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A visibility control system for collaborative digital table

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Abstract We propose a novel display system that presents information with different levels of visibility to multiple users for enhancing collaborative multi-touch on digital tables by controlling visibility with a revolving polarizer. We first
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  ORIGINAL ARTICLE A visibility control system for collaborative digital table Satoshi Sakurai Æ Yoshifumi Kitamura Æ Sriram Subramanian Æ Fumio Kishino Received: 10 October 2008/Accepted: 15 March 2009/Published online: 17 July 2009 Ó Springer-Verlag London Limited 2009 Abstract We propose a novel display system that pre-sents information with different levels of visibility tomultiple users for enhancing collaborative multi-touch ondigital tables by controlling visibility with a revolvingpolarizer. We first describe the system and its multiplevariations and then present several example applicationsshowing the technique’s benefits: the concealment andclassification of information for specific users. We thendescribe an example of an entertainment application and atangible-polarizer on display. In addition, we conductexperiments to learn the variations of visibilities dependingon the conditions for investigating application feasibilities.Finally, we discuss the future extensions of the configura-tion, its potential, and its limitations. The following are themain contributions of this paper: (1) the discussion andclarification of the importance for multi-visibility withmulti-touch, (2) the proposal of our multi-visibility systemand its usage, (3) the verification of multi-visibility controlthrough experiments. Keywords Display system Á Public display Á CSCW Á Entertainment 1 Introduction People engaged in communication/collaboration oftenmeet at physical locations to talk and enjoy themselves. Inmore than a few cases, they use large displays like pro- jected screens or plasma displays to effectively shareinformation. Such displays sometimes appear on the walls[10,32,37], but recently display table systems have also become popular [3,21,29,33]. By using a digital table, all the users around it can equally access information on thetabletop and work together effectively by utilizing theawareness information of all other users. Therefore, manytechniques and interfaces have been proposed to supportgroup work using such display systems.When people work collaboratively on digital tables,providing each user with an appropriate interface isimportant. As input devices, direct touch and tangibleinterfaces are often used for effective interaction; therefore,many digital tables allow multiple users to interact simul-taneously with these interfaces [3,16,18,35]. In addition, showing appropriate contents such as publicand private information to each user is important. Forexample, when we play a card game, all users have theirown cards and all users also share the cards on the table.Although personal displays are often used to deal withprivate information [20,22,31], problems sometimes exist: for example, when the relationship between public andprivate information is crucial. Although some systems andtechniques have been proposed to adequately deal withpersonal information in a public space [14,17], many limitations to the proposed solutions remain, includingrestrictions on the available display area, restrictionsagainst users freely changing their viewpoints and so on.Besides, these systems can only deal with binary visibili-ties: visible or invisible. If a display can continuously S. Sakurai ( & ) Á Y. Kitamura Á F. KishinoHuman Interface Engineering Laboratory, Osaka University,Suita, Osaka 565-0871, Japane-mail: sakurai.satoshi@ist.osaka-u.ac.jpY. Kitamurae-mail: kitamura@ist.osaka-u.ac.jpF. Kishinoe-mail: kishino@ist.osaka-u.ac.jpS. SubramanianDepartment of Computer Science, University of Bristol,Bristol BS8 1UB, UK e-mail: sriram@cs.bris.ac.uk   123 Pers Ubiquit Comput (2009) 13:619–632DOI 10.1007/s00779-009-0243-6  change visibility by controlling the brightness, it can beused for other purposes like the classification of digitalcontents.In this paper, we first discuss the importance of themulti-touch and multi-visibility systems and show relatedworks. Then we describe a novel display system andexample applications that was presented at [25] to providedifferent levels of visibility of digital content to differentusers who share the same display. We then extend this byverifying the feasibility of the example applications.Finally, we discuss the extended configuration of the sys-tems and their future possibilities and limitations. 2 Interfaces for collaborative digital table 2.1 Input for multiple usersWhen people work collaboratively on digital tables, thedigital contents can often be interacted by multiple usersaround the table. On the other hand, the users mightsimultaneously execute different tasks. Therefore, digitaltables are required to provide input interfaces with whichthe users can equally and simultaneously interact with thedigital contents on the tabletop.Direct touch and tangible interfaces are considered verysuitable for such systems because the tabletop surface iswithin the reach of the users around the table. Moreover,the users are provided intuitive controls of digital orphysical contents, preserving the inherent awarenesscapability of the digital tables. Therefore, many digitaltables adopt these techniques for inputs [16,18,35]. 