Reverse Engi of Asthetic Equipments

8/4/2019 Reverse Engi of Asthetic Equipments

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O R I G I N A L P A P E R

Reverse Engineering and restyling of aesthetic productsbased on sketches interpretation

Maura Mengoni Michele Germani

Received: 4 May 2008/ Accepted: 11 August 2008 / Published online: 19 November 2008

Springer-Verlag London Limited 2008

Abstract In the conceptual design stage, outcomes of

industrial designers work are generally represented by set ofsketches where curves, notes, shadows, and colors implic-

itly represent creative ideas. Signs and annotations are used

to synthesize and concretize the design intent that, finally,

will be transformed into the styling product visual appear-

ance. The loss of the original design intent may be due to the

complexity of the design process, and the involvement of

different actors. Our aim is to provide a method and relative

tools in order to interpret signs on sketches for eliciting the

design intent. The analysis result is a set of aesthetic fea-

tures that can be used for driving CAD modeling, in the case

both of Reverse Engineering applications and of product

modeling for restyling purposes. Sketches analysis is based

on a semiotic interpretation driven by the formalization of

the cognitive models used in the conceptual design phase.

The approach showed promising results on different styling

products test cases.

Keywords Styling products 2D sketches

Semiotic analysis Cognitive models

Reverse Engineering

1 Introduction

The word design has at least three important meanings:

as a process, as an object, and a discipline (Lawson 2006).

Product design, in broad sense, is associated with a set of

human activities aimed at developing an artifact; fromproduct shape ideation to product manufacture till main-

tenance and other after-sale services.

Restricting the interest to the phases strictly linked to

styling design, they typically include (but not are limited

to):

analysis of customers taste and needs. It includes the

study of socio-cultural trends in order to identify the

target market;

translation of the customers needs into marketing

specifications;

creation of the design concept (definition of theaesthetic and functional product features);

virtual and physical prototyping of alternatives

solutions;

analysis of technical and manufacturing feasibility

implying the choice of materials, colors, technological

solutions and, simultaneously, the identification of

possible manufacturing processes;

embodiment design and detailed design.

The growing complexity of modern products requires

an increasing degree of specialization to manage the

whole design process. The product design has to be

realized and evaluated involving several viewpoints and

disciplines. Industrial designers generally interact with

marketing departments, product engineers, and manufac-

turing engineers for achieving the final design solution. In

this collaborative process, communication problems

deriving from different technical backgrounds and indi-

viduals experience can emerge. This is particularly

evident when industrial designers and product engineers

interact. The difficult to effectively transmit the designers

creative ideas to other process actors results in a conflict

M. Mengoni M. Germani (&)

Department of Mechanics, Faculty of Engineering,

Polytechnic University of Marche, Via Brecce Bianche,

60131 Ancona, Italy

e-mail: [email protected]

URL: http://www.dipmec.univpm.it

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DOI 10.1007/s00163-008-0054-1


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that generates a high number of design iterations and

aesthetic errors.

In order to overcome the above-mentioned problems, we

state that it is necessary to formalize and, hence, preserve

designers intentions from the conceptual phases to the

following engineering developments.

We observed that communication problems are partic-

ularly critical in two different design situations: when thedesigner realizes a physical prototype and Reverse Engi-

neering (RE) techniques are applied to obtain its digital

representation, and when a product is subjected to restyling

(RS) in order to satisfy new emerging additional require-

ments. In both situations, designers generally conceive the

ideal product shape by realizing sketches or physical

models and CAD (Computer Aided Design) operators

belonging to the engineering department interpret the

designers intentions in order to develop the digital model.

Being forced to save time, they make autonomous 3D

modeling choices taking into account only the easier CAD

modeling strategy, and neglecting which shape featuresmust remain unchanged and must be modified according to

designers constraints. Errors are emphasized if the product

shape is characterized by a complex freeform surface.

The goal of our research is the development of a com-

putational tool for improving communication, avoiding

design intent misrepresentations, and, preserving it during

the whole design process. This goal can be achieved by two

main activities:

1. the definition of a method for the formalization of the

design intent based on the recognition and extraction

of the aesthetic features on the cloud of points (REprocess; the cloud of points is the set of points acquired

by a 3D scanning system) or on the CAD model (RS

process). The method aims at defining the optimal

design intent oriented CAD modeling strategies;

2. the study and development of rules and algorithms that

allow the future implementation of proposed method

within a CAD system.

In this work, we describe the approach and its experi-

mentation. The current process is not automatic as human

steering remains essential to interpret signs on sketches

(signs can be considered as lines, curves, words, etc.,

representing ideas on paper and they are collected into

sketches) and to identify the CAD modeling strategy. The

future method implementation will result into software tool

for automatically obtaining a 3D skeleton from 2D sket-

ches by a limited humans interaction and for automatically

realizing 3D CAD models based on design intent analysis.

We define the design intent as the set of the design

values represented by aesthetic features, and that must

remain unchanged throughout the whole product develop-

ment. Aesthetic features are conceived by the designer in

the first conceptual phase. From an operational point of

view, we define an 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 modeling the shape

according to the idea generation process. Styling curves are

the curves that characterize the aesthetic identity of

product.Aesthetic features and their use in surface modeling can

enable users to coherently and easily modify the model in

the following product development stages.

As sketches are the favorite channels of design intent

communication through the design process, we state that it

is important to study the creative process by analyzing

sketches evolution from a cognitive and a semiotic per-

spective in order to identify the coding/decoding rules to

link the design intent with the product shape attributes, to

highlight signs representing the styling curves, to formalize

the design intent and to translate it in the aesthetic features.

The proposed approach is illustrated by the restyling ofthe Intervista chair designed by Lella and Massimo

Vignelli and produced by Poltrona Frau. This particular

case study is chosen, because it requires first the applica-

tion of RE technique to obtain a digital model of the

existing product, and then the RS in order to answer to new

market requirements.

