68

Abstract

Digital technologies changed the design strategies. The new representation tools push to different relations between reality and its models, changing the meaning of original and copy, then the concept of reproduction. A case study stresses the concepts and meanings that express the relationships among model, copy and representation, then the ambiguity between image and shape. The experience concerns a reproduction of a wooden statue of a puppeteer, a monu-ment on the artist’s grave, which was replaced by a copy to allow the preservation of the restored memorial, now in the artist’s museum. The preservation’s needs impose a change in the material, therefore bronze is rather to other matter. The copying fusion in a mould, obtained by digital acquisition of three-dimensional digital laser scanner, stressed several problems, related to the material. Indeed there isn’t any technical problem, but a cultural one. The reverse modelling after 3D scanning could generate a mould with 3D printers. It is a routine process that gives exactly the same texture of the model, that is those of the original stuff, with the spurs left by the manual tools of sculpture. The high-pressure metal casting in silicon mould reprodu-ces whatever texture, but if you change the material, what do you have to reply at the working texture from the original one? Our feeling would perceive the copy as a false. To avoid this it must reshape the surface.

Key wordsRendering, Textures, Virtual Model, Digital Prototyping, 3D Printing

VIRTUAL IMAGES, MODELS AND COPIES: PROTOTYPING TEXTURES

Michela ROssI, Giorgio bURATTI

Department of Design, school of Design, Politecnico di Milano, Italy

E-mail: michela.rossi@polimi.it, giorgio.buratti@polimi.it

69

1. About copies

The preservation of cultural heritage includes a set of interventions that maintain or restore the distinctive characteristics of the asset. These affect the consistency, the functionality and the image with steps depending on the state of conservation and on the compomission suffered as a result of time and use. The complex relationship of cause and effect can be traced back to three essential elements:- the ravages of time on the degradation of stuff, which is visible on the surface,- the maintenance neglect,- the inadequacy of the materials with respect to use and durability.At this negative synergy time/maintenance meet all the actions that help to stop the process of materials decay, which can eventually determine the need for re-placement and/or reconstruction, due to the loss of original parts. This occurrence involves choices of method, involving enlarged cultural conditioning, because in the process of transformation (related to the time) settles the history of the artwork, which involves as well intangible assets, such as memory. The latter is the essential functions of any monument, which is defined as “concrete evidence and durable in honor or memory of people or events” (Devoto Oli). Once lost, they do have no longer any meaning. The conservation of a commemorating monument balances the “recall” by the arti-fact and its “history”, which also implies the concept of evolution, meaning change and transformation. The maintenance of the functionality that justifies the monu-ment, becomes the determining factor in relation to the dichotomy transformation/conservation. Actually, an artifact without use loses its own meaning and it becomes a memorial of something that no longer exists, then it dies. Therefore when the de-gradation exceeds the stuff limits, the replacement may be the only way to ensure the functionality, ie the maintenance of the original objective.In the case of the architecture monuments, the substitution is always partial and it may be distributed in time, sedimenting itself in the diachronic transformation. Con-versely, in the case of of smaller artifacts, it is the only alternative to the loss of the thing and its memory. Therefore the preservation provides for:- the conservative removal and the replacement of the original object with a copy.- the restoration with the gaps makeover and conservative carriage in a protected environment.Significant is the fact that the memorials, precisely for this reason, are made by du-rable stuffs such as the molten metal or the stone. When the material succumbs to the ravages of time, the problem how to arrange the

Fig. 1Original wooden sculpture, after the restauration, the digital copy, a rendering of the further bronze fusion