2.2 Displays for multiple usersIn addition to input interfaces, collaborative systems mustalso show appropriate information to each user. Forexample, users must often conceal parts of their individualinformation from some users while simultaneously sharingthat information with other users [4]. However, informationon conventional large displays is visible to the public andidentically presented to all users.One of the most straightforward methods is to use apersonal display to show private information to the corre-sponding user [20,22,31]; however, such methods have several problems. For example, it is not easy to moveinformation between separated display surfaces by directtouch or tangible interactions, and it is harder still to usephysical objects (consequently tangible interfaces) onsmall private display surfaces. Moreover, when the posi-tional relationships between private and public digitalcontents are important, observing them is difficult if thecontent is displayed on separated surfaces. Therefore, webelieve that some applications can benefit from a displaysystem that can provide private information on publicsurfaces. 3 Related work We first briefly present literature on multi-user collabora-tion around digital tables that enable multi-touch and thendiscuss more literature related to multi-visibility.3.1 Sharing a digital table with several usersIn light of the increasing demand for digital tables thatsupport group activities, a huge body of literature isdevoted to devices for collaboration [3,21,29,33], design guidelines [5,26], visualization techniques [36], and so on. When we hold discussions using digital content that isvisible on a tabletop, this information can be categorized aseither public or private information. Public informationmay be observed by all participants; on the contrary, pri-vate information should only be observed by one or asubset of the users. Consequently, private informationcannot be displayed on a large shared display because itwill be equally revealed to all users viewing the display.When dealing with many different kinds of informationobjects (e.g., icons), they must be classified into severalgroups to simplify tasks and add comfort. For group work involving multiple users, classification is often done, forexample, using a public canvas and private toolboxes. All of these objects are classified by their positions, orientations orcolors, which are determined based on their owners or thekind of tasks performed [15,27]. However, if each of the multipleusershasadifferentclassificationtask,thesituationbecomes complex. In this case, a kind of dynamic filter,which brightens/darkens the parts of information based onuser individual demands, would improve their work.Therefore, we consider a classification method in whichparticular objects can be observed brighter than others byonly one, or a subset, of the users in a shared space.3.2 Multi-touch devicesA number of approaches have been devised for imple-menting digital tables that allow multiple users to interactsimultaneously. These techniques can be roughly catego-rized into two groups: sensor electronics integrated into thesurfaces and camera-based sensors. The former includescapacitive [3,23], resistive [12], and IR sensors [8]. The latter uses a camera [1] as the sensor or an IR light sourceand a camera [7,18,24]. The continued improvement of  these devices enables greater accurate, higher frequency,and less latency while sensing many more touches/points. 620 Pers Ubiquit Comput (2009) 13:619–632  123  3.3 Multi-visibility displaysHere, we review the systems in which the visibility of information is controlled for all participants sharing adisplay. Lumisight Table is a square digital table that canshow different information to the users sitting on each of itssides [17]. The display surface consists of several specialfilms on which the information projected from a certaindirection can only be observed from specific directions.Four projectors, directed to each side and located inside thetable, project four different images to four individual users.Parallax barriers, normally used in stereoscopic displays,are used to show the different pixels of the same surfacedepending on user viewpoints. They can also be used forthe display surfaces that show different information todifferent users [28]. These systems, however, impose thefollowing limitation: users cannot freely change theirviewing position. If their viewing positions change, theymight see other users’ information.Hua et al. [9] proposed a collaborative system in whichusers can freely move their head positions and observeindividual information. In their system, a projector mountedon each user’s head projects information to the retro-reflective surface on which light is only reflected in thedirectionofthelightsource.Thisallowseachusertoobserveonly the information from the projector on his/her head.However, this requires placing somewhat heavy hardwareon user heads, which must also be wired for a power supply.Even if using small projectors like pico-projectors [19], theyare still obstacles. Besides, projection is prevented by userhands or physical objects on the screen. SharedWell, whichprovides bothpublicand privateinformation with userswhomove around the table [14], consists of a normal display anda display mask that has a hole in its center. Each userobserves part of the display through