The paper is structured as follows: first, we analyze the

RE and the RS processes to identify the common problems

running into the validation phase, and how the proposed

method can overcome them. The definition of aesthetic

features and how CAD systems can manage them are

provided in the following section. Then, in order to track

the styling design process, we study the cognitive models

of design and the interrelation between the act of creative

designing and sketching. In Sect. 6, we describe the gen-

eral approach and the related method to support aesthetic

features recognition on the product digital model, CAD

model surfaces reconstruction and the definition of the

geometrical constraints to manage shape modifications.

Finally, test cases and the experimental results are pre-

sented in order to evaluate and discuss the potential

timesaving and product quality improvement obtained by

the proposed method application.

2 Reverse Engineering and restyling processes

Despite the large diffusion of CAD systems in all pro-

ductive fields, industrial designers still prefer to follow the

practice that the creative idea, conceived during the con-

ceptual design phase, can be represented with the necessary

freedom only by hand-made sketches (Van Dijk and Mayer

1997). The sketched idea, then, is usually converted into a

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physical prototype. Designers prefer to realize full-scale

hand-made physical mock-ups that match the impressions

and emotions indicated by the corresponding sketches on

paper. Depending on the size of object, they sometimes

realize reduced scale prototypes (e.g., automotive or

architectural design).

Prototypes are made to concretize the ideas represented

by drawings, to evaluate the spatial effects of complextopological shapes and to assess the final product shape.

The RE process becomes fundamental to obtain usable

3D digital models for embodiment and detail design.

The RE process can be subdivided in the following

phases: physical prototype acquisition by using a 3D dig-

itizing system, elaboration of the cloud of points in order to

eliminate noisy data and prepare it for the following

operations, surfaces reconstruction to define the exact

geometry of the digital model, product engineering to meet

production and functional requirements, and, finally, vir-

tual product model validation performed by the whole

design team.Surfaces reconstruction errors, mainly aesthetic errors,

are due to the poor quality of initial styling information and

to the large amount of data that have to be managed, often

noisy, sparse or incomplete. The high density of data and/

or the incompleteness can hide aesthetic and functional

features to the CAD operator who is forced to make

autonomous modeling choices, pursuing the less time-

consuming approach. Furthermore, the CAD model is

subsequently modified in order to satisfy technical, func-

tional and manufacturing requirements. This generally

causes several process iterations, in order to preserve the

initial designers intentions, during the validation phase of

product model. Therefore, only if the reconstructed model

is coherent with the design intent, the validation phase can

be successful. We argue that the process iterations can be

avoided if the meaningful aesthetic properties of physical

prototype can be extracted from the cloud of points.

It is worth to notice that in the design development

another critical process is the restyling of existing products.

When a product, which gained success in the market and

represents the company brand, needs to be innovated to

meet new emerging market tastes, it can improved in terms

of additional functions, less expensive manufacturing

processes or innovative materials, of eco-sustainability, of

compliance with new normative standards or with a more

attractive style.

As current research (Norman 2004) has demonstrated,

emotions play a decisive role in the customer decision-

making process. The preservation of the design intent

depends on the maintenance of those aesthetic properties

that have previously appealed to consumers emotions. RS

can be performed by the designer or by the engineering

staff of the company manufacturing the product itself. In

the second case, the company staff makes individual

choices in the RS; the design intent may be lost due to the

difficulty to identify the design intent, and modify the

product shape according to it. As a consequence the suc-

cess of restyled product fails.

Restyling generally starts by the elaboration of a 3D

CAD model. When the company does not have the ori-

ginal CAD model, or when the model lacks thenecessary information to allow aesthetic features recog-

nition (e.g., the model has been stored in neutral formats,

such as IGES, STEP, STL, etc.), the RS process is not a

trivial task. In the first case, RE process is applied to

acquire the original product shape, in the second the

CAD model has to be elaborated in order to extract

styling curves, modeling strategies and design parame-

ters. We argue that the analogy between RE and RS

consists in the common properties of the cloud of points,

and of the CAD model that must be elaborated in RS.

Both of them implicitly contain the aesthetic information

that must be extracted in order to manage design mod-ifications, while the design intent is preserved. As a

consequence, in this paper we do not distinguish between

the first and the second type of virtual models; we

generally speak of 3D data sets.

3 CAD modeling based on aesthetic features

Nowadays, several design activities, from conceptual

design to detail design, are supported by CAD systems that

enable the creation of digital models from 3D datasets by

using strategies based on features.

The research on features can be roughly subdivided into

two main topics: Feature Recognition and Design by

Feature. This distinction can be applied also to the free-

form features (Fontana et al. 2000; Langerak and Vergeest

2006). If this definition is associated to aesthetic products

with complex freeform shapes, they are called aesthetic

features highlighting the stylistic elements they embody

(Germani and Mandorli 2004).

Design by Feature approach can support the creation of

aesthetic shapes by providing designers with features

reflecting the way of product shape generation. Available

computational tools are not yet adequate to model free

form shapes and intuitively control them in the first con-

ceptual phase (Sequin 2005).

Feature Recognition approach is more useful to manage

RE and RS problems, because it allows the extraction of

those features that must be preserved during the whole

design process. In this case, it is possible to use available

CAD modeling rules to recognize the styling curves on the

cloud of points, fit surfaces and set proper design param-

eters (Ke et al. 2006).

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Methods for aesthetic features recognition are a widely

explored topic in the CAS/CAID (Computer Aided Styling/

Computer Aided Industrial Design) research community.

Thompson et al. (1999) proposed and implemented a

method to recognize features from digitized data, but their

application was limited to regular features. Recently,

Langerak and Vergeest (2007) studied an approach for

recognizing free form features by using two differentmethods: template matching and feature line detection. The

first method uses shape matching to find regions on the

digitized data that resemble the shape of a feature shape

template. They establish a set of independent template

parameters to progressively search the shape similarity

between a shape and an instance of template. The second

approach is based on a slicing strategy to extract 2D pro-

files by taking as an input a feature library that is assumed

but not defined. The method automatically performs RE

activities, but does not specify how the user chooses the

intersection planes orientation to extract features.

