KNOWLEDGE AT YOUR FINGERTIPS

Multi-touch Interaction for GIS and Architectural Design Review Applications

Yvonne Jung, Jens Keil, Harald Wuest, Timo Engelke, Patrick Riess and Johannes Behr

Fraunhofer IGD/ TU Darmstadt, Fraunhoferstrasse 5, 64283 Darmstadt, Germany

Keywords:

Multi-touch, Multi-user, Tabletop interaction, Design review, Tracking, Visualization, X3D.

Abstract:

This paper introduces novel techniques of interacting and controlling 3D content using multi-touch interaction

principles for navigation and virtual camera control. Based on applications from GIS and for the architectural

design review process, implementation and usage of these interaction techniques are illustrated. A compre-

hensive hardware and software setup is used, which not only includes tracking, but also an X3D based layer

to simplify application development. Therefore it allows designers and other non-programmers to develop

multi-touch applications very efﬁciently, while allowing to focus on user interaction and content.

1 INTRODUCTION

Multi-touch technology is one of the most interest-

ing ﬁelds of research in today’s Human-Computer-

Interface area and interactive direct-touch tabletop

displays have been the focus of numerous research

projects. It is not only about interacting with more

than one or two ﬁngers simultaneously, but also work-

ing together collaboratively in a multi-user scenario.

Multi-touch techniques seem to be very promising

since they allow people to seamlessly interact with

what they see by simply touching it. Thus, new

ways of generating immersion are possible, like for

instance through tabletop interaction.

For most people, working on a table feels well

known and very intuitive, since they are used to work

on and surround tables every day. That is espe-

cially of concern when working with a lot of maps

or blueprints. These are normally plotted on huge pa-

persheets, spread on a table’s surface and discussed by

several people. However, many people are not capa-

ble to read and understand abstract maps or blueprints

decoded with a huge amount of information, hence

having problems to understand them.

By using both, blueprints or maps on the one hand

and their real-time rendered 3D representations on the

other hand, the beneﬁts can be combined. Regarding

an architectural design review process, one can have

the overview given by the blueprint and also the 3D

representation of the building itself. Hence, an archi-

tect now might achieve a higher and more sophisti-

cated comprehension of what he is looking at. The

proposed concepts of how to interact and navigate in

such a multi-touch environment are exemplarily dis-

cussed by introducing a GIS application as well as an

architectural design review application.

Our multi-touch table (Figure 1) is based on the

well-known frustrated total internal reﬂection (FTIR)

setup (Han, 2005). It is an optical method, that tracks

the user’s ﬁngers with computer vision techniques.

Therefore, the acrylic sheet of the touch table is il-

luminated by infrared light. Because of the refractive

index of acrylic relative to air, the light is subject to

total internal reﬂection. Whenever an object comes

close enough, the total reﬂection is frustrated, light

dissipates, and illuminates the object. Thus, a ﬁnger-

tip touching the surface is illuminated as well.

A camera, observing the table’s surface from be-

low, now can capture the resulting lightblobs of the

ﬁngers. In our setup we use a standard IDS uEye cam-

era with a resolution of 752x480 pixels. To improve

immersion, we chose a monolithic and reduced design

of the table, with a huge display of about 150x90cm,

which offers enough space for collaboration. The im-

age is created by a standard projector embedded in-

side the table, with a resolution of 1400x1050 pixels.

387

Jung Y., Keil J., Wuest H., Engelke T., Riess P. and Behr J.

KNOWLEDGE AT YOUR FINGERTIPS - Multi-touch Interaction for GIS and Architectural Design Review Applications.

DOI: 10.5220/0001814803870392

In Proceedings of the Fourth International Conference on Computer Graphics Theory and Applications (VISIGRAPP 2009), page

ISBN: 978-989-8111-67-8

Figure 1: Architectural design review on our multi-touch

table, presented at this year’s CeBIT trade fair in Hanover.

2 RELATED WORK

Multi-touch technology can be seen as the basis for

a lot of new techniques in the way, we are working

with computers. However, it is obvious that also the

corresponding applications may become increasingly

complex and different subjects have to be taken into

account, like ﬁnger tracking and gesture recognition,

software setup, and also more sophisticated graphical

interfaces and interaction principles as well. With his

FTIR setup (Han, 2005), Han made multi-touch re-

search interesting and public again, since this optical

ﬁnger detection method is not only low-cost, but also

easy to implement, and a robust setup for tabletop in-

teraction and Wall-like installations (Han, 2006).

