2D semantic sketcher for car aesthetic design

2D semantic sketcher for car aesthetic design Vincent Cheutet LISMMA 3, rue Fernand Hainaut 93407 St Ouen Cedex France [email protected] RÉSUMÉ.

Cet article présent une première étape dans l’élaboration d’un environnement de modélisation sémantique pour les premières étapes de la conception d’une voiture. Jusqu’à présent, la génération et la manipulation de forme en conception esthétique se basent toujours sur des outils géométriques de bas niveau qui proviennent du paradigme classique de la CAO (Conception Assistée par Ordinateur) D’un autre côté, la conception d’un produit est basée principalement sur la créativité des stylistes et sur des contraintes de haut niveau. Par conséquent, un tel environnement se base d’abord sur une structuration de la sémantique présente dans les premières esquisses 2D du produit. Pour manipuler ces esquisses, une grammaire de processus est intégrée pour décrire et manipuler les courbes. Enfin, cette grammaire a été implémentée grâce à un moteur de déformation adapté aux courbes NURBS. ABSTRACT. This paper presents a first step in the elaboration of a semantic-based modelling environment addressed to the conceptual design phase of a car. Up to now shape generation and manipulation in aesthetic design are still based on low-level geometric tools that come from the classical CAD (Computer-Aided Design) paradigm. On the other hand, the design of a product is mainly driven by designers’ creativity and high-level constraints. As a consequence, such environment is based first on a structuration of the semantics embedded in the first 2D sketches of the car. To manipulate such sketches, a process grammar has been integrated to describe and manipulate the curves. Finally this grammar has been implemented through the use of a deformation engine adapted to NURBS curves.

: conception esthétique, modélisation sémantique, grammaire de forme, esquisses 2D, opérateurs de déformation de courbes.

MOTS-CLÉS

KEYWORDS:

Aesthetic design, semantic-based modelling, shape grammar, 2D sketches, deformation operators for curves.

CPI2007 – Rabat, Maroc

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1. Introduction Styling activity is crucial in the conceptual design phase of an industrial product, where stylists express the product character through its shape, i.e. the feeling that the designer wants to give to the product, even if the product is not completely defined and the generation of a first model is mainly driven by designers’ creativity and highlevel constraints. Such requirements come usually from the so-called product briefing and from interviews of consumers. In fact, nowadays the importance of involving customers in the product development process by eliciting and integrating customers’ needs and feelings into product design is widely recognized [NAG89, NAG95]. Since the actors of the styling activity are stylists and do not have deep knowledge about geometric modelling and the use of CAD systems, it is important to find solutions to describe and manipulate a shape according to their know-how and their ways of works. The availability of powerful and flexible knowledge technologies has brought large benefits to the CAD (Computer-Aided Design) paradigm, allowing for the development of product modelling functionality able to incorporate prior knowledge into the product model, thus integrating all the information produced throughout different design phases. Unfortunately, currently available knowledge-based systems focus mainly on the functional elements of the design and do not support the management of the aesthetic knowledge; as a consequence, the industrial design process does not fully benefit of the technological progresses. In the car industry, the typical workflow [CAT04] begins with a first description of the proposed product solutions through sketches, very often hand-drawn. These sketches are the first materialisation of the mental representation of the product and so express the designer’s idea, and therefore implicitly contain the semantics of the context (figure 1). These sketches support also the dialog with the others actors of the product development process. Once one sketch is selected, it is scanned and converted into a digital format, often by a CAS (Computer-Aided Styling) technician that does not have any competencies in styling. The main objective of this step is to create a digital model that renders the impressions and the emotions produced by the hand-drawn sketches. Then, the leading curves drawn by the designer in the sketch are used as a framework on which the different surfaces are built up. It is not generally an easy task, since it requires several iterations before obtaining the shape desired by the stylist. This can be explained by: -

the lack of flexibility of the current tools, that consider the scanned geometric entities as constraints to precisely satisfy, without taking into account the uncertainties due to the considered phase of the product development process,

