Kinect Gestural Interaction for a Collaboration Game

João Paulo Prioste RodriguesUnder supervision of Rui Prada and Francisco Melo

Dep. Informatics, IST, Lisbon, Portugal

November 20, 2013

Abstract

Competition between players seems to be integrated in most video games in the video gameindustry. Yet, collaboration features can carry new game experiences that can interest some players.This thesis discusses some distinct Kinect and Wii Remote interactions, and relates them to if theycan enhance collaboration though a casual collaboration game called Geometry Friends. Naturalgestures, collaboration sense, gesture recognition, gesture exhaustion and space issues are some ofthe aspects compared between all interaction tested.

Keywords: Kinect, Nintendo Wii, Geometry Friends, Collaboration, Gesture Interaction,Gesture Recognition

1 Introduction

Video games are continuously evolving and sodoes the mechanisms to play them. New interac-tions with games, such as gestural interactions,brings new forms of how to play games and atthe same time bringing a lot of new revolutiongames to the market.

Many research has been exploring differentapproaches to play video games. But one featurestill remains in most video games: competition.Yet, competition is not the only option that videogames have. In collaboration games, the playersmay now play together to overcome challengesinstead of fighting each other.

Many new forms of playing video games wereintroduced in the past years, but it is stillunknown which is the best interaction mecha-nism to play them, and what are their limita-tions. Regarding this, can an interaction change,like changing a Wii Remote interaction to aKinect interaction, can also change the collab-oration feeling of Geometry Friends? Can nat-ural/playability factors affect this collaborationfeeling? For these problems, research aboutmeasures, problems, advantages and disadvan-tages that Wii Remote has when comparing with

Kinect was been explored. Different interactionstypes were implemented and tested using the Mi-crosoft Kinect and the Nintendo Wii Remote.All interaction tests were expected to gather in-formation about natural gestures, collaborationsense and playability. The final goal of this dis-sertation was to have a proper comparison tablethat manifest advantages and disadvantages ofeach interaction tested. All these interactions arecompared and discussed in this article results.

Figure 1: Players playing Geometry Friends

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2 Geometry Friends

Geometry Friends [1] is a casual game thatattains collaboration. The game runs in a two di-mensional world with a physics engine. The gameis played by two players, wherein each player con-trol a geometric shape, a circle or a rectangle.Each geometric shape has distinct possible ac-tions (Figure 2).

Figure 2: Geometry Friends game actions [1]

The circle is able to move sideways (left orright), jump and morph. Morphing the circleshape means increasing or decreasing its size,consequently increasing or decreasing its weight.Like the circle, the rectangle can also move toboth sides and morph, but the morph abilityworks differently. Using the morph ability, thesquare can stretch or contract its shape, alwaysmaintaining the same area.

Geometry Friends will have a set of differ-ent levels to challenge the players. In order tocomplete each level, players must catch all pur-ple collectible diamonds scattered across the level(an level example can be seen in Figure 3). SinceGeometry Friends is a collaboration game, mostdiamonds can not be possible to reach withoutthe cooperation of both players.

Figure 3: GeometryFriends Level Example

3 Related Work

3.1 Collaboration

Most of individual games have a winner anda loser, but in fact anyone has to lose in videogames. The collaboration games are based inthe approach wherein the only way to win is foreveryone to win. Still, collaboration features ingames can bring along many pitfalls. For exam-ple, the producer of the Legend of Zelda: FourSwords Adventures (2004), Eiji Aonuma, statedthat "although it is a game that four players haveto cooperate to solve puzzles, when you play it,you actually end up competing a lot more in thatgame than you do cooperating" [2]. Behavingcompetitively in a collaborative scenario is ex-actly what should not happen in a collaborativegame. To counter problems of competitiveness, acollaborative game should give players a certaintension, so that they can perceive both individu-ally utility and team utility [3].

