SPLINE AND IDEAL: FROM REAL TO VIRTUAL SCULPTURES, AND BACK

Transcription

1 SPLINE AND IDEAL: FROM REAL TO VIRTUAL SCULPTURES, AND BACK Eric Bittar 1, Olivier Nocent 1 and Anne Heff 2 1 CReSTIC-LERI, EA University of Reims Champagne-Ardenne, France {Eric.Bittar Olivier.Nocent}@univ-reims.fr ; 2 Barberey-Saint-Sulpice, Aube, France. Abstract: Key words: We present an original approach of virtual sculpting incarnated in a unified space-time interactive modeler named Splimo (for Spline Modeler), which is specifically designed for the creation of tubular spline works. Oriented towards artistic creation, it allows the realization of virtual transcriptions of real art pieces, as well as the creation of animated virtual sculptures, presented in an exhibition mode. Drawing curves in the virtual world is a way to be free of the constraints of the real sculptures, that s why we call it ideal. Splimo may however also be used as a virtual prototyping tool to experiment new shapes to be later designed in the real world. virtual sculpture, spline, interaction, computer art, virtual prototyping 1. INTRODUCTION The work presented in this paper results from the meeting of a sculptor, Anne Heff and two computer graphics researchers, around the theme of curves. This theme is central in the work of Anne Heff in the real world, under different forms. She started with 2D ink drawings of strophoïds. A strophoïd is a cubic curve of equation x(x 2 + y 2 ) = a(y 2 - x 2 ) which draws a loop. The artist then explored different technical approaches, always making loops, from knitting to ropes, from ropes to plastic drains, and from plastic to metal. She then shifted to monumental works in her recent exploration. She works for different institutions: libraries and media libraries, municipalities and art centers, in France and in Canada.

2 2 Eric Bittar, Olivier Nocent and Anne Heff The difficulty to create directly monumental works leads to the need of sketches, drafts and models. The virtual model is useful to allow the sculptor to create shapes in a free way, in an ideal world, without the constraints of the real world. It becomes also possible to build a virtual presentation of the model in its future location by using pictures of the place. It is for example the case of sculptures designed to be presented on buildings like city halls. With the spline curves, modeling becomes straightforward by hiding the mathematical complexity of objects representation. Furthermore, we constructed the modeler Splimo in a way that unifies space and time dimensions. Our implementation allows direct and real time interaction, and the sculptor draws easily geometrical curves as well as time trajectories. We will now present our choices compared with existing virtual sculpting approaches in Section 2, before detailing the functionalities of Splimo in Section 3, and showing results in Section VIRTUAL SCULPTING Some approaches to virtual sculpting emphasize immersion systems with 3D input devices [14], or even use specific devices like the ShapeTape [6]. We have made the choice of developing a software that runs on everybody s computer, with common devices. The main point is to allow interactive creation and modification of the data, whatever the choice of the structuring primitives: volumes [5], surfaces [1] [2] [12] or generalized cylinders and curves [6] [13] [14]. For us the choice of the latter was straightforward considering the kind of sculptures we wanted to achieve. Deformation is done with specific tools: handle and volume of attraction [1], 3D shaped tools like ellipsoids [5], direct manipulation of free-form deformations [7], forces [4] [12], direct curve drawing and editing. We chose to edit the curves both directly and with their control points and we will detail this choice in the next section. 3. SPLIMO The core of Splimo is based on Catmull-Rom splines [3], which are cubic interpolating curves. This choice was made according to previous works dealing with spline like objects animation for knitwear simulation [10] [11]. In these articles, we consider a spline curve as a succession of ns curve segments defined as functional combinations of a common set of n timedependent 3D control points p () t weighted by a set of parametric i functions