As mentioned before, Ke et al. (2006) proposed a pos-sible RE approach based on a 2D sketch space and

traditional surface modeling rules such as extruding,

revolving and sweeping the 3D profiles. In order to extract

the layered sectional profiles, their research method applies

the slicing technique to the unorganized cloud of points,

then estimates discrete circular curvatures and derivatives

the points contained in the interested sectional profiles. The

recognition of the slicing planes orientation, and the clas-

sification of the aesthetic features are driven only by human

expertise. Consequently, we argue that if the expertise is

not formalized, the choice and the identification of the

aesthetic profiles depend only on the sensibility of CAD

experts. A further study in this direction is proposed for the

creation of a framework for design intent management

based on precedents reuse. The method involves features

recognition in the freeform domain in order to obtain a

freeform shape parameterization according to the design

intent (Vergeest et al. 2006). In a previous research work a

3D problem is simplified into a 2D sketch approach

(Germani and Mandorli 2004). The functional definition of

the product allows identifying planes and 2D curves

characterizing the aesthetic regions of the model shape.

The dependency from the user makes the method scarcely

structured to objectify the design intent recognition on 3D

datasets.

On the other side we have observed that not only the

styling curves extraction is useful for the reconstruction

process, but also the identification of the proper modeling

strategy to fit surfaces and the corresponding design

parameters (Mengoni et al. 2006, 2007). It is worth

noticing that in order to preserve the design intent, the

modeling strategy would be reflect the process of idea

generation as it affects the way of free from shape

modification during the following engineering phases

(Cheutet et al. 2004).

4 Cognitive models of design to track sketches

evolutionary process

We have adopted well-known models of design, based onmultimodal perceptual representation and diagrammatic

reasoning, to track the creative design process in order to

correlate design meanings and the geometric attributes of

shapes, and to recognize the descriptive models of idea

generation by which designer conceived the product shape.

Design is a complex mental process that takes place in

the designers mind. It may be represented by a sequence

of distinct and identifiable activities that does not neces-

sarily occur in a logical order; problem and solution often

emerge together.

Many attempts have been made to objectify the design

process. Despite the diversity of nomenclature proposed inliterature (Dorst and Cross 2001; Gero and Kannengiesser

2004), certain major activities begin to crystallize. These

included:

Formulation implies the problem space framing and the

definition of design requirements;

Synthesis refers to the act of moving from the problem

space to the solution space. It is the activity that mainly

characterized the creative design process;

Representation consists of the act of externalizing the

conceived shape. Representation and Synthesis are

strictly interconnected: drawing affects synthesisperformance as it stimulates emergence and reinterpre-

tation. Drawing is not the only mean of design

outcomes representation, but one of the most important

in the first conceptual phase;

Evaluation is the first decision-making process in

which the designer elaborates a judgment by comparing

the achieved solution with design requirements and

formulates possible changes.

This model is less linear that it seems; continuous

interactions between all activities may lead to an accept-

able solution.

Cognitive science provides a novel standpoint to analyze

the design process. It attempts to uncover the mechanisms

through which designers achieve simple tasks for each

mentioned activity by processing a large amount of infor-

mation (previous case studies, internal and external

constraints, artificial and natural word images coming from

different domains, etc.)

Chandrasekaran (1999) proposed a cognitive model of

design, drawing a parallel between thought and perception.

He aimed at understanding the transition from formulation

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to synthesis. He proposed a computational framework

made of two different levels, the perceptual and conceptual

ones, containing logical structures and mental images;

ideas are externalized by designers through free-hand

sketches and textual notes. Multimodal perceptual repre-

sentation and diagrammatic reasoning is a cognitive model

that views a cognitive state as an integrated and inter-

linked collection of images in various modalities: theperceptual and the conceptual ones. Thinking, problem

solving, reasoning, etc., are viewed as sequences of such

states, where there is no an intrinsically preferred mode.

The perceptual level consists of several free-hand sketches

as design elaborations of the external images and previous

case studies. That phase structures the design space

(domain). In the conceptual level, the designers define the

design values drawing relationships between signs and

concepts.

By analyzing the evolutionary process of sketches,

signs, and annotations contained in each sketch, it is pos-

sible to recognize which sketches are realized either in theperceptual or in the conceptual level.

By investigating well-known techniques and procedures

of creative design, we adopt the five main descriptive

models of idea generation proposed by Cross (1997):

combination, mutation, analogy, emergence and first prin-

ciples (Fig. 1).

The identification of which strategies lash-up in the

designers mind, has several implications for CAD mod-

eling. For example, in mutation, Cross underlined the

necessity of recognizing which features of existing design

must be selected for modification or how the product

behavior can affect the deformation process. In analogy,

the difficulty consists in abstracting the appropriate fea-

tures of an existing design or natural shape in order to

model the new product model.

5 Sketches as traditional means of design intent

representation

Sketches analysis provides useful data for design intent

formalization in term of adopted creative strategies and

styling curves.

Sketches, as cited above, are a set of free-hand 2D signs,

and annotations correlated to the creative mental processes

of the designer. They support formal and functional rea-

soning, and they are the preferred media to communicate

ideas (Schutze et al. 2003).

Ferguson (1992) classifies three kinds of sketches: the

thinking sketch, the talking sketch, and the prescriptive

sketch (Fig. 2).

Thinking sketches refer to the designers making use of

the drawing surface in support of their individual thinking

processes. They are essential, abstract, vague, and dia-

grammatic. The industrial designer identifies clues that can

be used to form and inform emerging design concepts. He/

she creates series of rapid sketches to generate images in

his mind, to elaborate case studies and to adapt them to the

specific design problem. Their ambiguity stimulates re-

interpretation. They are characterized by monochrome line

drawing without shading or colors and have a uniformthickness.

