructional relations is a powerful medium to bring the right construction logic into the construction.

Next: Typical Applications Up: Constructional Relations Previous: Constructional Relations