Workbench
MichalKoutekandFritsH. Post
DelftUniversityofTechnology
email: fm.koutek,f.h.postg@cs.tudelft.nl
Abstract. Inthispaperwepresentadierentviewofuserinteraction
withvirtualworlds.Westartfromthequestion:howcanwebringmore
naturalobjectbehaviorintovirtualenvironments?Currently,objectsin
VRapplications often behave ina veryun-natural way. Incorporation
of physical laws inthe virtual environment, together with monitoring
naturaluseractionsandbehaviorisdesirable.Wepresentsomeprinciples
ofphysicallymorerealisticbehaviorofvirtual objectsandasetofuser
input techniques suitable for semi-immersive VR devices such as the
ResponsiveWorkbench.Weintroducespringsasanewtoolforassisting
directmanipulationofobjectsinVEs.
1 Introduction
Inuser interactionwith virtualworlds,consistentand realisticbehaviorofob-
jectsisveryimportant.Wewantobjectstorespondinanaturalandpredictable
waytoouractions.ButusuallyVEobjectsareweightlessandunsubstantial,and
theymovewithoutfrictionorinertia;thisleadstoaltogether'unphysical'behav-
iorandunpredictableresponses,especiallyinsemi-immersiveenvironmentssuch
as the Responsive Workbench (RWB), where real and virtual worlds co-exist,
andshould followthesamenaturallaws.
Absenceofweightandsubstancemaysometimesbedesirablewhileinspecting
anobjectoryingthroughanenvironment.Butespeciallyinmanipulationtasks,
mechanicallyrealisticbehaviorcanhelptoachieveconsistencyandpredictability.
Awaytoprovidemechanicalresponsesfromavirtualworldisthroughhap-
tic devices [8{11].But haptic devices usually giveforce feedback through an
intermediarydevicewithalimitedrange,andtechnologyforfree-eldhapticin-
teraction,whereobjectsmaybetouchedanywhereinspace,stilldoesnotexist.
Therefore,wewanttoavoidtheuseofaforce-basedhapticinterface.
We will look for a limited set of physical properties and laws of behavior
to makeuserinteractionmorepredictable and intuitive.Wewill introducethe
conceptsofforce,inertia,gravity,contact,surfacefriction,anddampingintothe
virtualworld.Thus, wewill havetoprovideavisualinterfaceto replacedirect
force input.Wewilldothisbytheuseofspringsattached toobjects,basedon
thefollowingassumptions:
{ alinearrelation offorce with spring compression /extension isintuitively
understood and visualized by the spiraling shape of a spring. Thus, even
withoutexertingrealforce,auserhasanintuitivenotionoftransforminga
changeofspringlengthtoaforce.
dependent. Specic mass of objects should be user specied. A massless
worldcanalwaysbecreatedbysettingallmassesto zero.
{ the table top of the RWB provides a natural ground plane for objects at
rest.
{ stabilityisintroducedbyfrictionanddamping,reducingexcessiveeectsof
inputactionsonobjects,andreducingundesiredoscillations.
{ physicalcontactofobjectsisintuitivelyequivalentwithgeometricintersec-
tion,andsound canbeusedtoprovidecontactfeedback.
Wewillpropose aset ofdynamic interactionprimitiveswith objects,based
on object selection and actions such as lifting/dropping, pushing/pulling, and
throwing/catching. We hypothesize that dynamics in interaction will provide
intuitivelyconsistentandpredictablebehavioroftheobjectsinthevirtualworld,
andthatinteractivemanipulationwillthereforebeeasierandmorenatural.We
will demonstrate the utility of theconcepts from an initial implementation of
a micro-world on the RWB table top bounded by walls around, consisting of
spheresandboxeswhichcanbemanipulatedusingspringsasmanipulatorsand
dynamicsto governbehavior.
Inthis paper, we rstpresentrelated work and providean overviewof 3D
interactiontechniquesin VR.Next,wedescribesomepossibleinteractiontech-
niquesfortheResponsiveWorkbench.Weconcentrateontheuseofdynamicsin
user'sinteractions.Onthecurrentimplementation ofamicro-worldwedemon-
strate thesuggested dynamic interaction in VEs. Finally, we present areasfor
future work.
