A Virtual Reality desktop configuration for free-form surface sketching

A Virtual Reality desktop configuration for free-form surface sketching
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  XIV Congreso Internacional de Ingeniería GráficaSantander, España – 5-7 junio de 2002 A VIRTUAL REALITY DESKTOPCONFIGURATION FOR FREE-FORM SURFACE SKETCHING F. Bruno, M.L. Luchi, M. Muzzupappa, S. Rizzuti Università della Calabria Dipartimento di Meccanicavia Pietro Bucci 44/c, 87030 Rende (CS), Italiae-mail: ABSTRACT In this paper a low-cost semi immersive Virtual Reality (VR) system for free-form surfacesketching is presented. The system is based on two 3D input devices (Microscribe 3D andSpacemouse) and a pair of shutter glasses for stereoscopic vision. This system has beenused in the Department of Mechanical Engineering of the University of Calabria to developand implement three innovative tools for sketching free-form curves and surfaces in VR  based on the concepts of extrude, revolve and skin. Key words: 3D input device, human-computer interaction, free-form surface modeling,virtual reality. 1   Introduction The introduction of Virtual Reality (VR) into the design process gives the designer the possibility of creating and manipulating 3D objects as if they were really in front of him. To create this illusion the system must show the 3D scene in stereoscopic visionand allow the designer to use both hands to manipulate the objects directly, in anintuitive and natural way. This makes it possible to reduce training periods and increaselabour productivity.At the moment there already exist some systems, such as ARCADE [ 1 , 2 ] or COVIRDS [ 3 ], that are under development for investigating the capabilities of the socalled VR Aided Design (VRAD) systems. The main aim of these systems is to givedesigners the possibility, in the near future, of creating and modifying a Digital Mock-Up (DMU) inside a Virtual Environment (VE). However the costliness of the specificVR hardware and software is still represents one of the main obstacles to the diffusionof VRAD systems. In fact, nowadays only a few important research institutes and somecorporations can sustain this expenses.With this in mind, in the initial phase of the research much attention was paid to thedevelopment of a low-cost VRAD platform on which some modelling tools are beingimplemented and tested.The platform belongs to the so called desktop VR configuration and it is similar tothe ones used in 3-Draw [4] and in JDCAD [5]: a monitor in combination with a pair of   2 shutter glasses Stereographics StereoEyes, used for the stereoscopic vision; aMicroscribe 3D and a Spacemouse, used to provide the 3D input.For this system a set of tools for free-form surface modelling has been developed,which aims to redefine the interaction techniques currently used on CAD and modellingsoftware, in order to maximise the advantages of the 3d input devices and stereoscopicrendering. Free form surface modelling is an application that requires a long training of high specialized technicians on the traditional CAD system. This lies behind theresearch into new free-form modelling techniques on VRAD system, to make themodelling itself easier and to increase the intuitiveness of the interface. 2   State of the Art One of the first approaches to modelling applications in virtual environments wasmade by Sachs, Roberts and Stoops [ 4 ]. They developed a system, called 3-Draw, wherethe user interface is based on the use of two electromagnetic trackers. They register the position and the orientation of a tray and a pencil. Sitting in front of a normal screen, theuser navigates the scene moving the tray with the non-dominant hand and defines free-form curves with the pencil. These curves are used to define the skeleton of a surface, asin a wireframe representation, but there is no possibility of creating a surface by thedrawn curves.JDCAD [ 5 ] uses a monitor as display device, a tracked pen for 3D input and a headtracker to establish the user’s point of view. JDCAD supports 3D-primitives and performs Boolean operations. The user can select an object type from a 3D-menu, drawa bounding box, and the system creates the primitive in the middle of the bounding boxwith the parameters derived from its size.One of the first modeling applications using immersive VR technology is 3DM [ 6 ],created for the use of Head-Mounted-Displays (HMD). The user can navigate in avirtual world and can construct objects from standard primitives such as spheres or cylinders. The menu for navigating is integrated into the 3D scene, but the navigationsystem doesn’t seem to be very easy. For modelling surfaces it uses triangle nets.Chu et al. [ 7 ] have developed a VR based CAD system. This application can be usedon a table-like back projection system, also known as Virtual Table (VT). The VT, incombination with shutter glasses, makes a large stereo display available. The 3D inputis provided by data gloves.Forsberg et al. have extended their SKETCH [ 8 ] system towards 3D and use twomagnetic trackers on a VT for object transformation with the non-dominant hand and3D-sketching with the dominant hand.Schmalstieg et al. [ 9 ] have developed a whole new user-interface design space for the VT using a transparent pen and a plexiglas sheet (pad). Subsequently Encarnação etal. [ 10 ] exploited this