This paper introduces the concept of procedural content generation for physical rehabilitation. In this initial study a ski-slalom game is developed on the Wii platform that procedurally places the gates of the game according to player performance. A preliminary game evaluation study is conducted on patients with injured legs and showcases the efficiency of the procedural gate generation mechanism tailoring the game difficulty to match rehabilitation goals. The study also validates certain usability aspects of the patients.peer-reviewe

2010 Workshop on Procedural Content Generation in Game

Dimovska, Dajana

Jarnfelt, Patrick

Selvig, Sebbe

Yannakakis, Georgios N.

OAR@UM

Towards Procedural Level Generation for RehabilitationDajana DimovskaCopenhagen Game CollectivePilestræede 45, 1Copenhagen, Denmarkdajana@cphgc.comPatrick JarnfeltCopenhagen Game CollectivePilestræede 45, 1Copenhagen, Denmarkpatrick@cphgc.comSebbe SelvigCopenhagen Game CollectivePilestræede 45, 1Copenhagen, Denmarksebbe@cphgc.comGeorgios N. YannakakisIT University of CopenhagenRued Langgaards Vej 7Copenhagen, Denmarkyannakakis@itu.dkABSTRACTThis paper introduces the concept of procedural content gen-eration for physical rehabilitation. In this initial study aski-slalom game is developed on the Wii platform that pro-cedurally places the gates of the game according to playerperformance. A preliminary game evaluation study is con-ducted on patients with injured legs and showcases the effi-ciency of the procedural gate generation mechanism tailoringthe game difficulty to match rehabilitation goals. The studyalso validates certain usability aspects of the patients.Categories and Subject DescriptorsI.2.1 [Artificial Intelligence]: Applications and ExpertSystems—Games; H.1.2 [Models and Principles]: User— Machine Systems—Human factorsKeywordsProcedural level generation, Wiihabilitation1. INTRODUCTIONThe Nintendo Wii game console is the best-selling con-sole available today [3] utilizing physical interaction via con-trollers such as the Wii Remote and the Wii Balance Board.TheWii Balance Board allows game interaction through footpressure and body weight balancing on the board. Thesefeatures push video game design to incorporate body move-ment as a key control modality. In particular, Wii Sports(2006) constitutes of different sport games such as boxing,bowling, golf, and tennis in which the player’s avatar can becontrolled via a physical motion, e.g. bowling using the WiiRemote. In We Ski (2008), the player is skiing by control-ling the pressure on the Wii Balance Board. On that basis,Wii technology gives players new motivations to immersePermission to make digital or hard copies of all or part of this work forpersonal or classroom use is granted without fee provided that copies arenot made or distributed for profit or commercial advantage and that copiesbear this notice and the full citation on the first page. To copy otherwise, torepublish, to post on servers or to redistribute to lists, requires prior specificpermission and/or a fee.PCGames 2010, June 18, Monterey, CA, USACopyright 2010 ACM 978-1-4503-0023-0/10/06 ...$10.00.themselves both mentally and physically. This has inspiredmany hospitals and rehabilitation centers in the USA [14] touse existing Wii physical games for rehabilitation purposes.The use of Wii technology for physical rehabilitation is alsoknown as Wiihabilitation.Existing commercial Wii games, however, are not designedfor rehabilitation which, in turn, may over-challenge peoplewith partly disabled extremities. Moreover, different pa-tients have different rehabilitation goals to reach and a fixedWii game does not tailor the gameplay for the patient tomeet specific rehabilitation milestones. A solution to thoseusability problems would be the development of an adap-tive Wii physical game that can adjust game parameters tomatch the patient’s physical ability level.The main aim of this initial study is the development andevaluation of an adaptive Wii game for rehabilitation, whichcan procedurally generate content in real-time and, further-more, provide effective training to each patient given thelevel of her physical ability. For this purpose, an adaptiveski-slalom game played with the Wii Balance Board is devel-oped and tested on patients of a rehabilitation center. Thegame procedurally generates the placement of the slalomgates based on heuristics of player performance. The pre-liminary user study presented here reveals that the game ad-justs the ski-slalom track effectively to match patient skills.This paper is novel in that it introduces the concept of pro-cedural content generation (PCG) and player modeling forphysical rehabilitation utilizing the Wii Balance Board.2. BACKGROUNDResearch in physical rehabilitation [1, 5] has shown thatexercises for patients recovering from physical disabilitiesand injuries are more effective when the exercises are in-tegrated within a game. Patients get more engaged andmotivated through a game environment and they tend toforget the pain caused by and the dull repetitive nature ofrehabilitation exercises. Physical interactivity, portability,low-cost and commercial