Interacting with Virtual Prototypes Coherently with
Design Intent
Maura Mengoni and Michele Germani
Polytechnic University of Marche, Italy
This paper argues that the design intent can be preserved along the product design
cycle only if the whole process is supported by proper methods and advanced digital technologies to model virtual prototypes managing the styling product coherence. In this context, the most critical phase is the transformation from the design
concept represented by sketches and physical prototypes and the engineered CAD
models necessary for testing and manufacturing. We aim at facilitating the design
intent preservation identifying the better digital technology usable to convert designers’ conceptual models in digital ones. The proposed classification is based on
the analysis of creative strategies adopted by the designer and on their correlation
with the modeling features in the computational systems. This goal requires the
definition of design intent transmitters that we define freeform aesthetic features,
and the determination of a benchmarking method to map computational systems
with the creative strategies. Experimental work through protocol studies has been
performed to validate the mapping method.
Introduction
Some artifacts are more appealing than others as the visual and tactile perceived attributes of the product appearance stimulate aesthetic impression,
symbolic association and semantic interpretation in the consumers [1]. In
particular, we state that the so-called aesthetic features that are tangible
expressions of the designer intentions stimulate perception.
The design intent can be defined as the set of semiotic contents expressed by the product shape that allow the discrimination among products
with the same functionality by inducing instinctive feelings and emotions
in the customer. The set of aesthetic features characterizing the design intent must remain unchanged throughout the product development process.
J.S. Gero and A.K. Goel (eds.), Design Computing and Cognition ’08,
© Springer Science + Business Media B.V. 2008
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In fact, the success of a product depends on the capacity of product shapes
to transmit emotions through aesthetic features [2].
Nowadays, freeform aesthetic features characterize many successful
products. They are complex topological shapes whose character is determined by the spatial curvature of the styling curves or of the enveloping
surfaces. Examples can be found both in industrial design (e.g. the car
body) and in architectural design (e.g. the Gehry’s envelopes). The preservation of the shape character (design intent) along the design cycle is even
more difficult in the case of freeform shapes.
The lack of design intent preservation is generally affected by the different viewpoints that contribute to the final design solution. According to
Mono [3], the aesthetic features are modified along the design process in
order to satisfy the engineering and manufacturing constraints. This can influence the way in which products are perceived; therefore design modifications must be monitored to be coherent with the original design intent. A
further cause of design errors is due to the heterogeneity of the representational means used along the design process. The product evolves from its
original vague concept, represented in the form of sketches and physical
prototypes, toward a more detailed design represented by a virtual prototype developed in Computer Aided Design (CAD) systems [4]. In the
transformation of the initial concept into a 3D CAD model, information
(e.g. size, dimensions, shape, materials, structure, etc.) increases contributing to the development of the final design solution. It progressively becomes from incomplete and approximated to accurate and detailed. During
the design modeling loop, the design intent is often lost as its preservation
depends heavily from the subjective capacity of individuals to perceive it,
understand it and represent it with the different means of design representation. As a consequence several design iterations are needed to achieve
the design solution validation and to avoid the aesthetic errors deriving
from freeform features modeling misunderstandings. Computational tools
are yet far away from supporting the whole design process development.
Therefore we highlight the need of structured methodologies to improve
the interaction with virtual prototypes coherently with the design intent
preservation.
Considering the vast amount of available digital technologies it is important to find the most performing tool able to robustly represent conceptual freeform objects resulted from different creative strategies, facilitating
the transformation from the designers’ intentions into a digital model. The
research focus, hence, is how to correlate and map the most effective digital technologies to model freeform aesthetic features with the creative
strategies. This also involves the definition of proper modeling strategies
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for each specified computational system and of a way to modify freeform
aesthetic features in accord with the engineering and manufacturing requirements while preserving the design intent.
The research objective can be achieved by:
• formalizing the design intent in the different means of the conceptual
design outcomes representation in terms of styling lines, aesthetic parameters, topological shape characteristics, etc.;
• identifying the main creative strategies to obtain freeform shapes and
correlating them with proper 3D modeling operations that can be performed in different computational systems;
• mapping the specified strategies with the digital technologies that can be
used to develop the final design solution without loosing meaningful information.
