Constraint Objects  --  
       Integrating Constraint Definition and Graphical Interaction


                        Ching-Yao Hsu, Beat Bruderlin




				UUCS-92-038

			       November, 1992


                       Department of Computer Science
                              University of Utah
                       Salt Lake City, UT 84112 USA



                                  Abstract


This paper is about an implementation of a new constraint-based technique 
for direct manipulation in interactive CAD, which will simplify the design 
process, especially in the early stages.
We introduce so called Constraint Objects and Parameter Objects which 
constitute an object-oriented view on constraints. They serve to simulate the
mutual degrees of freedom between objects for which a geometric relation 
(distance, angle, parallel, congruence, etc.) has been defined. A 2-D profile
editor has been realized for interactively constructing lines and circles in 
various ways. Each construction operation, implicitly defines constraints to 
capture the intent of the operation. These constraints are represented by 
corresponding parameter objects and constraint objects. A constraint solver is
applied to rewrite the set of constraints into its normal form if necessary. 
Finally, the resulting parameter-constraint-regular-object network serves to 
simulate the degrees of freedom of geometric objects during interactive 
dragging manipulations, and to make sure the existing constraints are not 
violated by subsequent operations.