availability make the Wii Remoteand Wii Balance Board the ideal controllers for developingphysical rehabilitation games [8].With the popularization of physical interactive game plat-forms considerable research has been conducted for captur-ing player immersion [11], engagement [2] and player affec-tive states [7, 16] in physical interactive games. On theFigure 1: A screenshot of the ReSkii game.other hand, very few studies investigate the design of adap-tive mechanisms for maximizing player experience in suchaugmented-reality game platforms [16].PCG in games has been used for a wide variety of pur-poses including terrain [6, 10] and space [9] generation, en-vironmental modeling [13], and race track [15] generation.However, to the best of the authors’ knowledge there hasbeen no study investigating procedurally generated contentfor Wii games and its impact to the rehabilitation process.3. THE RESKII GAMEInterviews were conducted with physiotherapists to assistthe design of the Wiihabilitation game. The design of thefinal test-bed game is based on the physiotherapists’ knowl-edge and observations about their patients recovering fromleg injuries. The following research hypotheses were formedon the basis of those interviews:• H1: Foot pressure performance is lower on injured orpartly-disabled legs (e.g. a patient with a right legdisability is expected to miss more right turns in aski-slalom game).• H2: Foot pressure performance is increased in easiertasks (e.g. easier right ski-slaloms will improve thescore of a player with a right leg injury)• H3: Gradually increasing the difficulty of a game taskwill force the player to exercise with the problematicleg thereby accelerating rehabilitation.In order to validate the stated hypotheses and demon-strate the impact of adaptation and PCG on rehabilitationwe developed a test-bed game. It was our aim that thedeveloped PCG game will not only accelerate the rehabili-tation process but also make exercises more fun to execute.The game should be able to recognize the patient’s playingstyle, identify the difficulties faced during the game and pro-cedurally generate a level that forces the patient to train herinjured leg(s) better.Skiing games are appropriate for both balance and persis-tence training (We Ski is already used in several rehabilita-tion centers globally) while theWii Balance Board is suitablefor engaging the lower part of the patient’s body in physicalactivity. Based on the above-mentioned design principles,we developed the ReSkii game (see Figure 1). ReSkii is asingle level ski-slalom game (see Figure 1) implemented onthe 3D engine Unity3D and played with the Wii BalanceFigure 2: The ReSkii level from top-down view.Board controller. Vital elements of the level are generatedprocedurally in real-time (during gameplay). In this initialstudy the position of the right (red) and left (blue) gatesare procedurally placed according to a player performancefunction.The performance function is built on the player motiontrack during gameplay. The ski-motion tracks are gener-ated by applying left and right leg pressure forces on theboard controller. In this initial study a simple evaluationof the player’s performance on left and right leg is imple-mented, that is based on the success of reaching left andright gates, respectively. The performance measure, namedpenalty score, is calculated every time the player passes through(successful pass) or next to (unsuccessful pass) a gate:P dir ={0, if successful passEdir, otherwise(1)where dir = {left, right} and Edir = √∆x2 +∆y2 +∆z2is the euclidian distance from the passing point to the closestpole of the dir (left or right) gate. The penalty score, P , isa value calculated independently for right and left gates andrepresents the success of the player’s skiing ability on eachleg which furthermore yields the basis of the PCG mech-anism proposed in this paper. Since the gates are placedalternately, left and right from the center of the track, per-sistent pressure force is needed on the leg in the directionof the gate in order to reach it. Therefore, the P values forleft and right gates encapsulate the players’ ability to havepersistent pressure on the corresponding legs; if a player hasa physical impairment on a leg, the penalty score for thatleg should therefore be higher. Alternatively, other perfor-mance measures could be investigated that would considerthe detailed motion patterns during gameplay (e.g. shiftingthe center of gravity from left to right and vice versa).The level terrain is generated once at the beginning ofthe game using four-point Catmull-Rom Splines which cre-ate the zig-zag pattern seen in Figure 2. Gates are placedat a fixed offset of three meters along the x-axis from thegenerated spline. The level is divided into a number of sec-tions equal to the number of track turns (Figure 2) withinwhich the P values are calculated and aggregated. The di-vision of the level into sections is essential for PCG sincethe zig-zag nature of the track does not allow visibility ofthe gate placement procedurality. At the end of each sec-tion the game dynamically adds alternating gates along thenext section with a