Also Microsoft’s Surface (Microsoft, 2008) and

Apple’s iPhone (Apple, 2008) are using this kind of

technology for bridging the gap between advanced

graphical user interfaces and today’s still limited in-

teraction principles. Hence, in order to overcome the

old WIMP paradigm that is still common for desktop

applications, in (Agarawala and Balakrishnan, 2006)

interesting new physical based desktop metaphors

like stacks of documents are discussed.

In (Jord`a et al., 2007) the collaborative generation

process of live music performance on tabletop inter-

faces is explores. Their ’reacTable’ is a special table-

top interaction device, which mainly uses tangibles

for interaction that are equipped with special markers.

With the ’DiamondTouch’ in (Dietz and Leigh, 2001)

a multi-touch table based on a capacitive system for

ﬁnger detection was presented, which is mainly devel-

oped for multi-user scenarios and collaborative work-

ing. The table itself features touch and gesture based

interaction techniques. Using so-called antennas, it is

also able to distinguish between different users.

In (Rekimoto, 2002) a sensor architecture for hand

and ﬁnger detection was introduced. It is sensitive to

human hands, ﬁnger gestures, and shapes. The sys-

tem thus can recognize a user’s entire arm. Reki-

moto is also one of the ﬁrst ones, who showed ba-

sic techniques to simultaneously translate, rotate and

scale 2D objects, and who presented gestures for ob-

ject manipulation. A similar approach for continu-

ously transforming objects in order to control Google

Earth is also described in (Kim et al., 2007), whereas

in (Wu et al., 2006) a set of design principles that sup-

port multi-hand gestural interaction is introduced.

In (Moscovich and Hughes, 2006) simple trans-

formations and form-based manipulations of digital

objects are described. However, the focus is on spe-

cial mouse-cursor-like techniques. A good overview

on tabletop interaction techniques in general can be

found in (Shen, 2007). Recently in (Jung et al., 2008)

multi-point interaction was introduced to the X3D

standard (Web3DConsortium, 2007), which also pro-

vides some extensions for GIS (Web3DCon., 2008),

by extending its pointing sensor component.

As can be seen, multi-touch applications show a

wide variety of different interaction methods and ges-

tures, which map ﬁngertips and movement to different

system functionalities. But in most cases they only ﬁt

to the special purpose of a task or tool. Designing

the application and its interaction principles becomes

often speciﬁc and tool-dependent, since there are no

basic rules or guidelines, yet. However, devices like

Apple’s iPhone introduce quasi standards, i.e. low-

level gestures, for (multi-) user interaction. Because

these techniques scale well, multi-touch in addition

inherently implies multi-user functionality: on bigger

systems it is now possible for many users to interact

with an application simultaneously. So, after a short

review of vision techniques, in this paper it will be

shown, how these methods also can be used for inter-

acting in and with 3D environments.

3 BLOB TRACKING

The task of the tracking process is to detect the il-

luminated ﬁnger tips in the grayscale picture of the

video camera, and to generate events if a ﬁnger is

touching, moving or disappearing from the surface.

The ﬁnger tracking procedure can be divided into two

problems: blob detection and blob tracking. Blob de-

tection handles the recognition of bright spots in the

image, which results in a set of 2D image position of

ﬁngertips. The detection of blobs in images has been

widely used for detecting active markers. Recently,

such methods have been applied to detect ﬁngertips

on multi-touch surfaces (Touchlib, 2008).

The ﬁrst step of the blob detection consists of

an adaptive background subtraction step, which pro-

duces a foregroundimage consisting only of bright re-

gions not present in the background model. From the

many background subtraction techniques which exist

(Piccardi, 2004), a simple running average turns out

to be most suitable for our demands. In every frame

GRAPP 2009 - International Conference on Computer Graphics Theory and Applications

388

Figure 2: Illustration of used gestures. a) General interaction principles: (i) one ﬁnger selects and drags an object, (ii) two

ﬁngers rotate an object or (iii) manipulate its scale by stretching the ﬁngers apart. b) Camera control: (i) the ﬁrst ﬁnger maps

to the camera’s position, (ii) a second ﬁnger in addition can control the camera’s orientation.

the value of every pixel of the background model is

updated by B

= αI

+(1−α)B

t−1

, where B

is the in-

tensity of a background pixel at time t, I

is the pixel

intensity of the camera image at time t and α is the

learning rate. For a fast adaption during the startup

a second learning rate has been introduced, which is

only applied during the ﬁrst seconds. At this time

no detection will occur. A pixel is regarded as part

of the foreground, if |I

− B

| > c, where c is a ﬁxed

threshold. The intention of detecting blobs in a fore-

ground image is to avoid the detection of false posi-

tives, which can occur from other light sources.