2D semantic sketcher for car aesthetic design

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-

the lack of tools that propose high-level manipulations of the surfaces. The shape modification is still realised through the low-level modification of the underlying geometric entities, like the modification of the control points of a NURBS surface. Some tools propose some surface deformation functions, but their use is still difficult and limited,

-

the difficulty to correctly transcribe the constraints and the objectives expressed by a stylist, since the mental perception of a shape by a stylist does not guarantee that it can be modelled in a 3D digital environment, without needing compromises allowing an acceptable digital model, i.e. with a smooth curvature distribution or satisfying dimensions of the product…

Figure 1. A car sketch (courtesy of Chevrolet)

The resulting 3D digital model is the input for successive design stages (e.g. manufacturing, assembly, simulation). Unfortunately, in this conversion from paper to digital model, the leading semantics is lost. Moreover, tools and methodologies that take into account stylists’ ways of working are lacking, and therefore the creation and manipulation of the digital model are time-consuming and cumbersome. Structuring the semantics expressed at the different steps and models of the conceptual design phase provides the basis to the development of more suitable modelling systems able to manage and use the involved knowledge. The paper is structured as follows: section 2 presents the formalisation of the knowledge related to the aesthetic elements embedded in 2D product sketches, whilst section 3 focuses on the curve manipulation component of the ontology. Section 4 proposes a set of aesthetic operators and section 5 introduces the partial implementation of the L-system through a shape deformation engine. Section 6 closes the paper with concluding remarks and a brief outline of the future steps.

2. Aesthetic design for Even if the final result of the design process is the complete and detailed definition of the surfaces defining the final shape of the product, the character

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evaluation and modification is performed by concentrating on specific curves of the object [CAT04], such as sections and reflection lines, which are normally judged in a planar view (paper or CAD screen). With the term treatment, stylists refer mainly to those curves contributing to express the global character of a car (figure 2). Such curves can be both real and virtual curves: in fact, they can be part of the contour but also lines related to the smoothness of the surface, as all the light lines.

Windshield line Hood line Crown line

Roof line Drip line Waist line

Accent line Character line

Wheel arch

Wheelbase line

Figure 2. Examples of car aesthetic key lines on a C4 (courtesy of Citroen).

In order to formalise the aesthetic context of a car design, a taxonomy of the Aesthetic Key Lines (AKLs) candidate to elicit some emotions and their aesthetic properties has been incorporated inside the ontology, where the Aesthetic Properties (APs) correspond to some terms used by designers for expressing desired curve shape modification [GIA04]. However, single curves are not usually enough to express a character and it is more common that some specific properties of specific curves together with special relationships among such curves define the predominant character of a car. In the figure 2, some of the AKLs belonging to the profile are depicted; in particular, the roofline, the windshield line and the wheelbase line are the fundamental ones. In fact, the roofline and the wheelbase both identify the packaging and start to suggest the style; while the windshield line contributes to the definition of the aesthetics and aerodynamics of the vehicle. In order of importance, the waistline and the accent line follow: the first is the boundary curve between the auto body and the glass of the windows; the second is usually a virtual line which describes the reflection of the light on the surface near the waist. The drip line and the hood line usually enforce the aesthetic behaviour of the roof line, while other character lines, both real and virtual curves, may be present to further emphasise the general style of the car. Finally, the wheel arch has a strong impact on the stability and compactness feeling of the car.


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Once defined the taxonomy of the AKLs, the Aesthetic Properties (APs) of such lines, which apply to AKLS, have to be defined. They are naturally related to the geometry, but in a complex way, and reflect the aesthetics of the shape. We have mainly concentrated on those properties corresponding to the terms used by stylists for expressing desired shape modification: in particular, the concepts of acceleration, softness/sharpness, tension, convexity/concavity, flatness, crown have been specified together with their measures [GIA04]. In this paper, we will treat only three of them, whose definitions will be briefly summarised. The term softness/sharpness is used in styling to describe the type of transition between two curves or surfaces; in general, a small radius designate a sharp transition, and a large radius a soft one. The convexity (concavity, if in the opposite direction with respect to the curve normal) is linked to the similarity of a curve to the enclosing semicircle; thus, the reference convex curve is either the semicircle, an arc of circle, or, more generally, the curve presenting the lowest variation in curvature according to the given continuity constraints. Finally, tension is a less obvious concept to define. Designers commonly associate it to the "internal energy" of a curve subject to continuity constraints at its boundaries, provided it is not a straight line. In particular, increasing the tension of the curve can be geometrically translated into a special evolution of curvature along the curve, i.e. a larger part of small curvature is generated.

