Proceedings of IDMME - Virtual Concept 2010 Bordeaux, France, October 20 – 22, 2010
Original Article
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Study on Virtual Clay Modeling System Using Refined Curves in Technological Aspects Hiroshige Kai, Hideki Aoyama Department of System Design Engineering, Keio University 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama, Kanagawa 223-8522, JAPAN Phone (81)45-566-1722 / FAX (81)45-566-1722 E-mail:
[email protected] [email protected]
Abstract: 3D CAD systems are powerful tools for constructing functional products, but they are insufficient for creating aesthetic designs like free form surfaces because they involve non-intuitive and complicated operations. In the current design process, a designer first shapes an idea by sketching, constructs a physical model made of clay, and then builds up CAD data by reverse engineering. In this research, we aim to streamline this process by utilizing virtual reality technology (VR). This paper proposes an interface system that can intuitively and simply create a clay model in virtual space by manipulating virtual hands and virtual tools comprised of VR devices. We have developed the proposed system and verified its availability. Key words: virtual clay modeling, virtual reality, reverse engineering, concept shape design, aesthetic design 1- Introduction
model from measurement data. For this reason, clay modelling is an undesirable design method in industry. With the rapid growth of virtual reality (VR) technologies, interactive 3D devices such as position tracker, data glove, force feedback device, head mounted display (HMD), and 3D glasses have developed, and these enable intuitive 3D operations in computer space, also known as virtual space. This paper proposes a virtual clay modeling system that can create clay models in virtual space by manipulating virtual hands and virtual tools derived from VR devices. It takes into consideration the function for designing car body shapes as a feature of the modeling process. It provides the functionality of intuitive design tools in clay modeling and allows the user to acquire CAD data simultaneously at the end of the modeling process without reverse engineering, thereby considerably streamlining the conventional aesthetic design process. Many related works about virtual clay modeling exist. Bordegoni et al. developed an innovative system for modeling industrial products based on haptic technology using a mechanical system [BC1] [CB1]. Dewaele et al. proposed a volumetric virtual clay model which can be sculptured by adding and removing material in real time, and can be deformed through interaction with rigid tools [DC1]. Ueda et al. developed a non-contact virtual clay modeling interface using camera images [UY1]. These works specialize in the interfaces for modeling a virtual clay model and mechanisms for a virtual clay modeling with unconstrained operability. The objective of this research is to develop a virtual clay modeling system which can form virtual clay using guide rails. The guide rails indicate key lines such as silhouette lines and characteristic lines. In this research, guide rails can be originally defined as a deflection equation and a refined curve which have non-uniform curvature change for making aesthetic form.
In order to develop products that meet customer needs timely and flexibly as customer tastes become increasing diversified and short-term, it is imperative for manufacturers to optimize product development processes and to reduce the cost and time of development. Toward this demand, 3D CAD/CAM/CAE systems help streamline the process of detailed design and production to a considerable extent, but their functions are not enough to assist basic design and aesthetic design. Conventional 3D CAD systems construct 3D shapes by uniting element configurations that are the basis of mechanical products, but they do not have the function for defining the shape of free form surfaces by sensuous operation. For this reason, in aesthetic design, designers shape an idea by sketching, construct the physical model using clay in real space, and check the designed shape with that model [Y1]. As shown in Figure 1, CAD model is constructed from positional data of discrete points measured using three-dimensional measurement instruments. Clay modeling in real space is a very efficient way of designing to designers because it can be created by easy 2- Basic Consideration in Development of Proposed Virtual Clay Modeling System handling and can be studied as a 3D shape, but the process requires considerable time, labour, and costs for creating and 2.1 –Modeling target measuring the physical model, and constructing the CAD
Paper Number
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IDMME - Virtual Concept 2010
Study on Virtual Clay Modeling System
True Sweep
Frequency of appearance
Figure 1: Design processes
Spring Steel Band
Min curvature radius Max curvature radius Inclination
Curvature radius
Figure 2: Clay modelling process
Figure 3: Distributional area and curve impressions radius
The system targeted in this research aims to design car body shapes and is equipped with the functions necessary for this. It however can be used not only for car body shapes but also for general design shapes.