varying distance among them. In partic-Figure 3: Control subject playing ReSkii.ular the P left value calculated during the previous sectionwill determine the distance Dleft between the right and leftgates while the corresponding P right value will determinethe distance Dright as follows:Ddir = P dirN (dmax − dmin) + dmin (2)where dir = {left, right}; P dirN is the normalized P valuethat lies between 0 and 1 and dmax and dmin are respectivelythe maximum and minimum distance offset values — dmaxequals 15 meters and dmin equals 5 meters in this paper.Equation 2 indicates that the distance offset between leftand right gates along the y axis is increased when the playeris performing poorly on the left and right gate respectively.On the contrary, good performance leads to y-offset (Ddir)decrease. This initial adaptation rule generates personal-ized gameplay via keeping the game challenge within theabilities of the patient. Furthermore, the matching betweenchallenge and player skills aims at maintaining the gameplaywithin the flow channel [4].4. USER STUDYTwo patients, a 66-year old male weighing 78 Kg and a60-year old female weighing 92 Kg, have participated in theexperiments reported here. The subjects played two gameseach. According to their physiotherapist, both subjects hada right leg injury at the time of the experiment. Moreover, amale subject, aged 25 and weighing 88 kg, with great phys-ical health (being an athlete and physiotherapist) played alevel of ReSkii and was used as the baseline/control subjectof this study. The player’s position and the raw Wii BalanceBoard data (pressure force on all four sensors of the board)are stored at a frequency of 60 Hz during gameplay. Thetime and the penalty score, P , are stored every time theplayer passes through or misses a gate.Results obtained from all four patients and the controlsubject are summarized in Figure 4 and Table 1. Figure 4depicts the ski routes and the penalty score over time for allsubjects that participated in the experiment while Table 1presents a number of statistical features for all games played.It is apparent that both patient subjects (in both gamesplayed) generate a higher penalty score for the right thanfor the left leg; not surprisingly this observation is consis-tent with their reported physical disability (right leg injury).On the contrary, the corresponding P values for the controlPenalty Left GatesPenalty Right GatesStartGoalTrack PlayedControlSubjectSubject AGame 1Subject AGame 2Subject BGame 1Subject BGame 2Figure 4: Track overview and penalty scores overtime for all experiment participants. Blue and redsquares indicate left and right gate.subject are close to zero. Moreover, as seen in Figure 4 highP values result to the generation of easier (smoother) levelsthrough gate placement. Low P values, on the other hand,generate rather difficult and noisy ski tracks.By looking at the figures of Table 1 one can deduct thattest subjects stand more on their non-problematic (left) legindicated by the average center of their gravity (G) through-out the game. It is also worth noticing that the two subjectsstand more on the left leg the more they play since G ishigher at the second game played.Surprisingly, it appears that both subjects put more pres-sure on average on the balance board via their partly-disabledright than via their healthy left foot on both games played.It was observed, however, that patients found it hard to keepa constant pressure with their injured leg; thus, in order toplay the ReSkii game well they had to press harder withthat leg within shorter time periods. That playing behav-ior yields higher maximum and average pressure recorded onthe right leg. The fluctuations of the pressure forces are alsoindicated by the higher standard deviation of the pressureforce values (σ{F}) on the right Wii Balance Board sensors.Testing of our research hypotheses formed in section 3 re-veals that H1 is validated through the increased penalty val-ues on the injured leg of the patients. H2 appears to be validsince playing performance increases with respect to the or-der of play for both patients; however, learnability and gamecontrol familiarity effects might also be present and need tobe investigated in a future study. Finally, H3 needs to bevalidated with additional experiments over longer periods oftraining.5. DISCUSSION AND CONCLUSIONSThis is an initial study investigating the interplay betweenprocedural level generation and player profiling on physicalWii games developed for physical rehabilitation. The adap-tive mechanism proposed adjusts elements of a skiing game(ReSkii) for the purpose of personalizing the rehabilitationprocess via PCG. For the gameplay experience to be per-sonalized, patient characteristics are captured during playand guide PCG. Results indicate that it is possible to buildsimple, yet successful, computational models of physical in-teraction of patients with partial leg disabilities and proce-Table 1: Subject (control, A and B) statistics of pressure force (F ), penalty score (P ) and center of gravity(G). Maximum, average and standard deviation of F and average P are calculated independently for