In Wolfenstein: Enemy Territory (2003), aplayer can play the game as a solo soldier andeven win the game singlehandedly, if skillfulenough. That is what collaboration games shouldavoid. These games should prevent players towin games alone, thus giving the fell that theyneed his partners to win the game. After play-ers perform their actions, they must be able toidentify payoffs back to their decisions and thesepayoffs have to engage the players with satisfy-ing results (if performed correctly) so that theycare about the outcome of the game [3]. Thispitfall applies to all games, but it is particularlyimportant for collaborative games. If players donot care about the outcome of their choices, theywill not fell motivated to collaborate with otherplayers. ’Replayability’ of the game is anothermajor problem that can any game can have andcollaboration games can have and may influencescollaboration.

3.2 Gestural interaction

Video games are controlled by their inputcontrols (e.g. motion controls) and these in-put controls are controlled by the players. Thismeans that changing game input controls changesthe way the players play the game [4]. In fact,many different interaction have been explored inmany different Kinect and Wii games. Exam-

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ples of these are "touch" interactions [5, 6] likein Kinect Sports Menus; hand interactions [7–11]such as The Gunstringer [12] or Child of Eden;full-body interactions [13–16] as in Dance Cen-tral; and even, yet not commonly used, collabo-ration interactions [6] and object interactions [17]like in Geek Run 1. It is remarkable that all theseinteraction comes with a common statement ofplaying games naturally as one could imagine itto do it in real life. Players can add skills to theirnatural abilities, likewise they need to learn newskills in order to adapt to a given environmentin their human life. If new skills are to be in-troduced to players in order to play the game,these new skills should be naturally learned byplayers or otherwise they may refuse to learn thedesirable skill [18].

3.3 Kinect SDK vs OpenNI vs Wii

Many application exist for Kinect (a lotfrom www.kinecthacks.net), but many rely uponKinect SDK or OpenNI, so these ones were cho-sen for implementation. Both SDKs (KinectSDK and OpenNI) can track 20 joints of the hu-man body and can track two players simultane-ous (with a little lag times in the mere millisec-onds) [19]. However, OpenNI’s tracking systemrequires the players to hold a calibration pose un-til it can truly identify and begin tracking all theplayers body joints. This is not necessarily withthe Kinect SDK since it uses a specialized sys-tem that compares known images of humans withthe Kinect data received [20], allowing to quickly(less than a second in most cases) generate allbody joints. This is great in a walk in/walkout situation, witch could not the granted by theOpenNI, since if some player would walk out ofthe Kinect range, that player would have to dothe calibration pose again. Because of this, theKinect SDK was chosen for further implementa-tion and testing for all interactions done.

Comparing with the Wii platform [21], theKinect has an impressive body tracking algo-rithms and unique controller-free experience,which is great for casual gaming parties andworkouts since it also promotes natural user in-terface [22]. Yet, it requires a lot of room toplay[23], hard-core gamers may not feel so muchsatisfied with insufficient precision[24], an it re-

quires to do standing poses which can lead togesture exhaustion[25].

4 SolutionFor motion control gaming consoles, many

studies and research have already compareKinect and Wii [26][27], as is here done. Butmost comparisons accord to some specific inter-action and most of all is a single-player research.What we want to research here, is to find out ifKinect can be compared with Wii in collabora-tion games.

After investigating how several researcherssolved their motion control [10, 14, 17, 26, 28, 29],collaboration issues [3, 6], it was developed sev-eral interaction for Kinect to compare with thecurrently Wii interaction.

4.1 Four different Kinect interactionsto compare vs Wii

Since non-button interactions have arrived,game developers start wondering about differ-ent interactions that can be used [12]. UnlikeWii Remote, Kinect offers a non-linear controlbase. The Wii Remote however, needs to nec-essarily use a Wii Remote control to play Wiivideo games. This feature open a wide varietyof possibilities resulting in proposing four differ-ent Kinect Interaction proposes to compare withthe current Wii interaction. The Wii Interactionwill not be changed from the currently GeometryFriends Wii interaction, that was already imple-mented.