To detect blob positions in the foreground im-

age we apply a two-pass algorithm (Rahimi, 2001)

to label connected components of pixels in a 4-

neighborhood. For every region of connected compo-

nents, the mean position, the mean intensity, the num-

ber of contributing pixels and the spacial covariance is

computed. To disregard blobs, which do not originate

from ﬁngertips, some tests like analyzing the round-

ness or the number of pixel are performed. All blobs

passing the tests are regarded as detected ﬁngertips

and are used as input for the tracking step.

For a meaningful interaction, the blobs not only

have to be detected, but also tracked from frame to

frame. Blob tracking assigns a unique ID to each blob

and tracks it from frame to frame. Thereby the move-

ment of a ﬁnger is detected. Since several blobs can

be detected in an image at the same time, a sophis-

ticated association of blob positions in a continuous

image stream beyond a simple nearest neighbor as-

signment is also needed. In order to be able to ﬁnd

the corresponding blobs, the ﬁrst approach is to look

near the old position. Depending on the frame rate

of the camera the ﬁnger might be found there. If the

ﬁnger is moving faster than this distance per frame,

the new position can be approximated using the ve-

locity. Sometimes ”tracking holes“ can occur due to

physical stick-slipping of the ﬁnger. In those cases it

vibrates while moving over the surface and thus might

not emit light toward the camera. Usually the ﬁnger

”returns“ in the next frames. The position therefore

will be hold at the last position, but furthermore is es-

timated the next n frames taking the former velocity

and acceleration into account. When the blob returns,

the ID will be reassigned, if not, it will be discarded.

When holding the ﬁngers down on the surface and

keeping them in one position, another problem lies

in the dynamic background subtraction, which adapts

the ﬁnger to the background. This results in a loss of

blobs when holding down for a longer period of time.

To prevent this effect, every blob that does not move

within a certain distance and frames gets a ’ﬁxed’ sta-

tus. This status ﬂag can be used to generate a binary

mask that helps the dynamic background subtraction

to ignore those areas in adaptation. Another advan-

tage of this ﬂag lies in the ability to prevent jittering,

which signiﬁcantly increases the user experience.

Because the positions of tracked blobs are de-

tected in the image space of the camera image, it is

desirable that the 2D positions of the blobs are trans-

formed into the image space of the projector image.

This transformation can be achieved by a simple ho-

mography H. If u = (u

, u

, 1) is the homogeneous

coordinate of the detected blob position, this position

can be transformed into the projector image space by

v = H ∗ u. To determine the homography H, the 4-

point method based on the Direct Linear Transform

algorithm is used. As many cameras with a wide ﬁeld

of view come along with a strong radial distortion, it

is necessary to consider these effects, too. If x is an

undistorted point and ˜x the distorted point, we approx-

imate the radial distortion by ˜x = x(1+ k

+ k

where r = kxk is the distance from the image center

and k

are distortion parameters.

4 DESIGN PRINCIPLES

Navigating through a 3D view of a building or land-

scape means controlling the virtual camera’s position

and orientation. But often people still have problems

to keep control while navigating with common VR

interaction devices, and thus, they usually have difﬁ-

KNOWLEDGE AT YOUR FINGERTIPS - Multi-touch Interaction for GIS and Architectural Design Review Applications

389

Figure 3: Interactive visualization of GIS data (different

types of 2D maps as well as the corresponding buildings

and the terrain in the 3D view) on the multi-touch table;

recently shown at the Expo Real 2008 trade fair in Munich.

culties to fulﬁll typical reviewing tasks, like moving

the camera around a 3D object while keeping it in fo-

cus, or moving it along a building’s facade. With e.g.

a space mouse a user has all degrees of freedom he

might need to take a look around. But controlling the

camera is still an issue and one has to be experienced

in using this device. So, instead of simply navigat-

ing and concentrating on the review process, users are

distracted by handling the interaction device.