In order to achieve the above mentioned goals, we assume a cognitive
standpoint to analyse the design process and the main creative strategies
adopted by designer to conceive freeform shapes. The semiotic analysis of
the evolutionary process of sketches and the role of physical prototypes in
the design outcomes representation allow the recognition of the meaningful freeform shape characteristics that reflect the design intent.
A benchmarking approach based on suitable metrics is adopted to experimentally verify the correspondence design technology vs creative
strategy using several digital systems and different styling products. After
an extensive treatment of the proposed approach the paper describes the
achieved preliminary results.
Research context
According to semiotics, the product can be seen as a sign whose expression form carries out the semiotic content that the designer wishes to
communicate to the consumer. In this context, design ideation can be conceived as an act of signifying, a unique and original construction of the
frame of reference that draws relationships among codes often belonging
to different domains. Codes provide a framework within which signs make
sense, and the meaning of a sign depends on the code where it is situated
[5], [6]. The semiotic standpoint highlights the communicative nature of
the design process as a process of design intent transmission from the designer to all persons that come in contact with the different representations
of semiotic contents, till the final consumer. Furthermore it points out the
main criticality for product success: as different viewpoints contribute to
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product design definition, the design intent can be lost across the design
cycle and the product fails in the market.
Jakobson [7] outlines that the coherence of the communication act can
be achieved when the encoding and decoding processes are regulated by
the same, or at least compatible, codes. As the spoken language, the design
language also consists of sets of signs and symbols that allow the transmission of the designer intentions to the users: the success of a product is
determined by the ability of the product shape and of its aesthetic characteristics to transmit the designer message (design intent). The main problem
is related to the different codes and conventions that each actor of the design process adopts to perceive, interpret and represent the design intent.
The design process is highly influenced by the technologies used to
support the creation and the representation of the design outcomes [8].
Representational technologies aim at representing the design models in
order to develop, design and visualize the alternative solutions, to share the
different viewpoints, to predict the impact of design changes on the product shape, etc. Different design models are realized to perform the specified tasks.
It is possible to distinguish between physical and mathematical models.
The firsts reproduce existing shapes in a full-scale or reduction scale while
the seconds are 3D representations of the design solution via specialized
software such as Computer Aided Design (CAD), Computer Aided Styling/Industrial Design (CAS/CAID) systems, etc., that allow different models representations.
In the first stages, design models, from sketches to physical prototypes,
generally called simplified models, are abstract, elementary, and intentionally incomplete with regard to the existing object. The different levels of
abstraction stimulate interpretation and the emergence of meaningful features. In accord with the degree of abstraction the design model can be
used for representing the designed object, communicate the design intent
to the users and for creating new emerging relationships. [9]. Designers
prefer traditional representation means instead of CAD tools as they evoke
a common imagery of grid snaps, tight tolerances, and exactitude, inhibiting the abstraction necessary for exploratory design [10].
As product design moves forward, design models detail in order to allow the construction of the final product. They progressively move from
simplified to nominal and expanded according to the amount of information and to the complexity of the design knowledge that the different actors, involved in the process, have. Nominal and expanded models are
represented only via advanced digital technologies, especially CAD-based.
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In the case of physical prototyping, models are usually 3D digitised with
the help of measuring systems (reverse design) and then processed with
specific software packages for surface reconstruction so that CAD models
are available at the end. Surface reconstruction is not a trivial task as the
recognition of freeform features on dense clouds of points requires a high
degree of specialization to avoid misunderstandings of the designers’ intentions [11].
In the field of direct design, several efforts have been made to create
computational tools to support designers in the conceptual stages.
CAS/CAID systems allow the conversion of digital sketches into threedimensional solid or surface models or both. They contain features both to
support the earlier phases of the design process and to model complex freeform shapes. Otherwise, the created 3D models are not usually precise and
need to be redefined by specialists. They are hard to learn and require a
high degree of experience. The modelling strategies they adopt, are far
from the design practice to freely sculpture clay [12]. Dynamic CAD modelling software packages and virtual reality-based systems are more similar
to the way in which freeform shapes are conceived. The first allows shape
modelling by transforming a primitive form by stretching its control vertices, by applying a forces matrix to a skeleton or to a metaball system, by
producing dynamic effects on the position of particles located in space, by
morphing an original form into a target one, etc. Unfortunately, the output
of the transformations does not contain any semiotic information useful for
the following engineering shape development and they are not integrated
in commercial CAD software tools [13]. The seconds try to involve the
sense of touch for virtual clay modelling and immerse the users in the virtual scene to reproduce very natural conditions. Haptic devices are used to
explore and create freeform shapes in the virtual environment giving touch
feedback relating to the “surface” users are sculpting or creating. The main
problems concern with the coordination of the user movements in a way
that are similar to those he/she traditionally has and the difficulty to reproduce devices really used in the industrial design to create physical prototypes [14]. Attracting the research attention, hand gestures capture
interfaces represent new way of virtually sculpturing clay. While designers
conceive the product shape the system captures the hands motion and
translates it in terms of 3D surfaces. The main problem deals with the difficulty to define a grammar of hand motion language and nowadays researches are far from a real application in the design practice [15].