4.1.1 Hand Interaction Proposed Solu-tion

Hand Interaction is the more basic form of in-teraction with the Kinect platform. Since handsare the basic utility for human life, it can de-liver great utility for gaming. From the existingKinect games that uses hands as the main or onlyinteraction for the game, it is remarkable thatmost of them use both hands to interact insteadof just one. Examples of these games are Childof Eden or The Gunstringer that makes use ofboth hands.

Regarding these studies .[8, 9, 12, 16], it wasdecided to use similar gesture as The Gunstringer

1http://geekrun.ch/ (last accessed November 19th, 2013)

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game have. Seeing The Gunstringer gestures ex-ample, it is remarkable that the right hand isused to do the more active actions while the lefthand is used to do moves that are less used.While the left, right and jump move (all doneby the left hand) are the less used actions in TheGunstringer, in Geometry Friends these are themost used. For this reason, the left, right andjump move was considered by the right hand todo it, instead of the left hand.

Figure 4: Hand Interaction Movement

Figure 5: Hand Interaction Transformations

4.1.2 Full-Body Interaction Proposed So-lution

Instead of tracking one particular point suchas hands, full-body interaction is more focusedon gesture using different body parts as one ges-ture. The proposed gestures for square and circlemovement were the player’s head and body (Fig-ure 6), since this gesture have been greatly eval-uated in many 3D environment applications [15,16]. Since movement did not require player’shands, both hands was used for transformations.

Figure 6: Full-Body Interaction Movement

Figure 7: Full-Body Interaction Transformations

4.1.3 Collaboration Interaction ProposedSolution

The collaboration interaction was meant tobring gestures of both players together as one.However, the movement of both circle and thesquare was proposed to be single based gesture,stating the important role of ’who I am’. Thedistance between players was used for the squaretransformation (Figure 8). Collaboration withhand gestures was used for the ball enlarge andjump.

Figure 8: Collaboration Interaction Square Trans-formations

Figure 9: Collaboration Interaction Enlarge Trans-formations

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Figure 10: Collaboration Interaction Full-Body Pro-posal

4.1.4 Tangible Interaction Proposed So-lution

The tangible interaction uses the Kinect as aform of bringing life to real life objects. Althoughthese objects have no controller chip added tocommunicate with Kinect, they represented themain interaction with the Kinect game. Thechoice of which object to choose was rather easyand logical, as Geometry Friends has alreadypolygons to choose from. Yet, since the poly-gons in the game use transformation actions, thiswould be rather hard to transpose it as real lifeobject actions, especially for the square trans-formations. With the help of two green rectan-gle objects (thus each rectangle representing halfof the square), the square transformation prob-lems were done for. Repeating the procedure ofthe collaboration interaction, the closer the twosquares are (the minimum would be at clashingboth rectangles) the thinner the square in thegame would be and vice-versa.

Figure 11: Circle Interaction

Figure 12: Square Interaction

4.2 Menu Interaction Proposed Solu-tion

As said before, the menus were evaluated too.Geometry Friends has two different menus, onewith line words: main menus; and another usingbig buttons: level select menus. In the level se-lect menus it was used the menu navigation of theKinect Sports, while the main menus was used asthe Dance Central menu navigation. There wasa little gap in the collaboration interaction al-though, where the second player used its righthand as a select action instead of the KinectSports and the Dance Central select actions. Inthe objects interaction, the circle was the cursorinstead of the player right hand.

5 Evaluation

Participants will be randomly chosen for eachproposed solution, but since this will be tested fora collaboration game there will be needed twoparticipants for each test. The mean of all testsgathered was around six tests for each interac-tion, gathering 12 participants for each interac-tion. The only restriction was that each partic-ipant can only be part of one of the proposedsolutions. This way, the participants will notconfuse or compare the controls from a previousexperience. Each test conducted to the partici-pants had three sections. There was a natural-interaction section, a gesture recognition accu-racy section, and a collaboration sense section.Every step of the participants was recorder, us-ing video capture of the game using Fraps 2 andan another video capture with a normal camerato record the players entire gameplay.