4.1 Simple Gestures

In typical design review applications the user is inter-

acting with only one 3D scene that contains the ob-

jects to be reviewed. But as mentioned 3D interac-

tion is still neither very intuitive nor does this type of

application allow to directly compare an abstract 2D

information with its concrete realization in a 3D dig-

ital mock-up. Thus, to overcome the aforementioned

problems, in our example applications we therefore

decided to combine 2D maps and blueprints with their

corresponding 3D rendering, utilizing multi-touch in-

teraction for navigation. Figures 3 and 4 show two

example applications from GIS and architecture.

Instead of only having one 3D rendering we are

actually using two. The ﬁrst one is used for render-

ing the virtual drawing table that contains the maps

or blueprints with which the users can interact very

similar to the way they are used to it from real life

experience as shown in Figure 1. The second render-

ing displays the 3D scene to be reviewed on another

object that also lies on the virtual drawing table, just

like a window into a second virtual world. A more

detailed explanation can be found in section 4.3. To

increase the ease of use, we chose well known low-

level gestures as mentioned in section 2. We tried to

keep the amount of simultaneously needed ﬁngers as

small as possible, since one can not control and use

several ﬁngers very well at a time, due to physical

and cognitive issues (Schieber and Santello, 1996).

The focus of most tasks relies on the foreﬁngers.

Using one ﬁnger touching an object a user can eas-

ily select and move it around. While stretching apart

two ﬁngers, the map can also be scaled/ zoomed and

rotated by the angle described by the two ﬁngers (see

Figure 2a). We do not distinguish whether one uses

ﬁngers of the same or of different hands, as we do

not distinguish between different users. There exist a

lot of ideas of how to manage workspaces and tasks in

multi-user scenarios, as e.g. described in (Shen, 2003;

Shen et al., 2004), but for our proposed scenarios spe-

cial kinds of application-based multi-user functional-

ities are neither needed nor useful. Hence, all users

can work in parallel and generally can use the whole

area of the table the same way, and also perform ac-

tions together like zooming or dragging. Users can

choose collaboratively, what might ﬁt to their needs

or to the purpose they want to perform.

4.2 Virtual Camera Control

The virtual camera can be controlled by simply posi-

tioning the ﬁngers onto the table’s surface. One ﬁrst

ﬁnger touching a map or blueprint controls the posi-

tion of the camera. Using then a second ﬁnger deﬁnes

the direction to look at, and controls thus the orien-

tation of the camera. Again, one can use ﬁngers of

one hand or of both hands. Mapping camera param-

eters onto the user’s ﬁngers supports him performing

navigation tasks, because now there is a pretty direct

representation for the camera through the ﬁngers, thus

helping to orientate oneself and to control navigation

within the 3D world. As can be seen in Figure 2b,

by moving one ﬁnger around the other, the camera of

the reviewing scene moves around the corresponding

point, while keeping the touched object in focus.

Furthermore, by interpolating smoothly between

succeeding camera poses, this offers a ﬂuent and con-

tinuous camera movement comparable to cinematic

camera movements. Especially in combination with

our second method for calculating the viewing direc-

tion, by simply orienting the virtual camera along the

direction of ﬁnger movement, this is, unlike using ar-

tiﬁcial interaction devices, a very intuitive and direct

means for navigating in a virtual environment. By

also updating the position and especially the height

of the virtual camera per texel according to the given

heightﬁeld of the maps, as shown in Figure 3, re-

alistic terrain following can be implemented as well

and leads to a much better experience than the tin-

kered sandbox landscapes one might remember from

school. Another important aspect is, that by scaling

a map, implicitly the velocity of camera movement

is also scaled. Thus, when zooming into a map the

corresponding camera movement slows down and au-

tomatically adapts to the scene, because within a cer-

tain time ∆t the path length d of the ﬁnger movement

in the map’s normalized image space is smaller when

GRAPP 2009 - International Conference on Computer Graphics Theory and Applications

390

Figure 4: The design review application. a) Illustration of ﬁngers and camera orientation (depicted by green blobs). b) Each

blueprint on the table corresponds to one level of the building, whose rendering is, similar to a photo, part of the table scene.

c) Closeup: the application provides basic visual feedback (ﬁngers/ blobs are denoted by colored circles and numbers).

the image is scaled larger and vice versa.

4.3 Examples and Discussion

The mentioned architectural design review applica-

tion shown in Figure 4 was developed for Messe

Frankfurt and shows its new exhibition hall. The ap-

plication consists of some transformable architectural

2D blueprints of several ﬂoors of the building and

a high quality rendered 3D view of the hall, which

changes according to the area that was touched in one

of the blueprints (with a third and fourth ﬁnger as vi-

sualized in Figures 2b and 4a). Several people can

move and zoom these plans simultaneously, but also

navigate through the model of the building.