In summary, it is worth to notice that in the industrial practice designers
prefer to adopt traditional means of representation, while CAD operators
are generally employed to translate the design concepts (as simplified
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models) into detailed CAD models (solid, surface, voxel-based in accord
with the particular used system) for the following phases.
Design errors and iterations derive from the communication problems
along the transformation from simplified into nominal ones. The question
is how to manage such transformation by available digital technologies
without loosing meaningful aesthetic information that is represented by
aesthetic freeform features. From an operational point of view, we define a
freeform aesthetic feature as a parametric description of a shape, containing the styling curves, the set of parameters and attributes that define them,
and the aesthetic constraints that allow the modeling of the shape according to the creative process the designer adopts to generate the design concepts. Such features and their use in surface modeling can enable users to
coherently and easily modify the 3D virtual prototypes in the product development stages.
We believe it is important to recognise the aesthetic freeform features on
freehand sketches realized in the first ideation phase and using them for 3D
modelling into a computational tool able to represent the design model in
the same way it is generated in the designer’s mind. This assumption needs
a practical solution correlating the designers’ creative strategies with the
more suitable design computational tool and a coherent a 3D modelling
method to preserve the design intent.
The problem formalization is reported in figure 1.
Fig. 1. The problem formalization
Approach to map creative strategies and digital design
technologies
The proposed approach is based on the following steps:
• classification of the creative strategies for freeform objects conceptualization and related means to transmit design intent;
• classification of freeform object modeling methods and technologies;
• definition of metrics to evaluate the technology in relation to the creative
strategy;
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• experimentation on test cases to determine the metrics value for different combinations (technology and strategy);
• determination of mapping matrix design technology vs creative strategy.
For each combination of creative strategy and computational design
technology it is necessary to define also a proper method for virtual prototype creation. In this section we resume the first two classifications while
in the next section we describe the metrics and their practical experimentation. In conclusion we report the preliminary benchmarking matrix.
The creative strategies classification
Designers rely on visual representations to generate and explore design
ideas. Prats and Earl [16] demonstrate the reciprocal relationships between
designers thinking and their representation: representations are consequence of thinking and thinking is stimulated by perception of representation. It is assumed here that there is also a relationship between concepts
representation and the creative strategy adopted by designers to conceive
the product shape. In particular freehand sketches serve as a tool to assist
analogical and metaphorical thinking while physical prototypes, obtained
by sculpturing malleable clay and foam, mainly support mutation strategy
application.
Cross [17] identified four main strategies that can be adopted to generate
design concepts: combination, mutation, analogy, emergence and first
principles.
A freeform shape is the result of the deformation of global shape and it
is generally characterized by a usually flowing asymmetrical shape or outline. We mainly focus on analogy and mutation as they are the creative
strategies to obtain freeform shapes because:
• the first is based on the similarity relation between existing shapes often
belonging to nature where forms are generally smoothed, no rational,
and irregular;
• the second is based on the variation of an original shape due to specified
strengths.
Freehand sketches and physical prototypes are central elements as concept means of representation respectively for analogy and mutation trategies.
Analogy
Among creative strategies, ‘similarity-based’ or “analogy-based” creativity
attracts the most research interest that recognizes the key role of the
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analogical reasoning in creative design [18], [19] (figure 2). Analogy involves accessing and transferring elements from familiar categories to use
it in constructing a novel idea.
The mechanisms of similarity-based creativity, analogical and metaphorical, can be referred to as either ‘juxtaposition of the dissimilar’ or
‘deconceptualization’ [20]. The underlying idea is to put dissimilar concepts or objects so as to find new perspectives and create new meanings
through their synthesis, moving away from similarity. Opposing, the “deconceptualization” mechanism preserves the associations but requires estrangement from the existing conceptualization and describing the object
as if it is seen for the first time. New, spontaneous associations can then be
originated either without any systematic guidance from outside or by creating by analogy and metaphor.