3. Curve manipulation through a process grammar To translate the designer’s knowledge in an adapted shape manipulation approach, we base our contribution on shape grammars [STI80]. Shape grammars have been extensively used in well-defined design domains, like architecture [CAG97], but also in product design [HSI97, SMY00, MCC04]. They provide the means to describe and manipulate shape in a concise and repeatable language because they describe a parametric geometry and establish production rules that operate on geometry to generate a family of shapes. Using such an intermediate layer between designers and the low-level geometry allows us to create a common platform for shape manipulation, where each actor of the design process has an appropriate interface, which is a mapping of his (resp. her) knowledge inside the chosen shape grammar. The process grammar developed by Leyton [LEY88] and extended by Cheutet et al. [CHE06a], dealing with 2D piecewise C2 curves, seems the most appropriate for the considered context: this grammar provides a description of the free-form curve geometry in terms of its curvature extrema and high-level operators to act on them, thus avoiding the manipulation of low-level geometric parameters. This process grammar defines a vocabulary enabling an exhaustive classification of the corresponding shapes. It is based on an intrinsic quantity, meaningful also for stylists: the curvature, and more precisely the curvature extrema and the inflexion

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points, called characteristic points (figure 3). In this way, a curve is described by the sequence of its characteristic points, called name, which form the grammatical description of a class of curves. Codons, defined as subsets of the name composed by three consecutive characteristics points, play a special role.

Figure 3. a) grammatical description of a smooth curve, b) curvature plots, c) definition of the symmetry axis

Since high capabilities for the specification of the aesthetic curves are usually required, it was necessary to extend the description and introduce quantitative properties, still related to the characteristics points. To navigate inside the shape description and access one of its elements, a formalisation of curve manipulation processes, called curve operators, has been proposed. Also the process grammar explains theoretically the sequence of operations enabling the transformation between two different curves. Among all the operators that have been defined in the grammar, the following operators have been defined to act on a C2 continuous subpart of the curve: -

six grammatical operators that modify the name of a curve, i.e. they act either on a curvature extremum or an inflection point to generate a new codon, along the direction of the symmetry axis: •

two grammatical continuations (CM- and Cm+) in which a pushing process is applied on a curvature extremum until the creation of two inflexion points (figure 4.a),


•

-

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four bifurcations (BM+, Bm+, BM- and Bm-), where the pushing process bifurcates to generate a new codon (figure 4.b and figure 4.c),

eight quantitative operators, still acting on a characteristic point without modifying the name but acting on the quantitative characteristics: •

four quantitative continuations (C*M+, C*m+, C*M- and C*m) in which a pushing process is applied on a curvature extremum along the extension of the previous symmetry axis (figure 4.d),

•

four displacement operators (DM+, Dm+, DM- and Dm-) in any direction (figure 4.e). c)

a) m+

M+

m-

M+

b) m+ 0m-0

M+

m+

m+

M + M + m +M +

m + m+ M + m+

m+

m+

e) d)

M+

M+

M+ M+ m-

M+

m+ m+

m-

M+

Figure 4. Examples of process grammar operators: a) Grammatical continuation in m+ b) Bifurcation in M+ c) Bifurcation in m+ d) Quantitative continuation in m+ and in M+ e) Curvature extremum displacement in M+.

