University of Maribor 1 CADUI'96 - 5-7 June 1996 - FUNDP Namur

An Interactive Constraint-Based An Interactive Constraint-Based Graphics System with Partially Graphics System with Partially

Constrained Form-FeaturesConstrained Form-Features

Borut Zalik

Laboratory for Computer Graphics & Artificial IntelligenceFaculty of Electrical Engineering & Computer Science

MARIBOR (Slovenia)

University of Maribor 2 CADUI'96 - 5-7 June 1996 - FUNDP Namur

IntroductionIntroduction

Problems which exist at the input of geometric modelling systems:

• the designers still have to use the most primitive approach of 'trials and errors'

• the engineering vocabulary is not supported

• spatial relationships cannot be expressed.

Two promissing approaches: constrained-based and feature-based design of geometric objects

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2D interactive constraint-based drawing system is considered.

• self-evident fact about geometric object are extracted automatically

• initial approximate data are used in the case of underdimensioning

• geometric constraints are specified in an interactive way.

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• geometry is divided into two groups– visible geometry (related with topology)– auxiliary geometry (related with geometric constraints).

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Constraints refer only to the auxiliary geometry

(points, lines, and circles).

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Constraints Constraints

A constraint describes a relation that should be satisfied.

• Structural constraintsThrough (li, pj) line li passes through point pj

HLine (li) line li is horizontal

Parallel (li, lj) lines li and lj are parallel

• Dimensional constraintsDistance (pi, pj, d) distance between points pi and pj is d

Angle (li, lj, ) angle between lines li and lj is

• Numerical constraints

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Problems at constraint descriptionProblems at constraint description

• An efficient constraint solver has to be implemented.

• Inserting exact number of constraints.

• In real applications a huge number of constraints have to be specified.

• Constraining of free-form shapes is usually not considered.

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Constraint SolvingConstraint Solving

• A local propagation of known states– supports an interactive design– easy to implement.

• An implementation with BCDG.

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Form-FeaturesForm-Features

• A form-feature is a group of geometric and topolo-gical entities with functional meaning in a certain context.

• Parametrisation of a form-feature can be different.

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Solutions:

• more than one constraining schemes is prepared

• transformation algorithms are used.

Our approach:

The form-features are not constrained completely at the time of forming but during the process of combining of individual form-features.

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Interactive Constraining ProcessInteractive Constraining Process

An untrained user describes the serif as a feature having three line segments and two curves.

Conclusion:

• The user describes the visible geometry.

• Description does not include spatial relations.

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The actions of the designer are observed by the system to generate all self-evident facts.

• Which facts are self-evident?

• Should we used an automatic detection of constraints based on the "small number "?

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Demonstration of our systemDemonstration of our system

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ConclusionsConclusions

• The system distinguishes between auxiliary and visible geometry

• By observing the actions of designer the self-evident information is extracted

• Bézier cubics are constrained in a natural way.• Well-constrained and under-constrained objects are

handled.

University of Maribor 22 CADUI'96 - 5-7 June 1996 - FUNDP Namur

University of Maribor 23 CADUI'96 - 5-7 June 1996 - FUNDP Namur