Fig. 2. Example of analogical reasoning strategy application in industrial design
case studies
The understanding of the similarity-based mechanisms clarifies the importance of associations for creative thinking. Creativity intensively makes
use of concepts kept in memory and bound by associative relations.
Freehand sketching on paper stimulates analogy transfer, formal arrangements, structure mapping, and knowledge acquisition. [21], [22].
Mutation
Mutation seems to play an important role in determining the sensorial and
emotional properties of the shapes (figure 3).
Advances in design and manufacturing digital technology have changed
both designers and users imaginary: un-forms, blobjects, and free forms resulting from the adoption of mutation strategies in design, are the more intuitive outcomes of the post-information age, and at the same time, they
look like primitive shapes as they draw several similarities with the or-
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ganic and anthropomorphic forms found in nature. Motion may be considered as a perceived property of a shape in the same way of colors, textures,
contours, etc. By mutation strategies designers imprint motion to shapes
that visually reflect the creative way of idea generation. Complex transformations such as melting, expansion, stretch or compression, contribute
to generate freeform shapes from primitive ones (e.g. sphere, cube, cylinder, etc.). Freeform shapes can be realized by sculpturing physical clay or
foam. Perception is crucial in design and prototypes provide designers with
a basis to stimulate perception. In mutation strategy application, physical
prototyping seems to be a preferred mode of design representation to
stimulate reinterpretation and emergence as sketches in analogy-based
strategy. Foam modeling and woodcarving give designers new expressive
freedom: they allow exploring material expression in the conceptual phase
of the design process in a natural manner.
Fig. 3. Example of the use of mutation based strategy application in product and
architectural design.
Freeform objects modeling methods
The definition of freeform aesthetic features is strictly linked to the CAD
environment as they can be regarded as the key elements of shape modeling: they can be obtained by enveloping surfaces starting from some essential curves, called styling curves, that bound the overall shape, or by applying
ing a sequence of deformations to a primitive shape.
A freeform virtual prototype can be generated [23], [24] by:
• editing a shape by deforming a primitive object (e.g. cube, sphere, plane, etc.) moving its control vertices;
• interpolating a 3D network of freeform curves that consist both of styling curves and characteristics curves such as boundary edges or internal
edges;
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• modeling by projecting planar sketches on different reference planes in
the 3D environment.
While in the first case deformations are managed by the variation of the
global shape control vertices, in the second case by the modification of enveloping curves parameters or of the control points of the generated surfaces.
Shape manipulations are often inaccurate and time consuming. They can
be improved only if the semiotic contents are made explicit and CAD experts are able to identify which modeling methods allow the representation
of the adopted creative strategy’s outcomes and which elements should be
modified.
The design technologies used for applying such modeling methodologies can be resumed as follows:
• surface-based CAD modeling systems that represent objects by NURBS
mathematical surfaces;
• dynamic and physically-based CAD modeling systems that represent
objects both as mathematical surfaces and voxel-based models.
A further main differentiation is relative to the available CAD systems
that are characterized by different user interaction approaches as cited in
section two, i.e. haptic systems, direct modeling etc.
Benchmarking method for design technology evaluation in
relation to the creative strategy
In order to identify which digital technology better supports virtual prototyping, we define a benchmarking matrix that correlates available computational systems with the different creative strategies and the corresponding modes of representation. For each combination we analyze also a
proper procedure for shape modeling.
Benchmarking is an accepted technique used to identify the strengths
and weaknesses of a wide range of processes, products and technologies. It
can be performed by defining a set of metrics as quantitative e qualitative
estimates of the evaluation subjects.
Metrics for computational design systems evaluation
Metrics are defined to assess the performance of the computational systems for supporting the transformation of simplified models in nominal
ones, for modeling freeform features and for interacting with virtual prototypes coherently with the design intent. We distinguish between design
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system metrics and design process metrics. They are chosen because the
firsts characterize the intrinsic features of the modeling system while the
seconds highlight how the process is influenced by the specific system.
The main identified Design System Metrics (DSM) are:
DSM 1: 2D sketches usability in the modeling system
Assuming that sketching activity is generally performed in the ideation
phase, both in mutation and analogy, the procedure for virtual prototyping
starts from the recognition and extraction of the design intent from freehand sketches in terms of styling curves and meaningful attributes.