approach for sketching solid geometries through gestural input.Dani and Gadh describe in [3] the System COVIRDS, Conceptual Virtual DesignSystem. This system aims at the combination of various VR-technologies, such asspeech input, gesture recognition as 3D-Input and Output during the conceptual phase.Then the user can create objects by saying e.g. "The cube has this width", where thewidth derives from the distance of his hands. The authors have also developed newinteraction techniques for creating and modifying free-form surfaces. Modifications can be performed with so called virtual tools: with a cone-tool, for example, it is possible tocreate a hole in a surface. The principal ideas described in [3] have not yet beencompletely implemented.In ARCADE [1] a pad and a pen, both equipped with 6 DOF magnetic trackers, asinput devices, and a VT as stereo display device, have been used. The functionalities areaccessible via the 3D main menu displayed on the pad. The user can create 2D  3  primitives, 3D primitives, curves and surfaces. He/she can perform Boolean operations,access the history of an object to change it partially or copy primitives. The objectmanipulation functionality is performed in a gesture-based manner. The authors alsodeveloped a fast algorithm to perform picking and snapping on the precise CAD model[ 11 ]. The 3D free-form surface modelling [2] is implemented in different ways, e.g.Coons patches from just one 3D outline stroke, skinned surfaces with immediate visualfeedback during creation, symmetric free-form surfaces by using the pad as a mirror  plane, subtractive sweeping, trimming, etc.The IRR (Immersive Reconfigurable Room) [ 12 ] is a full reconfigurable VR immersive system which aims at obtaining the visualization and interaction of full scalemodels. A free-form surface modelling application [ 13 ] has been developed for the IRR.Moving a tracked glove over the surface, the user can "spread" a profile on the surface,and so model the surface, according to, not only the tool shape, but also to themovement. The program also provides some utilities such as an interactive tool for shaping, input/output functions and mirror copying. 3   Motivation for the work  In the last twenty years engineering design has been radically changed by theintroduction of CAD systems in the design process. These systems have reached their maturity recently due to the transition from 2D to 3D and overall due to the introductionof parametric design systems. Nowadays the next important evolution may happen dueto the introduction of the VR. This, in fact, could give the designer the possibility of creating and manipulating 3D objects as if the objects are really in front of him. Tocreate this illusion the system must show the 3D scene in stereoscopic vision and givethe user the possibility of using both hands to manipulate the objects directly in a moreintuitive and natural way. In this way it would also be possible to reduce training periods and increase labour productivity. Even today the use of a physical prototypeduring the design process is very common. Generally, clay models are made to evaluateand develop the aesthetic aspects of a product, and to determine manufacturing aspectsand marketing opportunities. But by means of DMU it should be possible to save muchtime and money. In a VE the DMU can be observed in full-size from all the possible points of view. In the future it will be possible to change in real time the DMU insidethe VE and designers will be able to evaluate the results of DMU modificationsimmediately.Obviously the VR technologies are still under development: the graphic quality isalready good, but the lack of a plausible tactile-feedback represents the main challangefor researchers in the hardware technologies. Not only the research of new input/outputdevices is important, but also the development of appropriate software systems thatallow the use of the VR hardware in the best way. As shown in the previous sectionmuch effort has been addressed to developing new interaction techniques to define anefficient and intuitive interface between users and the VE.Free form surface modelling seems to be the application in which the advantagesoffered by the VRAD systems are more evident. The surface sketching software proposed in this paper is based on three tools. Though they seem similar to some thatalready exist in most CAD and modelling software, they differ from all of these becausethe interaction technique has been redefined in order to take advantage of the use of 3Dinput devices.This revisitation effort aims at satisfying two requirements: to increment theintuitiveness of the interface, making the man-machine interaction easier; and to give  4 the user, even during the modelling phase, control of the final appearance of thesurfaces, extending the concept of the ‘immediate visual feedback’ [2].The hardware configuration that has been adopted is defined in the literature as VR-desktop  and it is based on a low-cost graphic workstation, a pair of shutter glasses for  providing the stereoscopic vision and two 3D input devices.The use of the monitor as output device represents for many authors a limit to thefunctionality of a VR system, because the monitor offers a small field of view. Besidesthe immersivity of the system proves to be reduced. As a matter of fact, many VR systems are