2 3D Interaction Techniques
Interactionplaysaveryimportantroleinvirtualenvironments.Muchhasbeen
published aboutinteraction techniques in VR but the questfor truly intuitive
andnaturalinteractiontechniques isstillgoingon.
2.1 Related Work
Wehavestudied interactiontechniquesused in fullyimmersive(CAVE,HMD)
and semi-immersivevirtual environments (RWB). Surveys of interaction tech-
niques for VR workbench can be found in [1{3,5]. New methods for remote
translations in immersive VE, especially the CAVE, are presented in [4], but
someofthesemethodscanbeusedontheWorkbench aswell.Inthispaperwe
will not discuss navigationproblems and we will concentrateon selection and
directmanipulation ofobjects.
Theproblemsofmechanicsanddynamicshavebeenstudiedextensivelyfrom
many dierent points of view. It is outside the scope of this paper to review
this eld.Wedo nothavetheintentionto implement afullyrealistic dynamic
manipulation system, but rather a limited set of basic physics to assist the
manipulationof objectsinVE.
physicallawshavebeenimplementedintoavirtualworld,addressingthepoten-
tialvalue ofvirtual realityforscience education. Howeverdynamic factorsare
notappliedtouserinteraction.Anotherimplementationofvirtualmechanicsis
AERO [7],which isasimulationandanimationsystemofrigidbodies.Springs
areusedinthissystemforcollisionprocessingandforelasticobjectconnection.
Elasticityanddampingfactorsarealsodiscussedinthis paper.
In the VR literature, there is not much published work on incorporating
dynamics in userinteraction. Usually,haptics isused forthis purpose, [8{11].
We introduce springs asanewtool fordirect manipulation ofobjectsin VEs.
Manypeoplehavedonelab-exercisesinphysicswithpracticalexperimentsusing
springs(eg.liftingofanobjectattachedtoaspring).Ithasbeendoneinthereal
worldwith dynamic feedback. In this paperwewill use this asa metaphor to
improvetheuser'sfeelingofmechanicsanddynamicsinvirtualenvironments.
2.2 Interaction Techniques - Overview
We begin with a brief overview of selection and manipulation techniques in
virtual environments. After that we will suggest someinnovations. As we will
focusontheRWBenvironment,navigationisoflittleimportance.Firstwewill
discussselectiontechniques, [1], [2].
Direct picking is the most intuitive and easy way of object selection when
userhastoreachanobjectwiththepointer.Whentheobjectisnotwithinthe
reachofusershandorpointeranothertechniquemustbeused.Inraycasting,a
rayshootsoutfrom user'sngersorpointer andintersections withobjectsare
evaluated.Gaze-directedselection,usercanselectanobjectbylookingatit.The
pointing techniqueallowsthe usertoselectobjectbypointingat itwithnger
orpointerandaninvisiblerayshootsoutfrombetweenuser'seyesandpointer.
Virtual hands give user the possibility to reach distant objects by extending
virtualhandsfasterthentheuser'srealhands.
Once anobjecthasbeen selected,the usercanmanipulate it [1,4].When
anobjectisoutofreach,close manipulationbringsthisobjectneartotheuser.
Popping brings adistant object into user's hands and after manipulations the
objectgoesbacktoitsposition.Copying bringsacopyoftheobjectintouser's
hands,whiletheoriginaldistantobjectfollowsthemanipulationsperformedwith
the copy.Distant manipulation allowsuser to manipulate distant objectswith
toolsat a distance. In tele-manipulation, the user manipulates distant objects
just astheywerecloseto hisbody.This techniquecanbeusedin combination
withvirtualhands.