Previous research works deeply faced the problem of design intent formalization by analyzing the evolutionary process of sketches and by adopting image processing techniques for extracting the styling curves in the
different reference planes [25] (figure 4).
Fig. 4. Example of invariant curves searching by similarity measurement methods
The result of the design observation is that the styling lines look like the
invariant elements in the evolution of the free-hand sketches. The design
intent formalization is based on the research of these invariant lines in the
same reference planes by measuring the similarity between the early
sketches and the final ones. The matching result consists in the interpretative schemata that once extracted, are transformed into 2D B_Spline
curves and projected in the 3D CAD modeling environment. Textual notes
and graphic symbols play a crucial role in right scaling and positioning
both free-hand sketches in order to extract invariant curves and interpretative schemata in the 3D CAD modelling environment. As they contain
annotations about the overall dimensions of the product, they allow the dimensioning of all sketches in accordance with the 3D datasets measure, as
they give information about the reference view to which they are related,
they allow the comparison of early and detailed sketches and the recognition of the reference planes in the 3D CAD environment.
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DSM 2: ease of setting both geometric and non-geometric constraints
CAD models need to be constrained in terms of continuity, curvature and
tangency. Furthermore it is necessary to prescribe both qualitative attributes (fluency, sharpness, softness, etc.) and quantitative (dimensions).
DSM 3: ease and intuitiveness of freeform features modifications
The freeform features, generally, need to be modified according to the
manufacturability but such modifications have to be realized through intuitive 3D modeling functionalities maintaining the initial intent.
DSM 4: user interface usability
The modeling commands and the resulting entities should semantically
represent the designer’s intentions; the user interaction modalities should
be similar to the classical physical prototyping process.
The main identified Design Process Metrics (DPM) are:
DSP 1: time for freeform features modeling
DSP 2: number of design iterations necessary to achieve the solution validation
DSP 3: global number of aesthetic errors done by CAD operators
DSP 4: number of CAD models realized to collect all necessary information
Based on the observation of several design case studies, for each metric
we assign a value from 1 to 5 in benchmarking matrix, and the total
amount is used to qualify different technologies for the two analyzed creative strategies.
Experimental tests to metrics value determination
There are different methods of investigating the design process and the
weakness and strengthens of the adopted technologies. For example
“think-aloud” protocol is widely used in cognitive science, where participants are asked to talk during their work. In this experimental work we
adopt a more informal method based on interviews and post-hoc questionnaire. We state that these methods are less intrusive than e.g. “think-aloud”
and allow users to develop the specified task in their normal working
spaces without the pressure to be videotaped.
Participants are engineering and industrial design students with different
levels of product design expertise. Each of them has a good practice both
of CAD-based tools and of haptic systems as they had attended specific
courses. In particular tests are performed by students that have just at-
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tended a course in Mechanical Design and by students that are at the end of
a more advanced course of Industrial Design supported by CAD systems.
Students belonging to each course are divided into small groups consisting
of 4-5 members.
They were asked to develop a product design based on a specified brief
and to represent the achieved solution by the different modeling technologies cited below. The work lasted two weeks and at the end they had to
present a detail 3D model of the designed solution. They had at disposal all
technologies for design outcomes representation (3D scanners, physical
prototypes equipments, virtual reality lab, haptic devices, etc.) They were
encouraged to produce both hand-made sketches and physical prototypes.
The focus is on four different computational systems for representing
mutation and analogy strategy outcomes: CATIA by Dassault Systems that
provide parametric surface modeling, Free Dimension based on n-sided
surfaces (NSS) instead of non-uniform rational B_Spline surfaces
(NURBS), Freeform by Sensable coupled with an haptic device and Rhinoceros software by Mc Neel that allows the NURBS modeling. The
method is not limited to the considered elements. We choose them only for
exemplifying the methodology. The four selected systems are usually used
in real design processes.
Four different meaningful test cases are described in the following subparagraphs while the results are discussed in the next sub-section.
Case studies to represent mutation outcomes
Two different test cases have been studied: an armchair and a mini-car.
The first case study (CASE 1) is related to the re-styling of the armchair
“Intervista designed in 1982 by Lella and Massimo Vignelli and produced
by PoltronaFrau. Students were asked to modify the initial shape in order
to meet new ergonomic requirements and make the chair more appealing
while preserving the initial design intent.