2.4 – Virtual Clay Modeling System
The Virtual Clay Modeling System proposed in this research executes clay modelling by the next two functions. 2.2 –Design solution determination process (a) Function to build the virtual scraper (true-sweep) changing its shape arbitrarily. In conventional design processes, the designer constructs CAD (b) Function to build the virtual steel band scraper model from the abstraction level to concrete level through the changing its shape and setting its position arbitrarily. process from (a) to (c) as follow. The shapes of the virtual scraper and virtual steel band are (a) Realizing an idea by sketch defined by “deflection curve” and “rhythm curve” as follows. (1) Drawing thumbnail sketch from product image The deflection curve is a shape whose energy of strain is (2) Drawing rough sketch from thumbnail sketch minimum, and the rhythm curve is an aesthetic curve whose (3) Drawing rendering sketch from rough sketch condition of curvature change is constant. (b) Narrowing down design options by clay modeling (1) Creating several 1/4～1/5 clay models (scale model) 2.4.1 –Deflection curve based on sketch (2) Narrowing several scale models down to a few Equation (1) is a deflection equation for a simple support designs, creating 1/1 clay models, studying designed beam [BL1]. shape in detail, and finally deciding design solution (c) Constructing CAD model using 1/1 clay model based on M 1 d2y design solution by reverse engineering. (1) dx 2 EI 2.3 –Clay modeling process
Clay model is sculpted by a tool called “scraper”. The scraper consists of many types for various sculpturing purposes. A wide area is sculpted by the scraper in a process called “true-sweep”. As shown Figure 2, the modeler cuts and sculpts the clay by “true-sweeping” along the key-line which is settled on the target surface between two sides as the guide rail. The key-line is settled using steel band. For this reason, both the shapes of the steel band and true-sweep are the factors for deciding the surface shape.
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where E is the Young’s modulus, I is the second moment of the area that is a parameter of the bending strength of the beam, M is the moment distribution on the beam and is expressed by equation (2) as follows. M
m 2 m1 x m1 L
(2)
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Study on Virtual Clay Modeling System
Figure 4: System environment
(a)
Data-glove (b) Virtual hand Figure 5 Construction of virtual hand
where L is the length of the beam, and m 1, m2 are the bending moment on the two ends of the beam. The equation of a deflection curve (3) is derived from the solution of differential equation with boundary condition {( x, y ) = ( 0, 0 ), (L, 0 )}. y
m1 2 2 m1 m 2 L m1 m 2 3 x x x 6 EIL 2 EI 6 EI
(3)
3.2 – Specifications of devices used
2.4.2 –Rhythm curve
Harada et al. have shown that many aesthetic curves in artificial objects and natural world are curves whose relation between curvature and its frequency can be approximated by straight lines on the double logarithmic chart [H1]. The chart of this relation is called logarithmic curvature histograms, and curves having this characteristic are called rhythm curve [H1]. As shown Figure 3, the rhythm curve is obtained by determining the distribution curvature range (from minimum to maximum curvature) and inclination on the logarithmic curvature histograms. 3- Developed Virtual Clay Modeling System 3.1 – System outline
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Figure 4 shows an overview of the developed Virtual Clay Modeling basic System. The operation of this system is described as follows: STEP 1: An operator wears the data-glove equipped with a position tracker which can obtain 3D-position and direction and manipulates this system. The virtual hand is derived from the data of the position tracker and data-glove. STEP 2: The operation for the virtual clay is selected by virtual hands gesture. STEP 3: The shape of the virtual guide rail and its position in virtual space are determined from 3D-position and direction data by the virtual hand. STEP 4: The shape of the virtual scraper are determined from 3D-position and direction data by the virtual hand and it is constructed between virtual hands. STEP 5: Clay modeling is executed by manipulating the virtual scraper along the virtual guide rail and deleting virtual clay blocks according to the calculation results of interference computation between virtual scraper and virtual clay model. At this time, cutting sense is accessorily added to the operator by vibration in proportion to the cut amount with vibrator which is attached to the operator’s hands and controlled by the voltage through a DA converter. The processing results of the above are displayed on the head-mount-display. At this time, the operator can recognize the virtual world as stereoscopic vision by binocular display processing which provides two different parallactic images to the right and left display.
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The specifications of the devices used in this system are as follows. The magnetic sensor used for obtaining the data of 3D positions and orientations of the three axes is the LIBERTY produced by POLHEMUS. This sensor is used for detecting the positions and orientation of hands and viewpoints. The data-glove for obtaining data on bending angles of each finger is the 5DT Data Glove 14 Ultra produced by 5DT. This data-glove can detect a total of 14 angles; ten are the angles of the two joints of each finger, and the other four are the angles between fingers. The head-mount-display used for outputting the stereoscopic vision of virtual space to the operator is the AR-vision-3D produced by TRIVISIO. The PC used for operating this system has a Xeon E5420 2.50GHz CPU and VIDIA Quadro FX 3700 graphic board.