4. Interpretation of the semantic operators into grammar operators At a higher level, the extended process grammar has been incorporated to provide a complete environment for the 2D curve manipulation in the sketch-based context for aesthetic design of cars. Furthermore it will be hidden to the stylist, in order to provide him/her only with properties and operators suitable for a car aesthetic design environment. To achieve this result, a set of relationships between the aesthetic design fragment and the curve manipulation one has been established

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previously through a translation of aesthetic properties and operators into operators of the extended process grammar. Only three aesthetic operators are presented here (figure 5), the other one are presented in [CHE06b]:

a)

b)

c)

Figure 5. a) Example of sharp operator (sharper from the left to the right) b) Curves and their curvature plot, with tension increasing top-down c) Example of convex operator (more convex from bottom to top)


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-

Sharpening operator: it acts on a curve segment connecting two areas of small curvature, and in practice, it acts on a M+ or a m-. Making a blend area sharper means increasing the prominence of a corner between the two curves. At the same time, the process cannot generate undesired undulations; in other words, it cannot insert new characteristic points in the curve. The sharpening operator is directly translated by the operator C*M+ (or C*m-, depending on the initial configuration): the curvature value of the characteristic point increases (figure 5.a).

-

Convex operator: it transforms a curve to become closer to the enclosing half-circle. The first step is to apply on the middle curve curvature extremum the displacement operator Dm+ in the direction of the perpendicular bisector of the segment connecting the curve endpoints in order to equilibrate the left and right distance (from the lower curve to the middle one in figure 5.b): in this way, we obtain a more symmetric curve. Then, the inverse of the quantitative continuation operator C*m+ is applied along the perpendicular bisector to decrease the curvature variation of the characteristic point to approach a constant curvature area (from the middle curve to the upper one, that is a semi-circle).

-

Tension operator: it can be perceived when one curvature minimum with a small curvature value lies between two curvature maxima with high curvature values. “More tension” is obtained by applying the quantitative continuation operator C*m+ to decrease the curvature value of the curvature extremum to vanish (figure 5.c).

5. Towards the complete implementation of the process grammar The process grammar does not have any parameters directly linked to a geometric representation of type NURBS and so some ‘links’ have to also be created between this representation and the process grammar. Currently, these links are realised through curve deformation processes and geometric constraints [CHE06a] that have for objective to respect the properties of the grammar processes, i.e. number of characteristic points created or deleted, and respect the curvature distribution prescribed by each operator (figure 4). According to the process grammar, the geometric constraints allow defining some intervals of the curve where the operators are applied, i.e. some position constraints are applied on the curvature extrema bounding the modified area. Moreover, tangency constraints and curvature constraints are needed to preserve the continuity of the curve between an area modified by the user and the adjacent areas, unmodified. The displacement of a curvature extrema, describing the continuation

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process at this extrema, corresponds to a position constraint at a specific point of the curve. In the same way, to obtain the behaviour of a ‘bifurcation’ of curvature extrema, a set of specific geometric constraints are needed to guide the curve deformation process. Since there is no simple relation between the parameters defining the grammar operators and the one describing a NURBS curve, the properties of the grammar operators are preserved through a real-time evaluation mechanism of the curvature distribution. The objective of this mechanism is to determine if, at each shape deformation step, the shape curvature distribution answers to the applicability conditions of the operators used by the stylist. For instance, the mechanism verifies that the deformation process associated to the continuation operator in a curvature maximum does not create any inflexion point on the curve. With the geometric constraints described before, the deformation process establish a relation between the characteristic points of the shape and the vertices of the characteristic control polyhedron of the associated curve(s) by using a mechanical model of type ‘bar network’ [CHE06a] under constraints of mechanical equilibrium and geometric constraints (figure 6). blocked nodes

prescribed displacement initial distance

final distance

prescribed displacement fixed point blocked nodes

Figure 6. Two examples of deformation operators associated to the process grammar operators [CHE06a]


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6. Conclusions and future work This paper presents a first step towards an environment for knowledge-based modelling for the conceptual design phase of a car. It also provides the basic framework of a design environment for 2D digital sketches in which semantic-based and context-aware functionalities may complete the traditional modelling tools; in this way, stylists and engineers are allowed to create and manipulate shapes more easily. Such an environment takes advantage of a geometric process grammar to handle high-level shape manipulations. This environment is decomposed in four well-defined layers (semantics, grammar, constraints for deformation, geometry) that have each a specific point of view on the shape (figure 7). This layered architecture proposes the separation between the different representations and their properties, according to either the knowledge objects they contain or their mutual relations. This structuration may be interesting for the management of the product multi-views, with the multiple shape representations, with in particular the development of specific operators allowing to link, transfer and adapt representations et information between the different actors of the product development process.