With the slave method, the manipulated objectfollows translations of the
pointer. A disadvantage is that for largertranslations, the user must perform
a series of translate-release actions. Thestick method connects user's hand or
pointer with object by a ray - stick. The object is attached to this stick and
followstranslationsand rotations (center of rotationisthe user'shand). With
the3Dcross-hair method, theusercantranslateanobjectbydraggingtheray
ofthepointeralongoneofthecross-hairaxes.
Withtheymethod,thedirectionandthevelocityofapointerisappliedtothe
object.The throttle method usesa metaphor of amotorcycle throttle grip.By
rotatingthe pointer about thedirection of motion(the direction where points
thepointer)thevelocityofmotionisindicated.Forwardandbackwardmotions
arederivedfrom thedirectionofrotation.
Forusewithdynamicinteraction,selectionandmanipulationtechniquesmay
be adapted. We will now discuss the principles of some techniques which are
suitablefordynamicinteractionattheRWB.
3 Suggested Interaction Techniques
Beforewedescribethesuggestedtechniques,wehavetomentionthatourtrack-
ingsystem(tracker daemon)isabletomonitorandalsoevaluatemovementsof
theuser. Itcan detectwhen theuserisat rest, thestartand theend ofuser's
motion.Inaddition,somebasicgesturescanberecognized.Onthetopof this,
morecomplexinteraction techniques canbe built.A specicationofour RWB
environmentandthetrackingsystemisgiveninsection 5.1.
Wewill rstdescribeoursuggestedset of selectiontechniques for dynamic
interaction. Fornear objects we use direct picking. Fordistant objectswe use
ray casting. But if we will think of more natural selection method we should
look into the real world. How doweknow(or how doesthe arm know)which
objectwewant to manipulate? The answeris that it canbe derived from the
path of thehand, orfrom the direction of hand motion. This informationcan
beobtainedfrom thetrackerdaemonbuer. Whenwewantto graban object
we simply move our hand towards to it. This is the principle of the 'I want
this one' IWTO - method. Thepath information and current direction of the
movementofuser'shandarecombinedandinthepredicteddirectionaninvisible
rayisprojectedandintersectionswithobjectsareevaluated.Apossibleselected
candidate changes its colorand if userreally wants this object, just clicksthe
buttononthestyluspointer.Thisisanintuitivetechniquetakenfrom thereal
world,whichcanbeextendedforselectingdistantobjects.
After object selection we will now discuss object manipulation. In simple
caseswhen object iswithin reach of theuser's hand the slave method is used.
Translationandrotationareapplieddirectly totheobject.
Whenthe objectis distant, thestick method together with theray-casting
method could be used. Together with ray-casting works also the cross-plane
methodwhere theobjectisbeingtranslatedbytherayintersections inaplane
with z = const. Similar is the cross-surface method where the ray intersects
surfacesofotherobjects(eg.alandscape).
Whentheuserwantstomanipulateadistantobjectinhisworkingspace,he
cangrabtheobjectneartohimbyarecognizablegesture:afullyextendedhand
bendsin theelbowand movesinto hisworkingsphere.Thismethod workslike
amagnetonobjects.After manipulationanobject canbe releasedwithagiven
speedorsentback to itsoriginal position.Inthis wayan assembling taskcan
beperformed, when userbrings objectsinto his working space and puts them
Asdiscussedbefore,wewishto introducetheconceptsofforce,inertia,gravity,
contactand surfacefrictiontoprovideavirtualworld inwhichobjectsbehave
more naturally. Objects behave accordingto physical lawsand also the inter-
actionisphysics-based.Weareusingrelevantlawsofmechanicsanddynamics.
Basically,thethreeNewton's laws.
For objectcollisions,there areprinciples ofconservation ofmomentum and
ofenergy,andreversibleconversionbetweenkineticandpotentialenergy.
4.1 Why springs?
In VR it is just as easy to select and manipulate large and heavy objects as
small ones.This may sometimes be advantage, but it is not according to our
expectations. Therefore,wewilltrytogivetheuserafeelingofweightormass
ofobjects.