As previously described, using blueprints for nav-

igation is one of the key features of this multi-touch

application, and additionally an imitation and digital

enhancement of an architect’s native work environ-

ment. Working on a touch table is just like they know

it from daily work, except that the application shows

a valid real time visualization of the whole building.

Since building plans are usually decoded with spe-

cial symbols and a high amount of information, which

more or less only professionals can read and imagine,

the user now can see and ”grasp“ the high resolution

rendering of the blueprint he is looking at.

With one or two ﬁngers the user now simply

moves the plan around. While stretching apart two

ﬁngers, the blueprints can be scaled and also rotated.

The tile showing the 3D visualization lies on the vir-

tual table next to the blueprints, and is also modeled

as a textured plane, and thus can be transformed the

same way as the blueprints. But whereas their textures

are static, the texture displaying the architectural visu-

alization has to be dynamically rendered every frame

in a ﬁrst rendering pass according to the movement

of the virtual camera. Architects can thereby move,

rotate and scale the 3D visualization like any other

plan. For the ease of use, all selected objects come to

the top in the order they have been touched.

As shown in section 4.2, the view is controlled by

using two additional ﬁngers. To distinguish between

various ﬂoors or areas and to select them for naviga-

tion, ﬁrst two ﬁngers of the left hand for instance can

grab a plan and select it, just like one would hold a

sheet of paper while writing on it. Then, a ﬁnger of

the other hand points onto the construction plan and

thus controls the camera’s position. Moving around

this ﬁnger and using a second ﬁnger of the right

hand deﬁnes the viewing direction (see Figures 2b

and 4). The GIS application follows the same interac-

tion metaphor with the exception that each map cor-

responds to a different area of the landscape instead

of different ﬂoors, and thereby the viewing height is

not set per ﬂoor plan but per touched texel.

People who used these applications accepted very

well the way of selecting objects, and it felt quite fa-

miliar to them. Also, none of them had further prob-

lems in remembering or using the described gestures.

But there are still some issues: although the paper-

sheet metaphor is doing ﬁne, the gestures’ usage has

to be explained. One also has to ”ﬁx“ a plan during

the whole navigation-process and has to work with

three or four ﬁngers. While taking just a look around

works well, doing more sophisticated movements is a

bit complicated: when the third ﬁnger points onto a

certain object on the plan and the fourth ﬁnger shall

move around it, there is a physical limitation of hand-

and ﬁnger-movement, making it hard to move around

an object while keeping it in focus. Thus, by provid-

ing a default direction based on the direction of move-

ment, typical tasks like walking along a road, which

might be of interest in the GIS application shown in

Figure 3, can be accomplished more easily.

Nevertheless, the result is a much better percep-

tion and understanding of otherwise arbitrary look-

ing data – not only for the design and planning stage

but also for later presentation. Both applications are

created and setup using X3D and the InstantPlayer

(Avalon, 2008) for rendering and interaction. Not

only all the plans but also the 3D view are modeled as

textured ”Plane“ geometries in X3D. This way, trans-

lation, rotation and scaling of plans and maps work

KNOWLEDGE AT YOUR FINGERTIPS - Multi-touch Interaction for GIS and Architectural Design Review Applications

391

like in common multi-touch applications as already

described in section 4.1. By using special extensions

of the standard X3D pointing sensor component, like

the new ”HypersurfaceSensor“ node that is described

in more detail in (Jung et al., 2008), it is thereby easy

to design and implement multi-touch applications.

5 CONCLUSIONS

In this paper we have presented two applications as an

example of how tabletop interfaces and multi-touch

can be used in 3D environments. We have exemplar-

ily explained some concepts of how to interact and

navigate in multi-touch environments, which might

also be useful for other areas like shipbuilding or ve-

hicle construction. We have shown that multi-touch

interaction can be both, interacting with more than

one or two ﬁngers simultaneously, and also working

collaborative in a multi-user scenario. Moreover, we

have also pinpointed some problems concerning blob

detection and tracking and how to overcome them.

Multi-touch techniques are very promising since they

allow people to seamlessly interact with what they see

by simply touching it. It feels natural, and can lead to

more sophisticated interaction principles. Therefore,

new ways of generating immersion are possible.