Roof line

Semantics

Hood line Accent line

M+

M+ m+

0

0

Wheel arch

m+

Character line

M+

m-

M+

Process grammar

0

Target point

mLimiting points

Deformation constraints

Control points

Geometric model

Figure 7. A layered architecture of the 2D sketcher [CHE07]

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An interface expressing the classical needs of stylists for curve manipulation, which translates their way of working into the process grammar described, is under development. Subsequently, this environment will be well suited to generate a knowledge-based manipulation of digital sketches. Only the first step of the classical design workflow has been addressed at the moment, and the future work will include the generation of semantic-based modelling tools for the other stages, according to the knowledge and skills embedded in this design area.

6. Acknowledgements I would like to thank J.C. LEON from the laboratory G-SCOP (Grenoble, France) and F. GIANNINI and B. FALCIDIENIO from the laboratory IMATI (Genoa, Italy) for their contribution on this work as PhD directors. This work is currently carried out within the scope of the AIM@SHAPE Network of Excellence [AIM] supported by the European Commission Contract IST 506766.

7. Bibliography [AIM] AIM@SHAPE: Advanced and Innovative Models And Tools for the development of Semantic-based systems for Handling, Acquiring, and Processing knowledge Embedded in multidimensional digital objects, European Network of Excellence, Key Action: 2.3.1.7 Semantic-based knowledge systems, VI Framework, URL: http://www.aim-at-shape.net. [CAG97] G. Cagdas, "A shape grammar model for designing row-houses", Design Studies, vol. 17, pp 35-51, 1997. [CAT04] C.E. Catalano, “Feature-Based Methods for Free-Form Surface Manipulation in Aesthetic Engineering”, Ph.D. thesis, Genoa University, 2004. [CHE06a] V. Cheutet, “Towards semantic modelling of free-form mechanical products”, PhD thesis INP Grenoble (France) & Universita degli studi di Genova (Italy), 2006. [CHE06b] V. Cheutet, C.E. Catalano, F. Giannini, M. Monti, B. Falcidieno, J.C. Léon, “Semantic-based environment for aesthetic design”, proceedings of TMCE conference, Ljubljana, Slovenia, 2006. [CHE07] V. Cheutet, J.C. Léon, F. Giannini, B. Falcidieno, “Vers une structuration des représentations de composants dans une vue produit : application à la création de lignes clés esthétiques”, AIP PRIMECA 07, La Plagne, France, 2007. [GIA04] F. Giannini, M. Monti, G. Podehl, “Styling Properties and Features in Computer Aided Industrial Design Applications”, Proceedings of CAD conference, 2004.


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[HSI97] S.W. Hsiao, C.H. Chan, "A semantic and shape grammar based approach for product design", Design Studies, vol. 18, pp. 275-296, 1997. [LEY88] M. Leyton, “A process-grammar for shape”, Artificial Intelligence. Vol. 34: 213-247, 1988. [MCC04] J.P. McCormack, J. Cagan, "Speaking the Buick language: capturing, understanding, and exploring brand identity with shape grammars", Design Studies, vol. 25, pp. 1-29, 2004. [NAG89] 1989.

M. Nagamachi, "Kansei Engineering", Kaibundo Publishing, Tokyo,

[NAG95] M. Nagamachi, "Kansei engineering: a new ergonomic consumer-oriented technology for product development", International Journal of Industrial Ergonomics, vol. 15 (1), pp. 13–24, 1995. [SMY00] M. Smyth, E. Edmonds, "Supporting design through the strategic use of shape grammars", Knowledge-Based Systems, vol. 13, pp. 385-393, 2000. [STI80] G. Stiny, “Introduction to Shape and Shape Grammars”, Environment and Planning, vol. B 7, pp. 343-351, 1980.