Weproposetheuseofspringsasalinkbetweenuser'shand andamanipu-
latedobject.Bythis,wecanobtainanaturalvisualfeedbackduringmanipula-
tion.Whentheuserliftsaheavyobject,thespringwillextendproportionallyto
theobject'sweight.Also,accelerationanddecelerationofthemotionwillaect
thevisiblelengthofthespring.
4.2 MethodDescription
A specic mass is assigned to each object. From the volume of an object its
mass is calculated. Tomakeobjects move, forces are needed. In our case, are
actingthe gravityforceand the force exertedby the user.Counter-actingforces
are the friction force with the surfaceand the resistance force of the air. For
stabilityadampingforceisimplemented.Weuseasimpliedimplementationof
thephysicslawsinourvirtualworld.Wewillignoreair-resistance.Wewillalso
only use static friction , when user drags an object overa surface. At small
speeds isconstant. Forsimplicity, we will onlyconsider linearmotion here-
wewill notconsider rotational motion,torques, and angularmomentum.Here
istheoverviewofthelawsofmechanicsdiscussed:
Forcelaw:f
a
=ma=m dv
dt
=m d
2
x
dt 2
Gravitylaw:f
g
=mg
Frictionforce:f
r
=N=:mg
Springforce:f
s
= kx
Dampingforce:f
d
= c:
dx
dt
Momentum: p =mv
LinearMomentum:ma= n
X
F
i
gravity acceleration, =static friction coeÆcient, k= spring constantand c=
dampingfactor.
Duringusermanipulation, aspring isattached to the center of massof an
object.Thespringconstantkis calculatedforeach objectdierently. Therest
length of the spring x
0
is also calculated automatically from the mass of an
object.Theresultisthatforheavyobjectsastrongerandlongerspringisused,
and a weakerand shorter springfor light objects. If thesame spring constant
andsamerestlengthwouldbeusedforeveryobjectitwouldhaveawrongeect
onusermanipulation.Theuserwouldhavetolifthis handveryhighforheavy
objects with the weak spring. To avoid this, everyobjecthas its own type of
spring.Suchaspringisabletoholdtheattachedobjectwithanextensionof20
% of its rest lengthx
0
. The absolute massof objectsis application dependent
and directly aects the spring constant.The dierence between the springs is
visible, see g.1. Springs are normally only visible during manipulation of a
selectedobject.
Springconstant: k = m:g
x
m
= m:g
0:2x
0
where mg= kx
m
; x
m
= 0:2x
0
Fig.1.(a)Variousobjectsanddierentsprings (b)SpringDamperSystem
4.3 Spring Damper
Toreduceoscillationsduringmanipulationwithanobjectattachedtoaspring,
wehaveimplementedaspring mass-damper system [12].
mx(t)+cx(t)_ +kx(t)=f(t)
Herem is themassof theobject,x(t) is theextension/compression ofthe
spring, f(t) is the force acting on the object only in x(t) direction, k is the
springconstantanddisthedampingfactor.
Whentheobjectisliftedintheairandhangingonthespring,wecanrewrite
themodelthis way:
mx+cx_ +kx=mg; (x=x
s +x
d )
m
x
d +c
_
x
d +kx
s +kx
d
=mg; (kx
s
=mg)
s d
sionduringthemotion.Thedierentialsystemtosolvewillthenbe:
mx
d +cx_
d +kx
d
=0; x(0)=x
0
; x(0)_ =v
0
More about solutions can be found in [12]. The damping factor plays a
veryimportantrole.Itsvalueisderivedinasimilarwayasthespringconstant,
so that everyobject has its spring and damper. Damping refersto an energy
dissipation mechanism, either intentional or parasitic, such as air friction or
structuraldamping.Thedamperistheenergydissipatingelementofthesystem.
The damping factor measures the ability to damp themotion of the mass. In
g.2,theeect ofthedampingfactorcanbeobserved.
Fig.2.Dampingexamples
4.4 Spring Manipulation Techniques
An objectcan bemanipulated using aset oftechniquesasdescribed below. A
spiralspring isused asahandle, and theobjectwillshowitsmassandinertia
byitsbehaviouraccordingto thelawsofdynamics.