MO

DE

L |

D

Es

CR

IPTIO

N

70

copy that will replac the original, which involves the relationships between the model and the copy.There are famous cases, such as Michelangelo’s David: the copy exposed instead of the original in Accademia replicates in all respects the model and its features. Eventually it is identical in material such as in workmanship and it does not change al all the image of the monument and the environmental image. The copy, despite being, it is not percei-ved as a false and the result is valued as if it were true. What happens when you need to reproduce the original in a different material? The “copy” becomes a “false” or it needs to reinterpretate the model.A case is the example presented by the reproduction of a wooden memorial to be repla-ced in the Evangelic graveyard of the monumental cemetery in Parma.The pine sculpture standing on the tomb of the master puppeteer Giordano Ferrari repro-duces the appearance of the character Bargnocla (bump), the puppet that made famous this artist/craftsman who is the leading exponent of a puppeteer family, whose collection forms the heart of a small town museum, which is an international reference for scholars of popular theater. The puppet has an eye-catching bump on the brow, with the shape of the ham bone, just a craving for ham. The figure became very popular since its birth a hundred years ago (1914) and it is locally known and beloved not only by children. Some years ago, the City Council dedicated to its father the small museum named Castello dei Burattini that houses the rich collection donated by the family. The son created the statue just after the father’s death, reproducing the character that made him famous, in the common wood of the art. The material was consistent with the evangelical cemetery, where the graves are much more sober than elsewhere, and also with the art of the deceased, who worked the puppets by himself. The poor durability of soft wood of the statue, exposed for years to the weather without any protection, would have quickly led to the loss of the memorial, which was the only one in the evangelical cemetery in Parma. Only the museal preservation of the restored object would guaran-tee its survival, compromised wood pests. The Castello dei Burattini was meant as the natural place of the original artifact. With the agreement of the heirs, the City Council approved the conservative removal and the replacement in situ with a bronze copy. The original sculpture is made on the diagonal of a wood beam, it is about 100 cm high, and has a triangular base of 40 x 30 cm. The material is worked with skill and expressive efficiency, by big tip tools and the wooden texture shows the obvious sign of the carving. Before repairs, the surface showed evident drying cracks and other evidence of the stuff deterioration. The work required the reconstruction of some parts, such as the bump on the front of the puppet, which was lost, and the filling of the rotted center with resins.

2. The reply

The replacement of the removed memorial posed a principle and methodological pro-blem about the identity of the copy, which stresses the need to ponder on the concepts of original, model, prototype and reproduction. Therefore the consistent implementation of the copy, which is a real object, started with the ambiguous relationship between the digital reproduction and the virtual representation of the model.The preservation of the original required a copy in different stuff, using bronze instead of wood. To avoid the risk of a “false” the copy should be different from the model, even

71

if it will replace the first. To maintain the memory of the previous image brought to rethink the surface texture, which carries the traces of the working tools of the carving onto the typical veining of the wood. Virtual rendering disclosed the final image.The first necessary step was the digital survey. The main problem in the scan of sculptures is represented by undercuts, which are no longer a problem for the mo-dern techniques of high-pressure fusion. In the puppet the undercuts are often not easily accessible to the eye and the to hand and it was possible a digital retouching, directly on the model. From a technical point of view, the copy could be made iden-tical in the colors and/or in the original wooden texture, printing the model with ABS resin or milling a block with a machine for digital prototyping, no matter the stuff. The idea of performing a similar operation in a significant historic place, where any action is subject to the approval of the Monuments Superintendence would be a pro-vocation that was not even considered, but anyway this hypothesis has a theoretical value: the resin would allow a indistinguishable copy of the original, but because of the stuff would have been perceived as an insult to the place and to the memory. The people conceive the memorials of bronze or marble. Eventually in the XXth century graveyards the concrete is a widespread cheaper alternative, but a famous artistic foundry is sponsoring the operation and the a priori solution was the lost-wax casting with bronze, ensuring the uniqueness of the copy and the economic value of the sub-stitutive memorial. Among the traditional materials of funerary furniture, the bronze is undoubtedly the most durable and the high-pressure lost-wax casting in silicon mold allows the utmost tactile fidelity, ensuring quality and value to the product. Fusion requires a refractory mold and the cast of the original becomes the model, then the bronze copy of the real sculpture is the original monument. Starting objects are made with poorer stuffs easy to model, usually clay or wax, which shape the mold that print the final product. The mold may transfer to a different material the machining signs, keeping a memory of the original material in the fusion. The copying process is analogous, but If you change the material, it is logical to maintain such signs that are the expression of materials and their working? The copy shoud carry only the shape or even the tactile materiality of the surface texture, which is the very consequence of the material?The reproduction of the wood with a very different material, such as metal, needs to resolve the different image of the copy with a texture that recalls the original features avoiding the creation of a false. The bronze copy of the wooden original becomes a new original and the consistency to the material imposes a own surface finish: the copy is reinvented as a new original that derives from the removed one, becoming a quotation.The survey/casting methods depend on the copy material and the copying technique. In this case the bad conditions of the statue did not allow the cast with traditional techniques without an objective risk of irreversible damage to the original, sugge-sting the use of a digital survey with laser scanner, prior to simulate the final image with a render. The survey provides the main methodological and operational support as a discipline of knowledge that provides the foundation of the design choices for the reinterpretation of the original. The virtual representation show the new image on the digital model that allows thel prototyping, allowing the comparison of diffe-rent solutions in the color, which is a result of a chemical or heat treatment, and in

MO

DE

L |

D

Es

CR

IPTIO

N

72

the surface processing. The copying process builds on the survey aproach that becomes reinterpretation, while the digital restitution became a design project.The whole process shows two main phases:

1) – the copying: from the original to the model- Digital survey of the original (laser scanning)- Reconstruction of the vitrual model- Reworking of the virtual texture 2) – the prototyping: from the model to the new original- Creation of the mold from the digital model- Checking the textures- Prototyping the copy with poor working to cast the fusion mold.