Talking sketches refer to designers making use of the

(shared) drawing surface in support of the design team

discussion. Suwa and Tversky (1997) claim that designers

draw to externalize their concepts, and that drawings pro-

vide visual cues for revision and refinement of ideas.

Talking sketches, in some cases, can be generated as

consequent transformation of the first ones. The designer

modifies the thinking sketches by adding, deleting or

varying the initial curves drawn on paper. He/she generally

varies the line thickness of the sketched curves, highlights

the lines that better match his/her ideas, while modifyingthe others. These types of drawings may include brief

annotations such as the main dimensions of the product

parts.

Prescriptive sketches are used by designers to commu-

nicate design decisions to all persons that are outside of the

creative process. They are detailed and measured drawings.

They contain all the necessary dimensions and annotations

that allow the creation of physical prototypes and digital

models of the product design.

A further class, defined by Ullman et al. (1990), is the

storing sketches. They refer to the designers using the

drawing surface to archive design ideas for their own future

reference. Storing sketches have much in common with

prescriptive ones for the amount of annotations and graphic

signs, drawn by the designer to fix the most important

features in his/her mind, but are as essential as the talking

sketches. They both freeze, rather than develop, design

ideas. They are intended for retaining information, whereas

prescriptive sketches for communicating information.

The evolution from thinking sketches to prescriptive

ones is explained by Goel (1995); sketches successively

undergo two different types of transformation, lateral and

vertical transformation. In a lateral transformation, move-

ment is from one idea to a slightly different idea. In a

vertical transformation, movement is from one idea to a

more detailed and exacting version of the same idea. An

obvious change in thinking (divergence) is a lateral trans-

formation, while if the change is instead to a more detailed

version of the same idea then a vertical transformation

(convergence) has occurred. (Rodgers et al. 2000).

Lateral transformation stimulates emergence, while

vertical transformation inspires reinterpretation. Sketches

are thus central to the phenomena of emergence, and

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reinterpretation during early design activity. Emergence

refers to new thoughts and ideas that could not be antici-

pated or planned before sketching. Reinterpretation refers

to the ability to transform, develop and generate new

images in the mind while sketching. There is considerable

evidence to suggest that the production of design ideas

appears to depend heavily on this interaction with sketches(Kavakli and Gero 2001; Tovey et al. 2003).

Thus, also the evolutionary process of sketching has to

be taken into account considering every phase of its path.

By analyzing the design process of several test cases

from both cognitive and semiotic perspectives, we have

observed that thinking and storing sketches are generally

used in the perceptual level as they are essential, abstract

and stimulate lateral transformation while talking and

prescriptive sketches are realized in the conceptual level as

they are more detailed, contain much more information

(annotations, textual notes, graphic symbols) than the first

ones.

In our work, we focus the attention on prescriptive

sketches containing more analyzable information from a

computational point of view. The other classes of sket-

ches are arbitrary, vague, incomplete due to the nature ofthe mechanisms through which designers form their own

ideas (Gross 1994). Prescriptive sketches resume the

concepts, elaborated and externalized in previous sket-

ches, in an organic way. Thus, thinking and talking

sketches can be a very useful support to analyze pre-

scriptive ones in order to track the creative design

process, extract 2D styling curves, recognize the adopted

idea generation strategies and finally, link shape attributes

with design values.

Fig. 1 The five descriptive models of creative design

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6 Sketch-based RE and RS modeling: approach

overview

On the basis of the previous considerations, our approach

adopts two different standpoints: semiotic and cognitive.

(Fig. 3).

The first is used: (1) to interpret the structure of sket-ches, (2) to understand how product ideation can be

conceived as an act of signification, how creative outcomes

are affected by the interpretations of communicating

actors, how concepts are linked to signs on paper, (3) to

identify the generation/synthesis strategies by analyzing

graphic signs and annotations, (4) to recognize the styling

curves on prescriptive sketches and finally, (5) to study the

different channels of ideas transmission.

The result of the semiotic analysis consists in the 2D

interpretative schemata of the prescriptive sketches thatallow highlighting the meaningful styling curves succes-

sively translated into the 3D styling curves useful to create

the digital model of the product shape.

Fig. 2 Example of the three classes of sketches correlated to the increasing level of detail

Fig. 3 Schemata of the

proposed approach for feature

recognition

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On the other side, the cognitive perspective allows the

recognition of the ways of idea generation and of the link

between graphics signs on paper and design attributes. The

structure of the design process into perceptual and con-

ceptual levels provides a tool to achieve these tasks.

Extracted data are then elaborated to identify the proper set

of CAD modeling strategies to fit surfaces from 3D data-

sets and finally, the parameters and constraints that allowmanaging shape modifications while preserving the design

intent.

From an operational point of view, our approach starts

from the definition of aesthetic features as the set of shape

attributes determining style, and of their role in transmit-

ting and understanding the message implicitly contained in

the product shape. The design intent is generated by

applying shape generating strategies, geometrical and

functional relationships between forms, and by repeatedly

using a set of prominent forms. This can translated in the

set of aesthetic features that remain unchanged throughout

the product development process: the styling curves, therelationships between them (aesthetic constraints), and the

strategies to combine them in fitting surfaces.

Free-hand sketches have a central role in the identifi-

cation of the aesthetic information. Both thinking and

prescriptive sketches are sets of implicit and explicit

features.

In particular, explicit features are expressed as annota-

tions, generally contained mainly in the prescriptive

sketches. They may specify functional conditions (geo-

metrical and dimensional) and generic product attributes

(color, material, number of interfaces, etc.).

Implicit features are the translation of creative and

functional concepts in graphic signs in the hand made

sketches. These signs contain both the styling curves,

meaningful for the shape definition, and the main specifi-

cations to understand and computationally replicate the

process of form generation.