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Study on Virtual Clay Modeling System
three parameters (inclination, maximum curvature radius, Table1: Selected Kansei languages
Kansei languages which explain curve’s impressions
Complex Monotonous Peaceful Restless Static Dynamic Massive Lilting Weak Strong Boring Humorous Drape Vigorous Fine Rough
Input phase Kansei
Kansei
Kansei
……
The amount of the factor loadings Figure 6: Deflection curve 3.3 – Components of Virtual Clay Modeling System
The representation of virtual space is executed using graphics library OpenGL, and virtual clay is built using Zmap models.
Max curvature radius
Inclination
3.3.1 – Virtual hand
The virtual hands synchronized with actual hand motion play the role of operating the virtual scraper and guide rail. It is constructed on the basis of data from the magnetic sensor detecting the 3D positions and orientations (rotation angle of the three axes x, y, and z) and data-glove detecting finger joint angles (angles of the two joints of each finger and angles between fingers). As shown Figure 5(a), the second joint angle and the third joint angle per finger assigned to f 2 and f 3 are detected by the data-glove. As the virtual hands are built up, the joint angle f1 of the last link is derived from the angle f2 by Eq. (1) [RG1]. f1
2 3
f2
Min curvature radius
Output phase Figure 7 Neural network
Inclination
Max curvature radius
Min curvature radius
Logarithmic curvature histogram
(4)
As shown Figure 5(b), the virtual hand is formed from data on position and orientation from the magnetic sensor attached to back of the hand and data on finger joint angles from the data-glove. 3.3.2 – Shape definition of virtual scraper and virtual guide rail
In this system, the shapes of the virtual scraper and virtual guide rail are defined by the deflection curve or rhythm curve. (a) Deflection curve In equation (3), the bending moment on the ends of the beam m 1, m2 and the length of the beam L are simulated by data on position and orientation which are obtained from the magnetic sensor attached to back of the hand. A variety of impressive curves are defined by setting the strength parameter of a beam. Figure 6 shows an example of the deflection curve. (b) Rhythm curve As described in 2.4.2, the rhythm curve is derived from the Figure 8 Curve generation
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Pm 1, n Pm , n (a)straight line
(b)sigmoid function
(c)deflection curve
(d)curve generation system
Pm 1, n 1
Pm , n 1
Figure 9 Virtual clay modeling operation
Figure 10: Keyline selection
minimum curvature radius) on logarithmic curvature histograms. These three parameters can be determined by natural language [TA1]. In order to express the impression of the shape of the virtual scraper and virtual guide rail, 16 kansei wards (eight pairs) are selected as shown in Table 1. And in order to investigate the relation between kansei words as shown in Table 1 and the impression of curves, we preliminarily conducted a questionnaire survey on 18 types of curves by 5-level sensory evaluation using the semantic differential method and conducted factor analysis of the survey results. As shown in Figure 7, we built a neural network system whose input phase is the factor derived from factor analysis and output phase is the three parameters (inclination, maximum curvature radius, minimum curvature radius) dominating the curve shape on logarithmic curvature histograms. The results of the questionnaire survey are used for the learning data of this system. The three parameters (inclination, maximum curvature radius, minimum curvature radius) of the system dominating the curve shape on logarithmic curvature histograms are output by inputting curve impression expressed as a weight of kansei words, so that the curve which has a intended curve impression can be defined as shown in Figure 8. 3.3.3 – Building of virtual clay
Virtual clay is built using Zmap models. The building process of virtual clay using the virtual scraper is executed by replacing the grid heights data of the Zmap model with the locus heights data of the virtual scraper when the heights of locus surface made by the virtual scraper is lower than the grid heights of the Zmap model. As shown Figure 9, the m-th and m+1-th points making up the shape of the virtual scraper represent P m,n , P m+1,n for the n-th sampling time and P m,n+1 , P m+1,n+1 for the n+1-th sampling time in cutting situations. The sweeping locus surface made by the virtual scraper is defined by the four discrete points to calculate intersection points using the virtual clay Zmap model, and its intersection points data is replaced with the grid data of the Zmap model when the intersection points value is lower than the grid heights of the Zmap model. By this replacing operation, the manipulation of the virtual scraper can reflect the shape forming of virtual clay in real time.