The survey was carried out with the well-established technique of reverse modeling, using the scanner Faro, which offers a photographic image of the point-cloud, po-tentially more effective to render the virtual image. After the correction of the survey “noise” by the thinning of the redundant points, the virtual model allowed the com-parison of several hypotheses for different surface treatment, to be verified with the prototyping of fragments from the mesh model. The digital techniques offer different copy solutions, but it does not solve the basic problem of the choice of material, which is an essential character of the sculpture. This tactile art par excellence is conditioned from the manufacture and the materiali-ty of the surface that means a different working by carving or modeling. The choice of the finishing texture can be aided by the virtual rendering of the model, an then through the digital processing of the virtual cast from which one can realize the mold according to the indications specialized company.The research highlightes the limits of the virtual model and it stresses the importance of prototyping a mock-up. The whole process demontrates that the virtual image of the model is only a projective artifice on tte monitor display, which does not solve the solution of the tactile aspects of the real surfaces of the wor(texture). Actually these are the plastic result of the surface finishing, which can be generated and con-trolled digitally, only as a result of a 3D molding proofed on the mock-up.The issues raised by the search for a virtual view of the uneven surface, with no evidence of repetitive pattern that characterizes the renders, suggested to investigate

Fig. 2Three-dimensional digital texture and their finishing details:1 and 4 are generated with geometric approach; 2 and 3 using Image Sampler tool

Fig. 3The algorithmic definition, able to generate a generic surface’s texture, requires a correct identification of the geometrical problem. Any curve or surface generated in Rhino, and then in Grasshopper, constitutes a NURBS geometry type, that means described with a single equation that relates degree, control points and knots. These items allow you to maintain the accuracy of any shape. Modification of one of these parameters produces a local deformation linked to the representation of u and v. The geometric locus where u and v are constants is called isocurve or isoparametric curve. Surface textures obtained arise from identification of a series of isocurves on a surface subsequently modeled according to a sinusoidal pattern. Sine’s amplitude and vertices follow a random pattern, because the texture considered have no ordered but chaotic patterns. Local deformations, such as planar areas, are obtained by the use of attractor points, i.e. elements that can alter, in a specific range of action, the behavior of surrounding geometric objects.The algorithm measures the distance between point and perimeter of influence zone, and when the result is less than the established value, the vertices of the sinusoids are scaled according to a scale value proportional to point attractor’s distance.

73

the possibility of applying a generative software (Grasshopper) to model a “real” texture with irregular surface for the creation of the fingerprint of the mold, avoiding meanwhile the pattern-effect which results from the projective rendering.

3. The prototyping

The problems related to the irregular surface simulation, avoiding repetitive pattern that usually characterizes the rendering, suggested to investigate the use of a genera-tive software (Grasshopper) to shape the virtual model’s texture surfaces to be used M

OD

EL |

D

Es

CR

IPTIO

N

74

for the creation of the mold.The usual surfaces mapping allow to view any type of result on a smooth surface, anticipating the final finish effect , without a correspondence between the image and the texture of the real surface. A thorough analysis of any natural or artificial surface reveals how very complex structures exhibit ordered rules, though the identification of the same may be diffi-cult because they often do not depend on a single event. For example, the interaction between artifact’s geometry and chosen material characteristics influence and chan-ge the texture’s behavior, causing a continuous differentiation of the original pattern. In this specific case we have created algorithms able to impose formal laws who rule the material to reproduce the sensory and expressive surface qualities. The object’s surface finish design is attributable to the study of complex systems. Although there is no universally accepted definition, and though there are several proposals in lite-rature, these systems exhibit distinctive characteristics such as:

-¬ reiteration of a large number of items- appearance of emergent properties at the global level- self-organization properties

Parametric software allows the ability to work with complex objects identifying a series of steps which break down the problem in a limited number of simple le-vels. The basic design elements are interconnected algorithms that at every level influence each other generating other elements at a higher level. Step by step the whole geometry is created. Memorial’s textures were all made using Grasshopper’s potential, applying

two alternative procedures based on different concepts:

- geometric generation, based on geometric primitives analysis of the texture (sines and cosines in the cases studied) and behavior of created pattern that is modified according to used surface’s characteristics.