eachleg; the average center of gravity (G) is a value that lies between −1 and 1 the sign of which indicates theplacement of the center of gravity: left (−) or right (+) leg.Control Subject A Subject BGame 1 Game 1 Game 2 Game 1 Game 2Left Right Left Right Left Right Left Right Left Rightmax{F} (Kg) 64,34 53,31 52,50 58,95 54,65 55,87 52,54 62,04 51,59 54,95F (Kg) 38,89 42,45 34,38 40,08 35.29 39,08 45.02 44.30 44.17 45.12σ{F} (Kg) 3,33 3,29 3,74 3,93 5,48 5,59 1,99 2,04 1,96 2,01P 9.73 2.67 35.57 103.12 21.70 40.92 31.94 83.26 17.68 57.65G −0.19 −0.07 −0.28 −0.35 −0.43durally generate appropriate game levels for their needs.To increase the effectiveness of the training, patients’ phys-iological responses such as heart rate and skin conductancemay be measured and embedded as additional inputs to thelevel generation mechanism. Moreover, to keep the levelsinteresting and the game immersive for the player over ex-tended periods of rehabilitation, more elements can be addedor procedurally generated in the game such as narrative,level geometry, jumps and multiple-track choices. More so-phisticated algorithms for modeling playing behavior (e.g.via patient preference learning [12]) as well as richer PCGmechanisms (e.g. via ski track evolution [15]) will be inves-tigated in future studies and integrated in the ReSkii game.Such patient models could identify more types of physicaldisability a patient might have, measure the patient’s per-formance based on multiple gameplay characteristics andthereby evaluate better the “goodness” of content that isprocedurally generated within the game level.6. ACKNOWLEDGMENTSThe authors would like to thank the physiotherapists,Hannes Brunner and Majbritt Asmussen, of the T˚arnby Re-habilitation Center, Copenhagen, Denmark, and all subjectswho participated in the experiments.7. REFERENCES[1] J. Azpiroz, F. A. Barrios, M. Carrillo, R. Carrillo,A. Cerrato, J. Hernandez, R. S. Leder, A. O.Rodr´ıguez, and P. Salgado. Game Motivated andConstraint Induced Therapy in Late Stroke with fMRIStudies Pre and Post Therapy. In Proceedings of the2005 IEEE Engineering in Medicine and Biology 27thAnnual Conference, 2005.[2] N. Bianchi-Berthouze, W. W. Kim, and D. Patel.Does body movement engage you more in digital gameplay? and why? In A. Paiva, R. Prada, and R. W.Picard, editors, Affective Computing and IntelligentInteraction, Second International Conference, ACII2007, Lisbon, Portugal, Proceedings, volume 4738 ofLecture Notes in Computer Science, pages 102–113.Springer, 2007.[3] S. Carless. Independent Games Sales: Stats 101.Game Developer Magazine, 2009.[4] M. Csikszentmihalyi. Flow: The Psychology ofOptimal Experience. New York: Harper & Row, 1990.[5] J. E. Deutsch, M. Borbely, J. Filler, K. Huhn, andP. Guarrera-Bowlby. Use of a low-cost, commerciallyavailable gaming console (wii) for rehabilitation of anadolescent with cerebral palsy. Physical Therapy,88(10):1196–1207, 2008.[6] D. Ebert, K. Musgrave, D. Peachey, K. Perlin, andS. Worley. Texturing and Modeling: A ProceduralApproach. Morgan Kaufmann, 3rd edition edition,2003.[7] P. Jarnfelt, S. Selvig, and D. Dimovska. Towardstailoring player experience in physical wii games: acase study on relaxation. In Advances in ComputerEntertainment Technology, volume 422 of ACMInternational Conference Proceeding Series, pages328–331. ACM, 2009.[8] Y. Jeffrey and G. T. C. Nicholas. Using games toincrease exercise motivation. In Future Play ’07:Proceedings of the 2007 conference on Future Play,pages 166–173, New York, NY, USA, 2007. ACM.[9] M. Nitsche, C. Ashmore, W. Hankinson,R. Fitzpatrick, J. Kelly, and K. Margenau. Designingprocedural game spaces: A case study. In Proceedingsof FuturePlay 2006, London, Ontario, 2006.[10] J. Olsen. Realtime procedural terrain generation.Technical report, Oddlabs, 2004.[11] M. Pasch, N. Bianchi-Berthouze, E. M. A. G. vanDijk, and A. Nijholt. Immersion in movement-basedinteraction. Springer Verlag, 2009.[12] C. Pedersen, J. Togelius, and G. N. Yannakakis.Modeling Player Experience for Content Creation.IEEE Transactions on Computational Intelligence andAI in Games, 2(1):54–67, 2010.[13] P. Sweetser and J. Wiles. Scripting versus emergence:issues for game developers and players in gameenvironment design. International Journal ofIntelligent Games and Simulations, 4(1):1–9, 2005.[14] L. Tanner. Doctors use Wii games for rehab therapy.Associated Press, 2 2008.[15] J. Togelius, R. D. Nardi, and S. M. Lucas. Makingracing fun through player modeling and trackevolution. In G. N. Yannakakis and J. Hallam, editors,Proceedings of the SAB Workshop on AdaptiveApproaches to Optimizing Player Satisfaction, pages61–70, Rome, 2006.[16] G. N. Yannakakis and J. Hallam. Real-time GameAdaptation for Optimizing Player Satisfaction. IEEETransactions on Computational Intelligence and AI inGames, 1(2):121–133, June 2009.View publication stats

Towards procedural level generation for rehabilitation

https://www.um.edu.mt/library/oar//bitstream/123456789/22966/1/Towards_procedural_level_generation_for_rehabilita.pdf

Towards procedural level generation for rehabilitation

Abstract

Similar works

Full text

Available Versions

OAR@UM