based on rear projection display that allows a large visible area in table,wall or multiple wall configuration (CAVE). However, in order to encourage theintroduction of semi immersive VR system in medium-sized enterprises it is desirable tooffer a low cost system. As the system proposed is quite similar to a common CADworkstation a progressive migration from the traditional CAD systems to the VRADsystems is less "traumatic". To encourage this change it is important to underline thatthe VRAD systems have to be innovative, especially in the user interface, by offeringthe possibility of modeling the DMU directly in 3D in a very intuitive and natural way. 4   System overview The user interface of the software is very similar to the one used in a common Windowsapplication. The user can choose from several tools, using the mouse, through pop-upmenus or through the buttons on the toolbar. When the designer wants to use amodelling tool he/she can choose from three different options: the Microscribe 3D; theSpacemouse; or a mouse in conjunction with a construction plane which can be movedand rotated by the Spacemouse or by the keyboard.The Microscribe 3D is a device based on an electromechanical arm able to obtainthe position and the orientation of a pen placed at the end of the arm. It is commonlyused in reverse engineering applications to build a 3D model from a real object. In thisresearch the possibility of using it as a 3D input device has been investigated and theresults are encouraging. Compared to other 3D input devices, such as electromagnetic or optical tracker ones, the Microscribe 3D has some limitations in its movement and partially covers the monitor surface, but, on the other hand, it is very precise (0,23 mm),allows a quick set-up and is less expensive.As underlined in section 2 the principal instrument that the user can use to model inVR is a virtual pen, free to move in the space with 6 DOF. In this application the user moves the virtual pen by means of the Microscribe 3D, even if the rotation around the pen axis is not allowed. The user can also move the pen with the Spacemouse, but itworks like a joystick, and therefore it is inadequate for the modeling phase; on the other hand it's really useful for rotating and translating objects or the whole scene.The third modelling method is based on a construction plane, on which a pointer can be moved by simply dragging it with a mouse. The plane could be translated and rotatedeither by the Spacemouse, in a very easy and fast way, or by the keyboard. The use of the keyboard is less intuitive, but it allows the application to be run on machineswithout 3D input devices, which is really useful for the developers. Besides, the use of aconstruction plane, compared to the 3D input devices, makes it easier to draw a polylineor a curve lying on a plane.The software environment is based on Open Inventor 3.0 by TGS, a SoftwareDevelopment Kit that contains several functionalities for managing the 3D scene and for navigating in it. Initially it seemed to be more convenient to develop the application asan extension of the Open Inventor Scene Viewer. In this way it is possible toconcentrate on the development of the modelling tools, taking advantages of the basic  5 options such as cut&paste operations, color and material editing, lights, object importfrom external VRML or Open Inventor files, etc.. In this way time and energy were notwasted on recreating all these basic functionalities, indispensable for testing the systemin a proper way, in that they are suitable for the tasks of this work. b) Spacemousea) Microscribe 3Dc) Stereographics StereoeyesFigure 1: The set of devices employed in the system 4.1   Device set-up The first step for integrating the 3D input devices in a software environment consists inrelating the two different reference systems.The Microscribe 3D provides data about the position and orientation of the pen in itsreference system, positioned by default on its base and represented in ¡Error!Argumento de modificador desconocido. 2 with the X,Y,Z axes. Instead the position of the virtual pen is related to the scene reference system X’,Y’,Z’, thatmatches the monitor surface, as shown in ¡Error!Argumento de modificadordesconocido. 2. To obtain the correspondence between the Microscribe 3D pen and thevirtual pen movements the software has to know the mutual position of the tworeference systems X’,Y’,Z’ and X,Y,Z. The position of the physical pen, represented byvector P, can be defined by vector P' in the scene reference system. In this way, it is possible to superimpose the virtual pen on the physical one.The Microscribe 3D can be set up very easily. It requires just 3-4 seconds and has to be done only when the monitor or the base of the Microscribe 3D are moved. The user has to touch three points on the monitor surface with the pen: the lower left corner to setthe srcin; a point on the horizontal side of the monitor frame to set the X' axis and a point on the vertical side to set the Y' axis.The user can change his/her point of view (camera position) using three sliders of the Open Inventor Scene Viewer. Every time the camera position is changed thesoftware has to take into account the new camera position to maintain thesuperimposition between the virtual pen and the physical one.
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