Theactionsareperformedby selectinganobjectbymovingthestyluspen
towardsit,theIWTO method,see section 3.Ataclosedistance, aspring will
bedisplayed betweenthe objectand the user's hand position.Thespring can
beextended orcompressedby keepingthepen buttondepressedwhile moving
thepentowardsorawayfromtheobject.
Virtual forces are thus applied by the user to objects through the virtual
springs, which act as displacement-to-force transducers. The user will see the
extensionorcompressionofaspring,andforcesareinferredbythelinearrelation
of displacementand force. The springconstantsare calculatedas describedin
section 4.2. Theforces maybeexerted onanobjectin anarbitrarydirection.
Theyareassumedtoworkontheobject'scenterofmass,andthustheforceswill
induceonlylinearmotion,andnorotation,asnotorquesareinduced.Thiswas
doneforpracticalreasonsonly.Thelawsgoverningrotationalmotion,torques,
andangularmomentum areverysimilartothose forlinearmotion,andwill be
incorporatedlater.Althoughfrictiononthegroundplanecaninducetorquesand
rollingmotions,these eectsareignoredin thecurrentversionofthesystem.
Theexertedforces are decomposed into liftingforces (perpendicularto the
groundplane),anddraggingforces(paralleltothegroundplane).Toensureeasy
critically, which means that motion will essentially stop after one cycle. For
clarity,theobjectsareassumedheretorestonthegroundplane,althoughother
horizontalplaneswillhavethesameeects.
Fig.3showsthethree cases:lifting, pullingandpushing. Userexertsaforce
f
u
which is unknown. From the change of extension of the spring x
1
we can
obtainthespringforceF
s
whichmakesobjectattachedtothespringaccelerate
andmove.
Fig.3.Springmanipulationcases
Theset ofdynamicmanipulationtechniquesisdened asfollows:
{ lift:pullthespringupward,untilthespringforceexceedstheobject'sweight,
andthe objectgets anupwardacceleration,counter-acted bythe decrease
on the force caused by the shortening of the spring length. Oscillation is
damped.
{ drop:theobjectisreleasedfromthespringconnectionandfallsdownuntil
ittouchesthegroundplane.
{ pull: pull the spring in a horizontal direction away from the object until
theforceexceedsthestaticfrictionoftheobjectonthegroundsurface.The
object gets a horizontal acceleration and slides over the surface, counter-
actedbyfrictionforce, and thedecreasingforce from theshortening ofthe
spring.
{ push:thesameaspull,but thespringiscompressedtowardtheobject.
{ throw: theobjectreceivesthe initial velocity from theuser's hand and is
thrownin the givendirection,the springdisappears. Thetrajectory ofthe
objectin theairisthenonlyinuencedbythegravityforce andtheobject
isfallingtotheground.
{ catch: stoptheobjectmotionbyblockingit'smotionpath.Thespringbe-
tweentheuser'shandandtheobjectiscompressedandtheobjectdecelerates
andstopsmoving.Oscillationsareagaindamped.
{ swing: acombination oflift-pull-drop.The object behavesasapendulum
onthespring;theswingingmotionisdamped,theobjectismovedthrough
intheair,andisdroppedatthechosenposition.
motiondata from thetracker.With dropping thepenbutton is releasedwhile
the user's hand is at rest, for throwing the user releasesthe button while the
handismoving.Catchingyingobjectscanbedonebypositioningthehand to
blockthemotionpathandpressingthebutton.
Wherever spring actions are performed (too) far from the user's body, a
'shingrod'techniquecanbeusedtoattachandmovethespringandtheobject.
4.5 Collisions and Constraints
Fornaturalbehaviorin virtualenvironments,thecollisionsand constraintsare
essential.Ofcourse,wecannotstopuser'shandtomovethroughobjects.Butwe
can disableusermanipulation ofanobjectthroughanotherobject.Wecanuse
hereagaintheadvantagesofthe springmanipulation.Themanipulated object
willstayattheplaceofcollisionwiththeotherobject(seecollisionwithawall,
Fig.4) whileuserisstilltryingtopull.Inthiscase,thepullingwillonlyaects
thelengthofthespring,andnomotionisinduced.