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3.3.4 – Addition of the sense of cutting virtual clay
When cutting real clay, the operator has clues on cutting such as surface friction drag and reaction force between the tool and clay, and controls the handling of the scraper. In this system, vibration amplitude is sent to the operator in proportion to the cut amount via a vibrator attached to the operator’s hands. For the operator, this vibration simply and accessorily simulates the sense of cutting virtual clay using the virtual scraper. 3.3.5 – Stereoscopic vision
With this system, the operator can recognize the field of virtual clay modeling through video picture, which adopts stereoscopic vision, using a head-mount-display (HMD). It’s represented by binocular display processing which provides two different images from two different viewpoints to the right and left displays independently. For this reason, operator can manipulate the Virtual Clay Modeling System with stereoscopic vision. 4- Results
The general procedure for building a clay model with this system is described below. (a) Definition of the key-line shape for the virtual guide rail. (b) Setting the virtual guide rail by the shape of key-line. (c) Definition of the virtual scraper shape. (d) Operation of the virtual scraper along the guide rail. Figure 10 shows how to select four types of key-lines. The key-lines are selected by the simple gestures of the virtual hands in reference to the pull-down menu list on the display. For example, the gesture in (a) has a straight line connecting the left and right thumbs, the gesture in (b) has a sigmoid function, the gesture in (c) has a curve derived from a deflection equation, and the gesture in (d) has a rhythm curve from the curve generation system. Figure11 shows the aspects of the shape forming process of virtual clay by setting the selected key-line on the intended position and using it as the guide rail for cutting
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Select keyline → fixing
Study on Virtual Clay Modeling System
Cutting
Select keyline → fixing
Cutting
Figure 11: Modeling process
(c) This system adopts the stereoscopic vision feature and the feature of cutting sensory feedback with vibrator. In the future works, in order to improve the usability, intuitive modeling function and user friendly interface (UFI) will be developed, and the performance of the system will be evaluated. 6- References
[BC1]
Bordegoni M., Cugini U., Haptic modeling in the conceptual phases of product design, Virtual Reality Journal, Springer, Vol.9, 192-202, 2006.
[BL1]
Bedford A. and Liechti K.M., Mechanics of Materials, Prentice Hall College Div, 2000.
[CB1]
Cugini U., Bordegoni M., Touch and design: novel haptic interfaces for the generation of high quality surfaces for industrial design, The Visual Computer Journal, Springer, Vol. 23, 233-246, 2007.
[DC1]
Dewaele G., Cani J-P., Interactive global and local deformations for virtual clay, Proc. Pacific Conference on Computer Graphics and Applications, Vol. 66, 352-369, 2003.
Figure 12: Example of result
manipulation with the virtual scraper. The virtual scraper is supported by virtual hands and manipulated by the motion of [H1] virtual hands. When the deflection curve is selected for the shape of the virtual scraper, it becomes deformed in conjunction with the hand positions in real time. The shape can [RG1] also be fixed. Figure12 shows an example model generated using this system. [TA1]
5- Conclusions
Harada T., Study of Quantitative Analysis of the Characteristics of a Curve, FORMA, 12: 55-63, 1997. Rijpkema H. and Girard M., Computer Animation of Knowledge Base Human Grasping, Computer Graphics, SIGGRAPH, Vol.25, No.4, 339-348, 1991. Takeda K. and Aoyama H. Basic Study on Parameters to Quantitatively Evaluate Shape Impression. In Proceeding of the ASME 2008 International Design Engineering Technical Conference & Computers and Information in Engineering Conference IDETC/CIE 2008, 1-8(CD-ROM).
In this study, we proposed a virtual clay modeling system and developed for clay modeling by making use of virtual hands and virtual tool comprised of VR devices such as position tracker data-grove and head-mount-display. The characteristics of this system are as follows. (a) The shapes of virtual tools and guide rails are defined as [UY1] Ueda E., Yabugami K., Virtual Clay Modeling “deflection curve” and “rhythm curve”. System using Multi-viewpoint Images, 3dim, “Deflection curve” is a shape whose energy of pp.134-141, Fifth International Conference on 3-D strain is minimum, and rhythm curve is an aesthetic Digital Imaging and Modeling (3DIM'05), 2005. curve whose condition of curvature change is [Y1] Yamada Y., CLAY MODELLING: Technique for constant. Giving Three-dimensional Form to Idea, Car “Rhythm curve” can be generated by its impression Styling Extra Issue, Vol. 93 1/2, 1993. with kansei words. (b) The shape forming process of virtual clay is executed by the sweeping of the virtual tool along the guide rail.
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