2http://www.fraps.com/ (last accessed November 19th, 2013)

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The first section consisted in asking partici-pants to perform the movements as they felt wasmore natural. This was done before providinginstruction to the participants. After this sec-tion, the participants were informed about thegame controls, thus playing the rest of the gameas proposed. At the end, a questionnaire wasmade to participants in order to gather infor-mation about playability aspects, but mostly ofhow was his/her sense of collaboration about thegame. The playability questions gathered infor-mation about gesture recognition feeling, diffi-culty of the gestures, the environment physicalspace problems and gesture exhaustion too. Inthe other hand, collaboration questions gatheredinformation about the collaboration sense of theinteraction and aspects of the collaboration senseof the game itself as pleasure of winning, the re-peatability of the game, the feeling of being losingor the need of playing alone.

Before the game itself starts, players have topast through the menus to start playing. So,the menus interactions were evaluated too. Yet,they were not included in the natural-interactionsection, meaning that players were aware of themenu instructions when navigating the menus.Furthermore, there will be some additional ques-tion in the questionnaire regarding the menu in-teraction difficulty and collaboration sense.

6 Results

6.1 Menus Interaction results

Away from the in game experience, this sub-section describes the results of the menus experi-ence. As earlier explained in this document, thisexperience will not count with a natural factorand only one questions was done to evaluate thecollaboration sense factor. What is to be morejudge here is the playability factor, which gath-ers the Kinect fidelity to recognize gestures andthe gesture difficulty relatively this last one withthe time spent to complete the task, the numberof errors committed and finally some few ques-tion answering results. For last, it is to note thatthe hand interaction and the full-body interac-tion used the same menu interaction which thenwill be evaluated together. Table 1 resume all themain menu interaction results that were tested.

Table 1: Summary of all Main Menu Results

Comparing the different menus interaction,the one that achieve the most collaborationsense factor was the collaboration gesture witha 83,33% value. This value was expected sincethis interaction is the only one that needs a sec-ond players to navigate though the menus. Apartfrom this menu interaction, all others revealeddistinguishable values, almost in 10% intervals,with 70,83%, 62,88% and 50% corresponding re-spectively to the Wii interaction, the hand/full-body interaction and the object interaction.

Considering now the playability factor, theKinect accuracy revealed a perfectly 100% ges-ture recognition from the collaboration menuinteraction, and a 98% for the hand/full-bodymenu interaction. For the gesture difficulty met-ric, only the hand/full-body menu interaction re-vealed some side downs for some players indicat-ing that most players understand and used thisinteraction perfectly, but some of them (mostly,the female players) had some hardship to fullypass the menu navigation. Yet, it it to remarkthat this only happened in the main menu in-teraction, since all different interactions revealeda perfectly and remarkable 100% value for thelevel select menus (the reason why Table 1 onlygather the main menu results). With all down-sides from the hand/full-body main menu inter-action, the gesture difficulty meter estimate a49,01% evaluation. The object interaction hasonly tested by female players, and thus reveals avery disappointing evaluation. This may not bebecause of the sex factor, but it clearly revealsthat was the worst main menu interaction, sinceeven the hand/full-body interaction had betterresult. The collaboration interaction was the bestscored here with a 81,94% evaluation, winningthe 76,39% evaluation of the Wii interaction.

With an overview of all menu interaction re-sults, it clearly highlight the collaboration mainmenu interaction with an outstanding 87,15%evaluation, winning against the Wii interactionwith a 7% difference. The hand/full-body and

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the object interaction reflected a rather bad in-teraction to choose with many playability prob-lems. Nevertheless, it is not to forget that if themain menu interface has changed to a big buttonsinterface like the level select menus, all interac-tions are evaluated equally in playability (all have100%). This will put the hand/body, object andWii interaction in closer terms. Yet, the collabo-ration interaction would always have the trophyhere, because of the outstanding difference in thecollaboration sense.