Future work will focus on a more intuitive interac-

tion for the application’s camera movement function-

ality. Furthermore, we would also like to integrate

generic gesture recognition for e.g. selecting and ﬁx-

ing a blueprint. As discussed, the user then does not

have to keep an almost static position of his ﬁngers

during the navigation process, but can have some kind

of initiation and relaxation phase as described in (Wu

et al., 2006) while working with the application.

REFERENCES

Agarawala, A. and Balakrishnan, R. (2006). Keepin’ it real:

pushing the desktop metaphor with physics, piles and

the pen. In CHI ’06: Proc. of SIGCHI conf. on Human

Factors in comp. sys., pages 1283–1292. ACM.

Apple (2008). iphone. http://www.apple.com/iphone/.

Avalon (2008). Avalon. http://www.instantreality.org/.

Dietz, P. and Leigh, D. (2001). Diamondtouch: a multi-user

touch technology. In UIST ’01: ACM symposium on

User interface software and technology, pages 219–

226, New York, USA. ACM.

Han, J. Y. (2005). Low-cost multi-touch sensing through

frustrated total internal reﬂection. In UIST ’05: ACM

symposium on User interface software and technol-

ogy, pages 115–118, NY, USA. ACM.

Han, J. Y. (2006). Multi-touch interaction wall. In SIG-

GRAPH ’06: ACM SIGGRAPH 2006 Emerging tech-

nologies, page 25, NY. ACM.

Jord`a, S., Geiger, G., Alonso, M., and Kaltenbrunner, M.

(2007). The reactable: exploring the synergy between

live music performance and tabletop tangible inter-

faces. In TEI ’07: Int. conf. on Tangible and embedded

int., pages 139–146, NY. ACM.

Jung, Y., Keil, J., Behr, J., Webel, S., Z¨ollner, M., Engelke,

T., Wuest, H., and Becker, M. (2008). Adapting X3D

for multi-touch environments. In Spencer, S., editor,

Proceedings Web3D 2008: 13th International Confer-

ence on 3D Web Technology, pages 27–30, New York,

USA. ACM Press.

Kim, J., Park, J., Kim, H., and Lee, C. (2007). Hci(human

computer interaction) using multi-touch tabletop dis-

play. Communications, Computers and Signal Pro-

cessing, 2007. PacRim 2007., pages 391–394.

Microsoft (2008). Microsoft surface.

http://www.microsoft.com/surface/.

Moscovich, T. and Hughes, J. F. (2006). Multi-ﬁnger cursor

techniques. In GI ’06: Proc. of the 2006 conference

on Graphics interface, pages 1–7, Toronto, Canada.

Canadian Information Processing Society.

Piccardi, M. (2004). Background subtraction techniques:

a review. Systems, Man and Cybernetics, 2004 IEEE

International Conference, 4:3099–3104.

Rahimi, A. (2001). Fast connected components on images.

Rekimoto, J. (2002). Smartskin: An infrastructure for free-

hand manipulation on interactive surfaces. In SIGCHI

conf. on Human factors in computing systems, pages

113–120, Minneapolis, USA.

Schieber, M. and Santello, M. (1996). Hand function: Pe-

ripheral and central constraints on performance. Jour-

nal of Applied Physiology, (6):2293–2300.

Shen, C. (2003). Ubitable: Impromptu face-to-face collab-

oration on horizontal interactive surfaces.

Shen, C. (2007). Interactive tabletops. In ACM SIGGRAPH

2007 courses, pages 40–53, NY, USA. ACM.

Shen, C., Vernier, F. D., Forlines, C., and Ringel, M. (2004).

Diamondspin: an extensible toolkit for around-the-

table interaction. In CHI ’04: Proc. SIGCHI conf. on

Human Factors in comp. sys., pages 167–174. ACM.

Touchlib (2008). A multi-touch development kit.

Web3DCon. (2008). X3D-Earth. http://x3d-earth.nps.edu/.

Web3DConsortium (2007). Extensible 3D (X3D).

http://www.web3d.org/x3d/speciﬁcations/ISO-IEC-

FDIS-19775-1.2/index.html.

Wu, M., Shen, C., Ryall, K., Forlines, C., and Balakrishnan,

R. (2006). Gesture registration, relaxation, and reuse

for multi-point direct-touch surfaces. In TABLETOP

’06, pages 185–192, Washington, DC, USA. IEEE.

GRAPP 2009 - International Conference on Computer Graphics Theory and Applications

392