7 The proposed method: steps and tools

The different steps that lead to styling curves extraction

and to modeling strategy definition are:

extraction of the styling curves from 3D datasets;

creation of a 3D skeleton of styling curves and

grouping them according to the recognized ways of

design idea generation;

translation of the idea generation processes into a

proper set of surface modeling strategies and identifi-

cation of the design parameters and constraints to

manage design modifications coherently with the

design intent.

7.1 Extraction of the styling curves from 3D datasets

We briefly summarize the main steps to be carried out for

the extraction of the styling curves from free-hand

sketches.

It is worth noticing that different types of drawings are

associated with different stages of the design process. As

there is an intimate relationship between the process ofcognition, the act of sketching and the ways of ideas rep-

resentation on paper, the formalization of the creative

process starts with the analysis of free-hand sketches

evolution.

By observing several hand-made sketches realized in

different stages of the design process, we deduce that:

different free hand sketches reproduce the designer

ideas as they evolve in his/her mind;

prescriptive sketches contain all information necessary

for physical and virtual prototyping as they are used to

represent the final design solution. They can beconsidered the outcome of the vertical transformation

of some thinking sketches where the designer has

partially elaborated the design solution or he/she has

achieved some meaningful form curves (styling lines)

that will be strengthened in subsequent drawings. This

cognitive state of transformation corresponds to a

continuous growing of complexity and detail from

thinking sketches to prescriptive ones;

free hand sketches implicitly represent the designers

intentions;

form curves appear in all different types of drawings,

they are modified during the design evolution. Thedesigner generally increases the thickness of styling

curves that better concretize his/her ideas;

freehand sketches contain textual annotations to com-

municate and clarify design contents;

prescriptive sketches differ from thinking and talking

sketches for the increased level of details;

the visual assessment of sketches evolution and their

comparison show that there are some lines that remain

unvaried. They are the styling lines as defined by Tovey

et al. (2003). These invariant lines are all articulated in

the final sketches, while in the early drawings they do

not contemporary appear. They may occur in differentstages of the design process as soon as the design ideas

lash-up in the designers mind;

prescriptive sketches realized on orthographic/cross

sectioning views are the outcomes of the design

reasoning activities. Therefore, they represent the result

of the bedding and joining of meaningful signs drawn

in previous sketches;

As a result of design observation, we assume the styling

lines as the invariant elements in the evolution of the free-

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hand sketches. Design intent formalization starts by

searching these invariant lines, whose articulation gener-

ates the interpretative schemata of 3D datasets.

From these general considerations, styling curves

extraction is performed in accord with the following steps:

1. measurement of similarity between early sketches and

prescriptive ones. Chalechale method (Chalechaleet al. 2005), based on angular partitioning of two

images, is adopted to extract the invariant curves

between the first diagrammatic sketches, and the

detailed ones that are all scaled according to the 3D

dataset dimensions. Articulation of the achieved

invariant curves in the same reference plane generates

the interpretative schemata. The number of interpreta-

tive schemata is the same of the prescriptive drawings.

Textual notes and graphic symbols play a crucial role

in right scaling and positioning free-hand sketches for

similarity measurement. The objective is to extract

invariant curves and interpretative schemata with thesame dimensions of the shape in the 3D CAD

modeling environment. As prescriptive drawings con-

tain annotations about the overall dimensions of the

product and the reference view to which they are

related, they respectively allow sizing all sketches in

accord with the 3D datasets measure, comparing early

and detailed sketches and recognizing the reference

planes in the 3D CAD environment (Fig. 4);

2. replication of the interpretative schemata in the 3D

CAD modeling environment. The extracted interpre-

tative schemata are positioned in the corresponding

plane in the 3D dataset and replicated as B_Splinecurves. Each B_Spline curve is characterized by

parameters such as the position, the curvature and

tangency of the control points.

3. styling curves extraction. Each 2D B_Spline curve is

projected on the 3D dataset along perpendicular

directions to the identified reference planes. In case

of RE application, the projection of the interpretative

schemata detects a set of points that are imposed as the

control vertices of the 3D styling curves. On the other

hand, in case of RS, it detects a curve that is already

the searched styling curve. The result of this step is a

set of styling curves on the 3D dataset.

Styling curves extraction can be also successful in the

case of non-complete definition of form lines on paper. The

method actually provides a tool to support CAD experts in

surface reconstruction from 3D datasets by recognizing

meaningful lines to realize flexible digital models. During

the validation phase, the extracted styling curves can be

varied in order to improve surface quality: constraints and

parameters guarantee the coherence of modifications in

respect with the design values defined in the early stages of

the design process and, only subsequently formalized by

aesthetic features.

7.2 Creation of the 3D skeleton

Characteristic curves and character curves on 3D models

are meaningful geometric elements for aesthetic features

definition. The first are tangible curves, such as boundary

edges, internal edges and fillets edges. The second are only

visually perceived. They are the styling curves meaningful

for conceiving the 3D product model. Furthermore, there is

another type of curve that is important for surface model-

ing, but without a specific aesthetic meaning; they are the

cross-sectioning curves. They can be extracted by sec-

tioning the 3D dataset with perpendicular planes tocharacter curves. They are low-level geometric elements

that are related to the styling curves. They allow the direct

control of the product shape. They are particularly useful

when the projected schemata do not perfectly coincide with

the visually perceived styling curves on the 3D dataset as

Fig. 4 Searching for invariant curves (interpretative schemata) in the

Intervista product. The early sketches are compared with the

detailed ones in order to recognize the styling curves on the 3D

dataset. The interpretative schemata are extracted in the same

reference planes of the prescriptive sketches

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they provide a more detailed scanning of the 3D dataset

surfaces. The lack of the correspondence can be due to

small errors that may occur in the extraction of the inter-

pretative schemata from prescriptive sketches.