As the designers used sketches to represent their ideas, students first
analysed the evolutionary process of sketches and the textual notes and applied the method for design intent formalization in order to identify the
creative strategy adopted by designers to conceive the initial shape. They
observed that the freeform shape was obtained by enveloping a threedimensional profile along a curve that defines the backrest. Aesthetic features communicate concepts such as continuity and proportion.
Some students preferred to realize a physical prototype of the armchair
and deformed it by stretching the main functional elements until the backrest was divided in two main elements. This allowed the increasing of the
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sitting dimensions. As they adopted a metallic net for physically modelling, reverse engineering techniques cannot be used to scan the prototype.
As more extensively reported in the next sub-section, the more performing solution has been identified in Catia. In fact, the group who used the
Imagine&Shape package of CATIA (figure 5) has achieved a good result
in a short time and without relevant process iterations. An initial sphere
containing the “Intervista” chair has been set and progressively stretched
by pulling the control vertices of the global shape. Interpretative schemata
have been projected on it in order to drive geometrical variations. The resulting shape has been then exported into the parametric surface modeling
module for engineering developments.
Fig. 5. The re-styling process of “Intervista” chair by Imagine&Shape
The second case study (CASE 2) is related to the design of a mini-car
body. Some students preferred to directly realize a physical prototype by
sculpturing malleable clay and they simultaneously made freehand
sketches on paper to communicate the design outcomes. The resulting product model has been then digitised by a 3D scanner in order to obtain a
cloud of points of prototype that can be used as reference for virtual prototyping. Sketches realized in the ideation phase have been compared and
similarity measures have been adopted to extract styling curves. These
styling curves have been then replicated as B_Spline curves in the corresponding planes in the chosen 3D environment in order to manipulate the
digitised model coherently with the design intent.
As the shape has been conceived by deforming clay, the most effective
system resulted to be the haptic system.
The digitised model has been imported in Freeform software (figure 6);
it has been deformed and smoothed by a haptic device. The device lets
them to feel the surfaces continuity, touch them, and modify the model by
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pushing, pulling and dragging its surfaces in a natural manner. Instead of
working on construction curves, with the haptic device students can deform the model guaranteeing the continuity of surfaces. The styling curves
have been used to drive deformation. In particular they have been used to
subdivide the model into regions whose curvature can be modified. The
consequent global deformation is coherent with the design intent as the
styling curves remain the same while the surroundings areas are deformed.
Fig. 6. The design process of a mini-car by using FreeForm and the haptic device
Case studies to represent analogy outcomes
The mini-car design has been also used as third case study (CASE 3) since
visual analogy has been also adopted to conceive the product shape.
Catia and FreeDimension have demonstrated the better applicability.
Sketches realized at the end of the ideation phase have been compared with
the thinking sketches and similarity measures have been adopted to extract
2D styling curves on the different reference planes. These curves have
been then projected on the digitised shape using the Catia surface modeling environment in order to obtain a network of curves (both styling curves
and characteristics curves helpful for surface reconstruction) to model the
virtual prototype (figure 7).
Model modifications to achieve the final solution have been performed
by the variation of characteristic and cross-sectioning curves parameters
and by imposing constraints such as tangent and curvature continuity.
As analogical thinking was a prevalent strategy in one case of the minicar design and only sketches have been used to represent design outcomes,
the students successfully adopted an intuitive system that let them to reproduce the virtual prototype of the conceived shape in a manner that reflect the way the “fish” shape lashed-up in their mind.
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Fig. 7. The design process of a mini-car by using a parametric surface (NURBS)
modeler (CATIA by Dassault Systems)
Extracted interpretative schemata have been replicated in the reference
planes of a cylinder in FreeDimension software (figure 8). The boundary
edges of the cylinder have been deformed until they had matched the 2D
styling curves. The final shape is obtained by moving the control vertices
of obtained 3D styling curves and of additional curves that were created to
support freeform modeling and by imposing different vales of tangency to
the network curves. At the end curvature analysis has been performed to
check the quality of the achieved surfaces and improve it.
