Design Tooling - Bidirectional Design

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Information

Introduction
Beyond parametric modeling it is possible to introduce constraints to model geometry as for instance fix an angle to a certain value or the length of a line. In order to explore the sum of the constraints in a interactive fashion a solver is necessary for solving for the new state of the model. CATIA for instance has such a solver setup in its sketch environment. It operates on geometric entities and their respective constarints.

Designing backwards and forwards is a common activity. Bidirectional refers to the notion of designing an artifact to achieve a certain phenomena and then to be able to adjust the phenomena which in turn again updates the artifact.

The ability to interact and edit at any step in the dependency chain of a design model. One approach is to model the phenomena in the same representation as the artifact as for instance in the case of a mirror and the related light paths.

There is a number of metaphorical models for computation (Travers 1996)

Imperative model, functional model, procedural model, object model and the constraint model

Background

Metaphors in computation

There is a number of metaphorical models for computation (Travers 1996)

Imperative model - computer as sequential instruction follower

functional model - computation of values

procedural model - integration of imperative and functional model - decomposition of programs into parts

object oriented model - organization of the program around the data it manipulates - metaphor of objects

constraint model - combination of procedure with declarative information about what they do.

The constraint model is the one that most closely fits the bidirectional design model. It allows the specifications between objects while leaving the procdural details of how to enforce these relations up to the constraint solving system (Travers 1996)

Exercises

Framework
A possible design exercise could be to have the student identify a relationship between an object and a phenomena and identify the dependencies and criteria that link the two.

A second step could be to model them in a bidirectional relationship either in digital geometric form or as a physical constraint apparatus.

The computer is mostly perceived as a machine that gives results to clearly defined problems - the solver based constraint systems allow explorative design behavior while providing excact results none the less. The core challenge is to understand the relationship between the different design goals.

Implementations

Concert Hall Light Study sketch in CATIA

Implemented with the parametric modeler as CATIA for instance. A bidirectional solver allows for the interaction with any elements in a constraint geometry effectively allowing editing of the result or of the artifact. This is powerful when for instance non tangible phenomena like light sound or relationships between objects are modeled in geometric fashion and manipulated just as objects are.

Axel Killian 2002 in CATIA

- relation between mirror and object

- parabolic mirror and light rays

- Louvers and light paths

Sketchpad - Commercial software

Sketchpad is a dynamic construction and exploration tool that enables students to explore and understand mathematics

SketchPad Geometer(tm)

- mechanism

Graphic statics - Active statics - Edward Allen, Simon Greenwold

An example of designing in two domains in parallel. The user defines the topology of the mesh and gives geometric constraints and weights and a state of equilibrium is found through solving the equations of the statically determined model. This method only works for statically determined structures.

Edward Allen

Simon Greenwold Edward Allen

Axel Kilian

Graphic statics examples by Edward ALlen, technique originally developed around the turn of the century

Active statics by Edward ALlen and Simon Greenwold

Catia based example by Axel Kilian using bidrectional solver in the sketcher

Graphic statics in generative components/microstations - with 3d volumetric model attached

The graphic statics geometric pair of form polygon and force polygon allow the bidirectional exploration of the bridge structure and a 3D volumetric model provides an additional view of the build structure

Axel Kilian 2003

Graphic statics examples done interactively in Generative Components inside Microstation

A form diagram is linked to a force polygon. The combination of both drive a 3D volumetric model

The thicknesses of the members are proportional to the forces going through them

Digital form finding tool developed for/in Gaudi workshop spring 2004 with John Ochsendorf, Barbara Cutler, Marty and Eric Demaine and Simon Greenwold

An example of designing in parallel. The user defines the topology of the mesh and gives geometric constraints and weights and the solver searches for a state of equilibrium.

The model currently does not allow bidirectional moves since the solver is applied to finding the state of equilibrium among the forces present. The solver would have to be extended to allow for additional constraints to be modeled and manipulated byt the designer.

Axel Killian 2004

Connecting mass and strings generates form based on simulated gravity

Thickness attribute based on force present in the member