The 3D skeleton results from the extraction and elabo-

ration of styling curves, characteristic and cross-sectioning

curves. All curves are smoothed in order to avoid noisy

data and reach the required degree of surfaces quality.For complex free form shapes, all extracted curves are

grouped into smaller sets of curves in order to allow the

easily management of surface modeling. Each group rep-

resents different aesthetic and functional parts of the

product model.

Free-hand sketches are also useful to perform grouping

curves, for example, designers draw arrows to indicate

relationships or movements of product parts, in this way

the main functions and instructions for product use are

defined. It is possible to identify the main functional parts

of the product. Designers usually label design concepts and

parts names in their drawings. Moreover, the use colors insketches allows the identification of parts with different

aesthetic properties.

The extracted curves must also be grouped in accord

with the identified modeling strategies; it is necessary to

point out which curves belong to each strategy, and their

role in it.

As each modeling strategy corresponds to a different

way of idea generation, every group of curves reflect the

aesthetic and functional properties of the product design.

Therefore, the grouping of 3D skeleton curves in accord

with the modeling strategies is coherent with the design

intent (Fig. 5).

7.3 Translation of the shape generation process

into the set of modeling strategies

Our assumption is that the result of the parametric surface

modeling and the coherence of CAD models modifications

in respect with the design intent, strictly depend on the type

of strategy adopted to construct surfaces and on the

hierarchical chain of operations used to the whole surface

model.

In the context of freeform shapes, two prescriptive

models of creative design proposed by Cross (1997), are

representative of organic freeform shapes: mutation and

analogy.

Mutation can be realized by applying different trans-

formation actions:

fusion or melting: when two or more forms are

combined and their edges are strictly interconnected;

it is not possible to distinguish when the first ends and

the second begins. The product form is the result of the

interaction between the original shapes;

global deformation: when a form is modified by

applying forces along different directions and with

different weights;

morphing-like deformation: when a shape is generated

based on weighted average of two other existing

shapes.In all mutation strategies, the problem regards with the

definition of which surface entities are subjected to defor-

mation and of which constraints drive the deformation. The

choice of the firsts influences the second. Catalano et al.

(2004) classified freeform features by defining three dif-

ferent types of deformations and related constraints: point-

driven deformation, curve-driven deformation, surface-dri-

ven deformation. In order to apply deformation operations,

it is important to translate the mutation strategy into CAD

modeling operators and for each operator to identify which

parameters and constraints must be used to fit surfaces.

In creative and conceptual design, designers often look

to books, magazines, and other collection of images to find

forms they can adopt and adapt in designs. They use ref-

erencesimages of natural and artificial world from rocks

and flowers to boats and buildings, internal images that

come from their experience of realityas visual analogies

and metaphors. All images are arranged in different ways

according to the design values they aim at communicating

through the product shape.

Fig. 5 Projection of the

interpretative schemata on the

3D dataset (left) and the 3D

skeleton resulted from the

extraction of the styling curves,

characteristic and cross-

sectioning curves from the 3D

dataset (right)

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Among creative strategies, similarity-based or anal-

ogy-based creativity attracts most research interest

recognizing the key role of the analogical reasoning in

creative design (Goel 1995). It is a key point of the idea

generation as demonstrated by several works of contem-

porary designers where analogical reasoning involves

accessing and transferring elements from familiar catego-

ries to use it in constructing a novel product design. (e.g.,Karim Rashid, Future Systems, Frank OGehry, Michael

Graves, Massimiliano Fuksas, Philippe Starck, etc.).

The understanding of the similarity-based mechanisms

clarifies the importance of associations for creative think-

ing. Creativity intensively makes use of concepts kept in

memory and bound by associative relations. The nature of

these relations is neither logical nor mathematical, but

perceptual and experiential (Hofstadter 2001).

As analogy emerges as a relation of similarity between

two existing shapes, features or entities stored in long-term

memory, or rather between source and target (Goldschmidt

2001), a possible classification of analogical reasoningstrategies starts from the identification of the terms of

similarity:

analogy between forms or features;

analogy between opened or closed sections (entities).

In the first case, analogy can be created by morphing

two forms: the first can be retrieved by a 3D library where

the designer collects interesting forms, case studies and

previous works; the second can be a primitive shape or a

low elaborated shape. The problem deals with the defini-

tion of the relationships between geometrical properties of

the initial and the target forms, key reference points or

curves. This generation process is similar to the morphing-

like deformation process in mutation.

In the second case, when two forms are similar in the

morphology of some section curves, it is important to

identify the process through surfaces have been generated

from these sections:

process of sliding of one section along another, or

process of translation, rotation, extrusion along a

direction;

process of joining and connecting multiple sections.

In the context of freeform shapes, the similarity is set

between natural forms (i.e., organic shapes) or by applying

form generation processes deriving from nature.

Existing feature-based CAD tools difficulty support free-

form features design by providing functionalities able to

apply the above-mentioned generation processes, because

they allow only the transformation of existing surfaces by

arbitrary changing the position of their control points.

It is difficult to identify proper CAD modeling strategies

to fit surfaces reflecting creative processes and the

hierarchical chain of operations for whole product model-

ing, entities and parameters that can be modified and the

constraints that can be used to express the aesthetic prop-

erties of shape. Tables 1 and 2 try to map the ways of idea

generation with available CAD modeling strategies. The

first map identifies creative strategies within available

CAD modeling functions, while the second one provides a

list of geometric entities, parameters and constraints thatcan be set for each CAD modeling strategy encompassing

the corresponding creative technique. The proposed CAD

modeling strategies can be found in different feature-based

CAD systems, and is not limited to the adopted technology

for method validation.