Fig 8. The design process of a mini-car by using a parametric surface (NSS) modeler (FreeDimension by FreeDesign)
The final test case (CASE 4) consists in the design of a hairdryer. The
creative process started from sketching on paper several hypotheses until a
satisfying shape had answered to all design requirements in terms of size,
dimensions, ergonomics, aesthetics, etc. A trajectory curves is used to
sweep a rail along it in order to conceptually generate the hairdryer body.
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The resulting shape visually recalls a strange animal’s snout. Sketches
have been compared and interpretative schemata have been extracted. The
models have been successfully realised by Catia and Rhinoceros (figure 9).
we report only the last case.
Fig. 9. The design process of a hairdryer by using the NURBS modeler Rhinoceros by Mc Neel
Sketches have been imported into Rhinoceros in the corresponding
views and replicated as planar B_Spline curves Each curve has been successively projected in the 3D space in order to obtain the 3D model.
Results discussion
On the basis of the experimental work and, hence, of interviews and posthoc questionnaires analyses it has been possible to compile the following
benchmarking matrix (table 2). The Design Systems Metrics values, from
1 to 5, are weighted (wi), from 0 to 1, in relation of the importance of metric typology for the specific creative strategy (i.e. DSM 1, that is 2D
sketches usability, assumes a weight 0 for mutation, instead assume a
weight 1 for analogy). In table 1 are reported the weights values.
Mutation strategy requires a software system able to deform shape in a
very natural manner by looking at the styling curves for driving modifications. Both Imagine&Shape Catia module and FreeForm allow users to set
the styling curves in the corresponding planes and elaborate the forms they
work with as in physical sculpturing. The difference is that while in the
first modifications are managed by the position of the control vertices of
the global shape whose dimension can be set, in the second deformations
are very similar to those performed on clay but dimensional control is not
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allowed. The haptic system results highly usable and, definitely, the most
effective for modeling of objects created by mutation strategies.
Table 1 Weights (wi) relative to the different Design System Metrics for the mutation and the analogy.
DSM 1
DSM 2
DSM 3
DSM 4
MUTATION
0
0,7
1
1
ANALOGY
1
1
0,7
1
Table 2 Benchmarking matrix between the considered available digital technologies and the creative strategies.
Regarding objects based on the analogy strategies, FreeDimension represents a software system able to model freeform surfaces with a minimal
toolset instead of more complex parametric software packages (e.g.
CATIA) where modifications need a high degree of specialization.
Advantages consist in the usability of the modeling tools to obtain freeform aesthetic features from the deformation of the styling curves and in
the ease of modifications by controlling curves parameters and constraints.
Otherwise, it lacks of real dimensional control and evaluation tools, getting
closed to conceptual tools. It is difficulty usable for realizing a detailed virtual model, as it is not integrated within existing software packages for engineering developments. On the other side, CATIA modeler allows the surface reconstruction of the objects by interpolating the styling curves
network and imposing constraints (e.g. tangency continuity, control points
position, etc) to guarantee the preservation of the design intent. Modifications can be performed only if proper parameters and constraints are set.
Time spent for shape modifications is longer then with FreeDimension, but
at the end it is possible to develop the detail design without using additional tools.
Students’ interviews highlight the ease of use of Rhinoceros and that the
way of surface modeling is very similar to the creative strategy (analogy
Interacting with Virtual Prototypes Coherently with Design Intent
695
and sweep-like technique) they adopted. The main difficulty consists in the
modifications of the shape according to engineering requirements as it is
not possible to manage and control styling curves parameters and impose
aesthetic constraints.
Conclusions
The method proposed in this paper allows the identification of the more
suitable 3D modeling computational system coherently with the creative
strategy adopted during product ideation. Through this approach it is possible to improve the effective preservation of the designers’ intent represented by sketches and physical prototypes along the 3D virtual prototype
realization. The study is based on a benchmarking matrix where metrics
values characterize the different design technologies in relation to the mutation and analogy taken as meaningful examples of creative strategies.
The experimental case studies allowed a preliminary classification of technologies that has to be extended and improved. In particular, we verified
that surface-based CAD systems can be successfully used to model shapes
conceived by analogical reasoning strategies as they allow the definition of
the styling curves starting from sketches analysis, while the haptic systems
easily support the mutation principles by modelling shapes obtained by
sculpturing physical clay.
The future work will be focused on the refinement of metrics in terms of
weights used in table 1, on the analysis of further design software systems,
and, finally, on the definition of a more extensive number of case studies
to achieve a more robust and accurate characterization.
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