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.