3 Spline and Ideal: from real to virtual sculptures, and back 3 j bi ( ω ). The position p of a point belonging to the segment j, of parametric coordinate ω at time t, is given by: n j i i= 1 i [ ] p( j, ω,) t = b ( ω) p () t 1 j ns, ω 0,1 The sculptor may therefore easily and intuitively draw a curve and modify it via its control points. We added the possibilities to displace at one time one or more curve segments to speed-up the deformation of a set of curves, and the possibility to add/delete a control point on any curve. As stated in [6] existing work on the design of 3D curve editing tools have indicated that to ensure accuracy it is preferable for 3D curves to be created and manipulated in 2D orthographic views of the 3D scene. We provide the sculptor with four views. Three of them are orthographic, and the fourth is a perspective view (see Figure 1). In this view the control points may also be edited: they move on virtual horizontal planes. Figure 1. Splimo user interface 3.1 The core: spatial splines The splines are used to draw generalized cylinders. All their parameters may be edited: number and location of the control points, radii and stiffness along the curve, material properties and texture. At any point, a new branch may be created. Two non-neighbour points may be merged, whatever their locations are: at an extremity or in the middle of curves.

4 4 Eric Bittar, Olivier Nocent and Anne Heff 3.2 Animation: spline trajectories Figure 2. Left: top view with camera path. Right: Camera view at the end of the animation A curve may also be chosen as trajectory for lights and cameras, and the speed on the path may be tuned on each segment. The sculpture is then shown in a special mode called exhibition mode. The exhibition mode allows the visitor to see the sculpture under different points of views, with two options: by selecting interactively the observation angles or by following the trajectory designed by the sculptor (Figure 2). This latter is more powerful than the presentation mode the commercial NURBS modeler Rhinoceros [15]. If we refer to the typology of presentations in three groups of time-spatial works-of-art [8], the first option corresponds to the 1 st group : the sculpture remains still. As the 2 nd group is made of works the elements of which can be moved by spectators, it doesn t correspond to any of our two options. The second option is closer to the 3 rd group, which includes the animated works, with the difference that here, only the camera is animated, and the sculpture is still. So we may consider that the sculpture and its presentation form together the work-of-art. 4. FROM REAL TO VIRTUAL SCULPTURES Let s now show as examples the virtual transcriptions of two sculptures. The Fork (Fourchette) is made of two plastic drains of different radii (Figure 3). The virtual Heart (Coeur) is shown as well as the picture of the real sculpture loaded in Splimo to help the design (Figure 4). But Splimo is more than a tool dedicated to static transcriptions of real sculptures: it integrates time-dependency and interactivity. Theses features offer many new ways of creation for an artist like Anne Heff. She is now able to think at her sculptures as evolving entities by using animated control points. Moreover, in the exhibition mode, the user (the artist or even a visitor) is able to grab any control point in order to deform the virtual sculpture.

5 Spline and Ideal: from real to virtual sculptures, and back 5 Figure 3. Fork Sculptures: real and virtual one Figure 4. Heart Sculptures: virtual and real one. The picture of the real sculpture is loaded in Splimo to help the design of the virtual one.

6 6 Eric Bittar, Olivier Nocent and Anne Heff 5. CONCLUSION AND PERSPECTIVES We have presented an original work which is the result of the meeting of computer graphics researchers with a sculptor. Splimo is an interactive easyto-use modeler designed specifically to build virtual sculptures made up with tubular splines. It is a tool suited for elaborating sculptures in the ideal virtual world. These works may be transcriptions of real sculptures, or virtual creations. A specific exhibition mode allows the animated presentation of the works, where the camera follows a virtual path. 5.1 Back to real sculptures Splimo may however also be used as a virtual prototyping tool to experiment new shapes to be later designed in the real world. It will help enrich the real world with sculptures exposed in or on private or civic buildings. As mentioned before, we also proposed a physically-based spline model for simulation [10][11] called Dynamic Material Spline. We intend now to incorporate mechanical aspects of DMS like strength of materials, length, volume and weight computation into Splimo, to enhance the correspondence between the real and the ideal worlds. Even if we are now dealing with physically-based continuous deformation energy for knitwear simulation [9], we have to improve our model to make it run in a real-time framework like our spline modeler. ACKNOWLEDGEMENT The authors whish to thank all the persons who applied their skill and imagination to the making of the software : the main programmer, Irénée Caroulle ; the prj04 team, Sylvain Deprez, Benoit Gambier, Erwan Guehenneux, Remy Lesiak, Benoit Prevost ; and Mickaël Thierry, who fought tirelessly against shifty bugs.

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