Fig.4.Collisionwiththewall
Thebasicconstraintisthegroundplane.Noobjectcangetthroughit.The
groundcanalwaysproduceenoughnormalforce to supportanyobject. Calcu-
lation ofcollisionswiththegroundis thebasicattributeofourmini-world.We
alsohandle collisionsbetweenobjectsinside theworld(spheres,boxes). Elastic
collisions are simulated. We use there the lawof momentum conservation and
thelawofenergyconservation.
5 Experimental Application
Totesttheconceptsofthispaperwehaveimplementedanexperimentalapplica-
tionwhereusercanperformdynamicspringmanipulationwithvirtualobjectsin
amini-world.Themini-worldconsistsofseveralboxesandspheres,surrounded
bywalls,seeFig.5.ThebasicconceptoftheRWBisthattheuserisstandingin
therealworld,wherethephysicallawsapply,andispartlyimmersedinthevir-
tual mini-world which is projectedon theResponsiveWorkbench. Finally, the
user can also observe a visual feedback of dynamic behaviour of manipulated
objects.
5.1 RWB Environment
TheVRsystemwhichweareusingisbasedontheSGIONYX2,with4xMIPS
R10000,1xIR2graphicspipeandastereoprojectiontable.Forimplementation
wehave used C++, Iris Performer and OpenGL. Our ResponsiveWorkbench
is equipped with aFastrak[13] trackingsystem, to trackthe position and the
orientation of user's head and hand (stylus pointer). A Tracker daemon reads
periodicallyatarateof50Hz thetrackingdataandstoresthem inabuer.
Velocity and accelerationvectorsof the user's hand are calculated.During
our experimentswehavedone measurements onthe trackerdata, g.6. These
dataareusedtogetherwithtimeinformationforcalculationofthespringforce
andforthesimulationofthespring-dampersystem.
0 5 10 15 20 25 30 35
Time samples -49.5594
0.4406 50.4406 100.4406 150.4406
cm
Legend Lifting
x y z
0 5 10 15 20 25 30
Time samples -106.0479
-56.0479 -6.0479 43.9521 93.9521
cm
Legend
Throwing
x y z
Fig.6.Trackerpositionsduringlifting andthrowingmanipulations
5.2 Dynamic ManipulationExamples
Thegures7and8,illustrateaprocedureofdynamicmanipulationofanobject
in themini-world.Onthese pictures,asimplemanipulationtask isperformed:
1)liftingoftheobject,2)translationalmotionintheairand3)droppingofthe
object.Whiletheobjectisin theair,thereisalsoaswingingmotion.
Ingures 9, 10and 11, we show somepictures taken of theactual system
implementedintheRWBenvironment.
Fig.8.Movinganddroppingofobject
Fig.9.RWB-overview:thedynamicmanipulationwithobjectsinthemini-world
6 Conclusions and Future Work
We have developed a scheme for introduction of dynamic object behaviourin
manipulationofobjectsinvirtualenvironments.Initialresultshaveshownthat
object'sbehaviourappears morenaturaland predictable thanthe 'unphysical'
objectsin mostvirtualenvironments.Also,it seemsthat thedynamicsgivesa
coherent'lookandfeel'toa3Ddirectmanipulationuserinterface.Wecaneasily
dene a long list of extensions for future work in this area. The set of basic
user operations will have to be extended, especially if we want to introduce
rotational motions. The numerical computations for dynamic simulation can
becomeverycomplex,andmayputseriouslimitsonperformance.Bothaccuracy
and geometric constraint maintenance should beintegrated. A problem which
needs attention is th dierence in direction between the gravity elds in the
virtualandrealenvironment,dueto theangleoftheRWB'stable top.
Fig.10.RWB-detail:Liftingofasphere Fig.11.RWB-detail:Draggingofabox
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