6.2 ’In game’ Results

This subsection will finalize the results sec-tion with an overall fun factor results gatheredfrom every results indicated in the past subsec-tions. To make things clearly and understand-able, table 2 relates all overall results for everyfactor evaluated. The fun factor has separatedfrom the three main factors of this dissertation(natural, playability and collaboration sense) sothat is does not influence the dissertation objec-tive, making it clear the final percentage resultswithout the fun factor.

Table 2: Summary of all Factor Results

From this table, we can see that the object in-teraction has the better final result with or with-out the fun factor. This was expected since thisinteraction is especially designed for only thisgame particularly. Yet, without the fun factor,the difference is not so great as expected, sinceonly a 2,54% difference from the Wii interactionand a 3,76% difference from the full-body inter-action is detected. It fact, the object interactiononly wins here, because of the high natural factor,since the playability factor is the worst betweenall interaction and the collaboration sense is thesecond worst. In the same theory, the Wii inter-action achieves the second place though the goodnatural factor too.

Comparing between the different Kinect in-teractions tested, the full-body interaction is

clearly the winner in here. Yet, with the fun fac-tor the full-body interaction became very close tothe collaboration interaction (1,74% difference).The lost in the collaboration sense (not expected)reveals less reasons why this interaction should behighlighted, since it has expected with the collab-oration need of the interaction gestures, the col-laboration interaction will come up with a bettercollaboration sense. The hand interaction wasexpected to achieve a better playability factortoo, however it reveals a lost when comparingwith the full-body interaction as the collabora-tion interaction lost in the collaboration sensefactor. Furthermore, the full-body interaction re-vealed a much better natural than the other twoKinect interactions. It is also remarkable thatthe full-body interaction has the better playabil-ity and collaboration sense evaluation from allinteraction tested.

In overall, the object and the Wii interactionrevealed a good and better natural factor thanall Kinect interaction done, and as concluded inthe natural sub-chapter, the Kinect console re-inforcement of many natural interactions possi-bilities are hard (yet, surely not impossible) toenhance in a non real-life environment or non-avatar control. Yet, only an nearly 8% differencediffers from the full-body interaction and the Wiiinteraction. If a natural factor was not evaluatedhere, the Kinect full-body interaction would havethe best score here, and even the object interac-tion would be surpassed by all other interactions.However, with the fun factor appreciation, thefull-body interaction have a clearly downside hereto the collaboration, object and Wii interactionswhich both had superior evaluation. Finally, itis clear in table 2 that the hand had the worstcollaboration sense factor and fun factor of allinteractions.

7 Conclusions

To make a good structure of a collaborationgame is not an easy task. Even if the objective ofthe game is for everyone to win, it does not meanthat a collaboration feeling is achieved. It hasrevealed that the Geometry Friends can achievetrue collaboration sense and this sense can varywith an interaction change. It was not found adirect interrelation between the playability andthe collaboration sense of the different interac-

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tion, meaning that problems like gestures recog-nition and gesture exhaustion does not have a di-rectly interrelate with collaboration sense of theplayers neither have a true interrelate with funtoo. Playability is a factor to surely be takenseriously it the gestural and motion control in-dustry, but not the only important feature. Forexample, the introduction of the second playeruse for the menus navigation, surely has helpedand persuasive the players in the gesture diffi-culty evaluation. It can not be forgotten thatthe idea of collaboration or cooperation modesin games are to reach a better performance thanin a single-player mode, not equal or worst.

A change of the interaction can truly changethe perspective about the game. Yet, partici-pants proclaimed difference problem issues aboutthe game, such as being to hard, taking to muchtime to complete a level, repeatability, etc. It wasalso acknowledged that players needed more timeto accustom to the Geometry Friends physics andoverall gameplay with the Kinect interactions.This means a proper change in the game inter-action should come with a change of the gameitself. If the game is non real-life environment ornon-avatar control, players tend to have difficul-ties to get used to the virtual game. This rela-tionship can be seen when comparing the natu-ral results with the gesture difficulty of the threeKinect skeleton interactions. This concludes thatsome more tutorials and difficulty changes shouldbe evaluated to be changed when changing thegame interaction to a gestural interaction.

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