The role of object shadows in promoting 3D visuali

1TheRoleofObjectShadowsinPromoting3DVisualizationGEOFFREYS.HUBONA1DepartmentofInformationSystemsVirginiaCommonwealthUniversity,Richmond,VA23284PHILIPN.WHEELERCenturyComputing,Inc.,8101SandySpringRoad,Laurel,MD20707GREGORYW.SHIRAHandMATTHEWBRANDTGoddardSpaceFlightCenter,Code522,Greenbelt,MD20771______________________________________________________________________Morepowerfulcontemporarycomputerhardwarehasenabledawidervarietyoftechniquestodepictcomputer-generatedobjectsinthreedimensional(3D)space.Theuseofstereoscopicviewingandmotionhavebeenextensivelystudied.However,theeffectiveuseofcomputer-renderedobjectshadowstoprovidespatialinformationabouttherelativepositionandsizeofobjectsinvirtualspacehasnot.Subjectsperformtwotaskswith3Dgeometricpatternsofobjectspresentedonacomputerscreen:(1)positioningtheobjecttocompleteasymmetricalgeometricfigure;and(2)resizingtheobjecttomatchthesizeofotherobjects.Performanceaccuracyandspeedarerecordedunderthefollowingconditions:(1)objectscastingshadowsonandoff;(2)oneortwolightsources;(3)stereoscopicandmonoscopicviewing;and(4)differentbackgroundsforthecastshadows:flatplane(i.e.floor);‘stair-step’floorwithnowalls;andfloorwithwalls(i.e.room).Theuseofobjectshadowsasdepthcuesenhancespositioningperformance,butnotresizingperformance.Moreover,theobjectshadowsarenotaseffectiveasstereoscopicviewinginfacilitatingperformance.Furthermore,performancedegradeswiththestair-stepshadowbackground,andwhenthenumberoflightsourcesincreasesfromonetotwo.CategoriesandSubjectDescriptors:H.1[ModelsandPrinciples]:User/MachineSystems;H.5[InformationInterfacesandPresentation]:UserInterfaces;I.3[ComputerGraphics]:Three-DimensionalGraphicsandRealismGeneralTerms:Experimentation,HumanFactorsAdditionalKeyWordsandPhrases:3Duserinterfaces,shadows,depthperception,cuetheory______________________________________________________________________1.INTRODUCTIONTherehasbeenadramaticincreaseintheperformancecapabilitiesofpersonalcomputersandmodernworkstationsinrecentyears.Theincreasingpowerofcomputingmachineryhasenabledtheproliferationofthree-dimensional(3D)userinterfacetechniquesforavarietyofbusiness,entertainment,andscientificapplications.However,akeychallengefacing3Dinterfacedesignersistodevelopeffectivetechniquestodepictobjectsin3Dspaceonaphysicalmediumthatisinherentlytwo-dimensional(2D):aflatcomputerscreen.Itisespeciallyimportanttodepictspatialrelationshipsamongobjectsin3Dspace,particularlywithrespecttodepth(e.g.thezdimension)sothatuserscanlocateandmanipulatetheseobjects.Tothisend,interfacedesignersusevariousdepthcuestomakeobjects‘appear’3Dona2Dscreen.Thesetechniquesarebasedonpsychologicalresearchabouthumanperceptionandthecognitiveprocessingofvisualinformation.1Primaryauthor’scurrentaddressis:2215HanoverAvenue,1stFloor,Richmond,VA232202Thevisualcuesthatenablehumanstoperceivedepthhavebeenextensivelydocumented.However,theuseofthesecues,andparticularly,thecombinationsofthesecues,toeffectivelyconveydepthinformationincomputer-generatedscenesremainsanongoingtopicofresearch.Oftendepthcuesareplacedintwocategories:(1)primarycuesincludingbinoculardisparity,convergenceandaccommodation;and(2)secondarycuesthatincludeperspectiveandelevation,size,texture,shadingandshadow,motion,referenceframes,andothers(seeKelsey[1993]foracompletediscussionoftheseprimaryandsecondarydepthcues).Binoculardisparityisaprimary,physiologicalcharacteristicthatenablesthestereoscopicviewingofobjectswithinalimiteddistance.Stereoviewingfacilitatesspatialtasksbyprovidingpowerfuldepthinformationabouttherelativesizesandpositionsofobjectsinspace.However,therearepracticallimitationswithrespecttotheuseofstereovision-enablinghardwareandsoftwareincomputerapplications.Itisrelativelyexpensive,requirestheconcomitantuseofoftencumbersomehead-mountedapparatuses,anditisnotsupportedbymostexistingsoftware.Forthesereasons,itisusefultoresearchvisualtechniquesthatmayserveaspracticalalternativestostereoviewinginconveyingdepthinformation.Theuseofrealtime,movingobjectshadowstoconveydepthinformationaboutthecorrespondingobjectsin3Dspaceisatechniquethathasnotbeenextensivelystudied.Inparticular,therelativecontributionsofmoving,real-timeobjectshadowsinfacilitatingperformancewithinteractive,spatialtaskshasnotbeenfullyexamined.Inpart,thisisbecausecontemporaryhardwarehasjustrecentlybecomepowerfulenoughtosupportpracticalapplicationsinvolvingthecomputationandrenderingofrealtimeshadowsforuser-controlled,movingobjects.Thepurposeofthisstudyistoinvestigatehowthepresenceofshadowscreatedbydifferentlightsourcesandcastbyobjectsondifferentbackgroundsprovidesdepthinformationabouttherelativeplacementandsizeofobjectsin3Dspace.Itisofparticularinteresttocomparetherelativecontributionsofobjectshadowswithstereoscopicviewinginbenefitinginteractivetaskperformancesinvolvingthepositioningandresizingofobjectsin3Dspace.Thisarticlereviewsthemorepowerful3Ddepthcuedisplaytechniquesandthenpresentsaformalexperimentalstudythatexaminesthepowerofcastshadowsinprovidingdepthinformationneededtocomplete3Dobjectpositioningandresizingt