Fig. 4The image sampler tool associates a input to each point of a grid, a numerical value computed in accordance with an image file. Height of the generated surface’s vertices is related to pixels color coordinates of the graphic image file: higher will be the numerical value of the coordinates or the palette index, greater will be the height’s dimension. The mesh is then created directly corresponding to image file’s pixel. The outcome depends mainly on two parameters: image resolution, which influences the texture’s morphology on the x and z axes, and the definition of chromaticity coordinates, that influence elevation. In the examples we used channel Brightness, which distinguishes color changes from white to black, associating value 1 to white, to black value 0 and to intermediate gray tones the open interval of real numbers. Higher is the resolution of the graphic file, denser (and accurate) the obtained surface’s mesh

75

- Image Sampler generation, a tool capable of converting an image’s color infor-mation into numerical data, with which it’s possible to obtain a three-dimensional digital equivalent of a two-dimensional texture, and modify it to the task.

The Image Sampler use enables a fast process but does not automatically solve the problem of seams between the different patch, unless you change the original two-dimensional image, effectively hiding the seams. Generation by geometric primiti-ves requires longer time in speculation and analysis, but originates more efficient three-dimensional adaptive textures.

The benefit connected to realization of three-dimensional digital textures is the pos-sibility to inform directly the production systems so as to achieve a short time fi-le’s translation in a actual object. It to be shaped for removing material by a CNC machine, or by a material deposition system typical of a three-dimensional printer, the same finish is obtainable in hundreds of formal variations. The model created, however complex, is an algorithm’s result. It’s therefore possible to alter the genera-

Fig. 5Subdivision of the model, optimization for 3d printer

MO

DE

L |

D

Es

CR

IPTIO

N

76

tive input modifying geometry and obtaining, from the same relationship structure, required corrections. If to display and render a digital model it isn’t required special measures in case of prototyping is necessary to carefully plan the transition from the virtual model to physical object. Artifact must be relatively sturdy, with a stable base and monolithic. In this case, the object exceeds in size the printing- machine possibilities, therefore the plaster copy destined to the creation of the casting mold can only be realized assembling different portions of the model, each of which must respect precise con-ditions:- to be defined by a closed shape, all surfaces that compose the body must match each other, without cracks or interpenetration- must withstand the mechanical stresses related to printing operations, elimination of powder residue and so on.

The resulting model will facilitate subsequent manuals operations executed by a sculptor specialized in funeral art that ennoble the melting surface of the “copy” with his artistic hand. The trial posed by the problems encountered during small mo-nument’s reprocessing has transformed a modest work for third parties in a applied research subject to further interesting developments. Although with current tools digital processing requires a lot of RAM memory and high-performance computers, combining generative software and new methods of digital production pave the way to new formal experiments and several application scenarios arising from the ambi-guous relationship between reality and its image, pointing out that the representation is an active part of creative process correlated to the project.

77

List of References

[1] BURATTI G, Superfici complesse e trasformazioni parametriche: Algori-thmic Aided Design in Il disegno come ricerca. Strumenti grafici e modelli rappre-sentativi per il progetto, Maggioli Editore, Milano 2012.[2] BURATTI G. Generative Algorithms and associative modelling to design articulate surfaces in M. Rossi, edited by, Nexus Ph.D. Day Relationships between Architecture and Mathematics. Milano 2012, McGraw-Hill.[3] DEL CURTO, FIORANI E., C. PASSARO La pelle del design, progettare la sensorialità. Milano, Lupetti, 2010.[4] FRAZER J.H, Design Thinking: Creativity in three dimension, http://www.ellipsis.com.[5] ISSA R. Essential Mathematics for Computational Design, Robert Mcnell&Associate, 2009. [6] IWAMOTO L., Digital fabrication: Architectural and material techniques, Princeton, Princeton Architectural Press.[7] LORENZ N., The essence of Chaos, Washington: University of Washing-ton Press, 1996.[8] MEREDITH M., From Control to design: parametric Algorithmic Archi-tecture. Actar, 2008.[9] MIGLIARI R., Geometria descrittiva vol II, tecniche e applicazioni,Novara, Cittàstudi edizioni, 2009.[10] POTTMAN H., Asperl A., Architectural geometry. Exton, Bentley Institu-te Press, 2007.[11] SEMPER G., Der Stil in den technischen und tektonischen Künsten, oder praktische Aesthetik, Münich: 1878.[12] TEDESCHI A., Architettura Parametrica. Introduzione a Grasshopper. Potenza, edizioni Le Penseur, 2011.

MO

DE

L |

D

Es

CR

IPTIO

N