the hole and uses thatpart as his/her private area. The overlapping sections of twoor more private areas are used as shared public areas.Although users easily understand the relation betweenpublic and private information, they cannot collaborate overa large space. They also have difficulty in reaching thescreen surface due to the bulky mask.Some systems separately provide two different types of information to users, regardless of their viewpoints. Somesystems display two images, one for each user, in a time-sequential manner on the display [2,30]; these system show images to one user who wears synchronized LCDshutter glasses and show other images to other users.Adaptation to more than three users is difficult due to theflicker, and these show both images to other users who arenot wearing glasses. Yerazunis et al. [39] displayed maskedand private images in a time-sequential manner. The userswearing LCD glasses observe the private image, and theusers without glasses observe the masked image. However,the system cannot flexibly control visibility; for example, itcannot change a certain user’s private image to anotheruser’s private image. Besides, LCD glasses need synchro-nization signals that can be blocked by user hands whenreaching for the screen or gesturing.Some tabletop systems [11,13] can show independent images on both the tabletop surface and other surfaces onor above the tabletop. These layered images are equallyobserved by all users; on the other hand, our method pro-vides different images to different users. 4 Multi-level visibility control system 4.1 Implementation and technical featuresBefore describing the challenges and the benefits of ourvisibility control system, we describe its implementationand technical features.Figure1shows an overview of our system. Two DMDprojectors (projectors 1 and 2) cast unpolarized imagestoward the tabletop’s screen through a mirror. In front of projector 1, a linear polarizer labeled the projector-pola-rizer, which is located perpendicular to the projection axis,can be revolved around the projection axis by an attachedmotor based on control signals from a PC. If a userobserves the screen through another polarizer called the eye-polarizer  , the observed brightness varies based on h ,which is the relative angle between the two polarizing axesof the eye-polarizer and the projected light. Figure2showsthe relationship between the observed brightness of thescreen through the eye-polarizer and h . The observedbrightness can be varied by rotating the projector-polarizerand changing the observing direction. Therefore, bystanding at different locations around the tabletop, eachmultiple user observes the same content projected fromprojector 1 with different brightness. This technique iscalled visibility control using a revolving polarizer [25]. Fig. 1 Design of implemented systemPers Ubiquit Comput (2009) 13:619–632 621  123  When observing without the eye-polarizer, the observedbrightness is constant regardless of the observing directionand the polarizing axis of the projector-polarizer. Theobserved brightness without the eye-polarizer also dependson the observing direction, because it varies based on thediffusion property of the screen and the incidence directionof the light. In contrast, since there is no polarizer in frontof projector 2, its projected light can be observed withconstant brightness independently of the observing direc-tion. When observing through the eye-polarizer, thebrightness is constantly halved. Tangible-polarizers, which are circular pieces of pola-rizer put on the screen, can also be used in this system.They can be rotated by user hands and can be used asalternatives to the projector-polarizer. When projector 2projects images inside the tangible-polarizer, the observedbrightness through the eye-polarizer varies based on theobserving direction and the polarizing axis of the tangible-polarizer. Meanwhile, in this paper, projector 1 does notproject images inside the tangible-polarizer, because it is socomplex to use projector-polarizer, tangible-polarizer, andeye-polarizer at same time.For the implementation, we used a Toshiba TDP-TW350(J) (3,500 ANSI lumen, 1,600 9 1,200 pixels)product for projectors 1 and 2, a Stewart Techplex 200 forthe tabletop screen, and a plastic polarizer LN-1804P(38.00 ± 0.25% transmittance of unpolarized light, morethan 95% polarization degree) for each of the polarizers.The screen size is 1,200 9 900 mm, and the height fromthe ground is 900 mm. We implemented the eye-polarizeras glasses with a vertical polarizing axis. We also used anInserSense IS-600 Mark II SoniDisc (180 Hz, 7.0-mmerror, 4–10 ms latency), which is an ultrasonic sensor todetect the positions of the eye-polarizer glasses and thetangible-polarizers. We attached two IS-600 tags to eachtangible-polarizer to detect the rotation on the table surfacein addition to the position. A two-phase unipolar step motor(7.5 ° per step) rotates the projector-polarizer and is con-trolled by a motor driver of our own composition thorougha RS-232C connection from the PC. The motor can revolvethe projector-polarizer 360 °  /s. This is enough fast to con-trol the projector-polarizer based on user movementsaround the table.In this paper, our system is not equipped with touchsensors or tangible interfaces to emphasize its multi-visi-bility. But they can be easily integrated except for thosetechnologies that need opaque surfaces.4.2 Benefits of multi-visibility control systemOur