Free-hand sketches evolution analyses may be useful to

understand designers intentions and the ways of free-form

shapes generation. For example, designers draw arrows and

lines to indicate the main directions of the orthographic and

sectional views, the main movements or relationships

between product parts. They also draw lines characterized

by conventional styles to indicate symmetry axes, parallelplanes, perpendicular lines, etc., but also trajectories of

sliding, of translation, etc. They use to underline key ref-

erence points or curves. Designers, who prefer aesthetic

proportions and balance, usually sketch lines to form grids

that may constitute geometrical constraints for the fol-

lowing engineering developments. The use of big arrows

generally means the weight of forces applied for shape

deformation. Designers usually write words, stick images,

make collages, in order to recall precedents and meaningful

reference images that can be useful for similarity entities

recovery. Finally, designers write numbers to prescribe the

overall dimensions and to identify product parts interfaces;

this data is necessary not only for scaling sketches

according to the 3D datasets dimensions in order to extract

the interpretative schemata, but also to identify parameters

and constraints for coherent shape modifications, such as

position parameters, dimensions, angles of revolution, etc.

Once the styling curves have been extracted and

grouped, modeling strategies choice starts from the anal-

ysis of sketches evolution in order to search for which

geometric entities should be engaged in fitting surfaces. For

example, if a revolution axis and a styling curve nearby is

identified in the free-hand sketches, the revolution sweep

strategy should be adopted, or if the product shape results

from the deformation of a grid of curves, the grid strategy

should be applied. Possible parameters and constraints

have been defined for managing modifications (Table 2),

so the whole parametric model is achieved by identifying

the proper chain of CAD operators (Fig. 6).

While the extraction of styling, characteristic, and cross-

sectioning curves is quite automatic, the identification of

the CAD modeling strategies from sketches evolution is

still manual. The proposed method provides a framework

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to support CAD experts to analyze the design intent in

order to identify the proper chain of operations to fit sur-

faces and manage the following engineering modifications

on digital prototypes. Tables 1 and 2 allow mapping

strategies with available modeling techniques.

8 Experimental validation

In RE context, the validation of the proposed method has

been preliminarily carried out using two test cases: a mouse

prototype for left-hand users and a telephone design. Each

product is characterized by free-form shapes.

Small groups of engineering students (groups of 45

members) have been chosen to design each prototype.

Students have a practice of RE methods akin to that of

designers. This particular experimental condition has been

chosen, because it allows the analysis of the whole ideation

process in a fully monitored context where the adopted

cognitive models of design, the classified strategies of

creative design have been tested. During the conceptual

design phase, students explored ideas by free-hand

sketching on paper searching for a meaningful shape that is

able to represent their personal product concept. After

continuous design reviews, the product shape seemed to

reflect the design values expressed by students both in a

verbal manner and in the annotations and graphic signs

drawn on paper. Physical models have been realized by

sculpturing malleable clay both to actualize design ideas in

concrete terms, and to evaluate the spatial effects of the

conceived products shape. In order to obtain digital mod-

els, students used commercial technologies for digitizing

both the mouse and the telephone physical mock-ups

acquisitions were done by a contact system (Modela MDX-

15 by Roland). A 3D CAD system (CATIA v.5.14 by

Dassault Systemes) has been chosen to perform the fol-

lowing main steps of the proposed method. The system has

been chosen, because it supports simultaneously the fea-

ture-based modeling and the parametric surface modeling

and it provides tools both for points cloud data elaboration

(post-processing data points), for surface modeling and for

structural, ergonomic and manufacturing simulations. This

CAD system allows furthermore managing attributes to

link the semantic contents with the graphical entities.

Similarities measure between preliminary sketches and

prescriptive ones has been performed. The result is a set of

Table 1 Classification of the creative process of free form generation. Mapping with 3D CAD modeling strategies

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interpretative schemata of sketches that consists of 2D

styling curves on the same reference planes of the pre-

scriptive sketches.

In order to extract the necessary curves for 3D skeleton

creation, students followed the following steps:

1. analysis of textual notes contained in the prescriptive

sketches to gather the position of reference planes;

2. identification of the corresponding planes in the

digitized data CAD environment in order to position

the interpretative schemata and replicated them as

B_Spline curves;

3. projection of B_Spline curves on the clouds of points

along perpendicular directions to the reference planes.

The result is the set of the styling curves;

Table 2 CAD modeling strategies using commercial CAD/CAID systems. Definition of the parameters and constraints to control shape

modifications for each strategy

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4. extraction of cross-sectioning curves and characteristic

curves to obtain a 3D skeleton of each product model

to fit surfaces.

The analysis of free-hand sketches evolution and the

structuring of the whole design process according to the

perceptual and conceptual levels proposed by the adopted

cognitive standpoint showed that:

1. in the case of the mouse design, analogy strategy has

been adopted, creating a relation between different

natural forms. Analogical reasoning leads to theidentification of some meaningful curves on natural

shapes, such as flowers and leaves, that are replicated

on the particular design context: in nature, visible

curves correspond to the structure of the natural

element.

2. in the case of the telephone design, mutation has been

adopted to conceive the shape. A primitive form (oval

shape) was progressively hollowed according to the

specified ergonomic requirements: students attempted

to facilitate the phones hold. In the product brief was

specifically explained that key numbers need to be

clear visible: the reasoning relationship between theadjective visible and the image of an eye appear in

the first sketches. The hollow followed the shape of a

big eye.

By mapping the idea generation techniques with the

CAD modeling strategies (Tables 1, 2), surface recon-

struction has been performed and aesthetic constraints have

been imposed (Figs. 7, 8).

On the other side, method validation has been performed

on real industrial design cases. The restyling of two

existing products (Intervista chair by Lella e Massimo

Vignelli and BIBI seat for Distillerie Nardini theatre by

Massimiliano Fuksas) have been developed in collabora-

tion with Poltrona Frau, a worldwide leader in the field of

furniture design and manufacture. Its products are charac-

terized by high aesthetic values.

The first test case is well explained across the paper in

order to illustrate the proposed method.