review highlights the two main challenges in designingdynamic visibility control: (1) the ability to simultaneouslycontrol information visibility to multiple users on the samesurface without limiting viewpoint movements or pre-venting touch and tangible interactions; (2) allowing thevisibility control of each part of the information to beperformed independently to show both public and privateinformation. Moreover, if the visibility is controlled con-tinuously, we can visualize the classification of information(for example, as one’s own and other’s) by assigning adifferent visibility to each part of information. In addition,the visibility statuses must be known and changed intui-tively by the users so that they can easily control theinformation visibility. For example, they can select to showor hide certain information from others.Our system described in Sect4.1allows multiple usersto observe digital contents projected from projector 1 onthe tabletop surface by varying the levels of brightness tocontrol what they see. If the observed brightness of thepolarized image for a certain user is low enough because itwas cut at the eye-polarizer, the content is invisible to him/ her, but visible to other users at different locations.We can realize more elaborate visibility control usingprojector 2 and projecting non-polarized content. Thecontent projected from projector 2 is observed by all usersregardless whether they are wearing polarized glasses withthe same brightness and their direction around the display.The combination of the contents from the two projec-tors*** produces various information representations. Forexample, the content projected from projector 2 can beused as public information while the content from projector1 can be used as private information. If the content fromprojector 1 covers and conceals content from projector 2,the content from projector 2 is also concealed from userswithout glasses. Only users wearing polarized glasses whoare standing at appropriate directions can observe the Fig. 2 Relationship betweenobserved brightness and relativeangle h . a h = 90 ° , b h = 45 ° , c h = 0 ° 622 Pers Ubiquit Comput (2009) 13:619–632  123  content from projector 2 because the brightness of thecontent from projector 1 is adequately reduced. By com-bining these controls, we can assign different visibilitiessuch as public for all users, hidden from certain glassedusers, or independently shown only to certain glassed usersto each part of the display.Moreover, content brightness can be continuously con-trolled so that the system can show the content from pro- jector 1 with any degree of brightness to specific userswearing glasses. Therefore, for example, if the contentfrom projector 1 is reduced by 10%, while content fromprojector 2 is halved at a certain user’s eye-polarizer, allcontent remains visible but is recognized as darker andbrighter parts. This can be used to classify information thatonly specific users can observe. In addition, we can intro-duce tangible objects on the table to control the polariza-tion direction by hand to determine the polarizing axis andto control it intuitively. 5 Example representation We now describe suitable productivity and entertainmentapplications that can potentially benefit from our system.5.1 User-specific concealment of informationGroup collaboration studies have shown that often groupsmeet to discuss a group activity and then disperse to carryout individual tasks [34]. Our multi-level visibility controlsystem can aid users in such situations.Particularly,inlargedesignprojectswheremultipleusersare working on different aspects (for example, user A isdesigning the interiors of the CEO’s office while user B isdesigning the corridors on the same floor), it is often usefulfor users to work together on a shared surface so they canmaintainawarenessofothers’activities;butatthesametimeitisalsousefulforbothuserstofocusontheirsegmentoftheactivity without being disturbed by external factors.In the example above, the system can filter out infor-mation about the corridors to user A so he can focus on theCEO’s office, but whenever he wants to know how user Bis designing the corridor spaces, he can get an overview of the design’s status by removing his polarizer glasses.Figure3illustrates a simple example of concealment.Two users standing along adjacent edges of the tablewearing eye-polarizers are looking at the screen. Projector2 only projects information relevant to user A. The entirefloor plan is projected from projector 1 and has a polar-izing axis that is parallel to user B’s eye-polarizer. Thehorizontal lines in Fig.3b show the image of the polar-ized part and its polarizing axis of the projected image.As a result, user A only observes the information pro- jected from projector 2 (images from projector 1 are fil-tered out). Conversely, user B observes all the displaycontent. If the application turns the polarization axis of the projected information by 90 ° by revolving the pro- jector-polarizer and keeping the input to each projector,user A observes all information and user B observes onlythe corridor. In addition, by appropriately assigning eachprojector with pieces of information, the system canswitch the visibility of the information on arbitrary parts Fig. 3 Typical example usingprototype display. a Overview, b polarized part and axis, c userA’s view, d user B’s viewPers Ubiquit Comput (2009) 13:619–632 623  123
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