The BIBI restyling started from the companys needs to

redesign an office chair that would preserve the original

BIBI design intent; some aesthetic features must remainunchanged while new additional functions appear. The

company had the CAD model of the BIBI but in a neutral

format (e.g. IGES). The RS was carried out by CATIA v

5.14 that is used by Poltrona Frau technical staff for

everyday works.

In order to recognize aesthetic features, sketches and

preliminary prototypes have been analyzed. The way of

idea generation resulted in the combination of three dif-

ferent techniques: analogy with the drop shape, mutation as

the melting of the drop shape when it comes in contact with

the seating, and first principles in terms of the structure that

determines shape. Some meaningful curves have beensimultaneously extracted by similarity measurement.

The translation of interpretative schema into 3D styling

curves over the BIBI digital model and the application of

the identified CAD modeling strategies mapped by using

Tables 1 and 2, allowed the creation of a parameterized 3D

model that was modified according to new emerging ideas

for the office chair (Fig. 9).

In order to qualify the styling design process improve-

ments by means of the proposed method application,

Fig. 6 Grouping the extracted curves in accord both with the

modeling strategies and with the aesthetical and functional properties

of the product parts. The parameterization of Intervista surface

model. As balance among the product part represent one of the

designers intentions, shape deformation is constrained to a grid that

was drawn on the early sketches. The restyling process is due to the

need for an improvement of the ergonomic characteristics: enlarge-

ment of the dimensions of the seat, lowering of the backrest

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Fig. 7 The application of the method for the RE of the mouse prototype

Fig. 8 The application of the

method for the RE of thetelephone prototype

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performance measurement is essential. A list of process

metrics has been established to evaluate the method effi-

ciency in terms of:

1. time needed for surfaces reconstruction and product

validation as we have observed that design iterations

and aesthetic errors are mainly due to the difficulty of

design intent transmission from the designers to CAD

experts;

2. number of hours needed for prototyping new physical

mock-ups when validation fails and for re-engineering

the product model. The design process generally needs

both raw prototypes to define the product aesthetic

shape and well-finished prototypes (with real colors,materials, additional functions, etc.) for final decision-

making activities. If validation fails, the number of

physical prototypes increases. The number of hours for

prototyping demonstrates the time to market increase

or decrease;

3. hours for CAD model modifications after product

engineering. During the engineering development, the

product digital model is generally modified according

to structural, technical, manufacturing requirements.

Modifications are carried out by CAD experts with the

support of the whole technical staff. If shape variations

are not coherent with the design intent, product

validation fails. This metric measures how coherent

is the final product shape with the initial designers

intentions;

In order to measure the adopted metrics, the RE and RS

are performed both with the traditional and the proposed

method (Table 3).

By analyzing the first validation results we can infer the

following considerations:

CAD modeling time increases due to the addition of

more steps such as the recovery of the sketches, the

extraction of the interpretative schemata and the styling

curves and the analysis of the creative design process,

method application eliminates the need to construct

additional physical prototypes when validation is not

successful

design iterations are reduced.

Globally the new process allowed saving nearby 30% of

time for restyling, and 40% for RS.

Fig. 9 The BIBI restyling: from the interpretation of sketches and physical prototypes of the existing product to the generation of new design

concepts, to the extraction of the aesthetic features and the redesign of the new office chair

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9 Conclusions

The proposed method uses sketches analysis from a cog-

nitive and a semiotic standpoint for interpreting the

creative design process and the outcomes of the synthesis

activity in order to extract aesthetic features from 3Ddatasets. The method improves the performance of two

different critical design situations: the transformation of the

product physical mock-up into a CAD model by applying

RE techniques and the RS of existing products when new

additional requirements emerge.

The main research objective is to guarantee the preser-

vation of the design intent from the first conceptual design

phases to the following engineering developments. The

design intent is formalized in terms of styling curves and

CAD modeling strategies. The hand-made sketches evo-

lution represents the starting point for aesthetic features

identification. A detailed analysis of design models and of

sketches role in developing the product design concept is

carried out in order to theoretically establish solid basis for

the proposed method.

Different application examples in the industrial design

field have been described. They have been used to dem-

onstrate the approach applicability in case of free-form

shape products. The preliminary validation has shown

positive results in terms of increased timesaving,

improvements of models surfaces quality, and of the

accuracy and flexibility of the obtained CAD models,

coherently with the design intent.

On the other hand, the approach needs of further

methodological and technical improvements. From the

methodological point of view, further research work will be

concentrated on: a better and robust link between somedescriptive models and aesthetic features, between 3D

skeleton attributes and CAD modeling strategies, the study

of further methods for semi-automatically identifying aes-

thetic features from sketches analysis, rationalizing the

rules to develop algorithms useful for the software imple-

mentation into a commercial design system, and, finally,

making an extended protocol analysis to validate, and

eventually improve the CAD modeling strategies deduced

by sketches analysis.

From the technical side it is necessary to study and

develop the integration of the procedure within a CAD

system in order to support CAD operators with a tool for

automatic sketch analysis, for aesthetic features recognition

and, finally, for automating the right modeling strategies.

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Table 3 The results of the validation phase

Surfaces

reconstruction

or CAD

modeling time

Time for physical

prototyping for the

final approval (number

of physical mock-ups)

Time for surface

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the new mock-ups

Validation

time (h)

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Armchair Intervista

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prototype)

1.25 3 32.25

RE/RS performed by the

proposed method

9 5 (1) 0 0.5 1 15.5

Mouse prototype

Traditional RE process 6 4 (2) 12 (6 h for each

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1 5 28

RE performed by the

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Telephone prototype

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RE performed by the

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8 0 0 0.15 0.5 8.65

BIBI chair

Traditional RS process 24 12 (2) 12 (6 h for each

prototype)

2 5 57

RS performed by the proposed

method

20 6 (1) 6 1 2 35

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