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3/23/2005 © Dr. Zachary Wartell 1
“Visual Cues for Perceiving Distances from Objects to Surfaces”
Helen H. Hu, Amy A. Gooch, Sarah H. Creem-Regehr, William B. Thompson
Presence, Vol. 11, No. 6, December 2002, 652– 664
Presentation: Revision 1.0
3/23/2005 © Dr. Zachary Wartell 2
Introduction
• perception of impending contact between moving object and surface
• important during manipulation tasks• relevant visual range - “personal space” (Cutting
& Vishton)• paper presents two experiments in object-
surface distance perception:– 1) sub’s control movement of object – 2) sub’s just watch movement and report
object-surface distance• experiments IV: stereo, shadows, interreflections
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Introduction (cont.)
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Prior Work: Does stereo help? It depends…
• stereo not better than mono , [Kim87, Reinhart90, Barfield95]
• stereo → learn task more quickly [Drascic91]• stereo →performing task more quickly
[Spain90,Drascic 91, Yeh92,Hsu93,Ware96]• stereo →performing task more quickly (Cole90,
Barfield95]• generally stereo increases in effectiveness:
– as task is more difficult– as visual scenes has fewer other depth cues– for “personal space”
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Prior Work: Shadows
• shadows important – [Yonas78,Kjelldahl95, Kersten 97, Madison 2001]
• shadow effectiveness – varies widely between tasks [Wanger92]– somewhat between individuals [Hu00]
• interaction of shadow & other depths cues– shadows sometimes override conflicting cues
[Bolj99]– shadows sometimes degrade task accuracy
and speed when added to stereo [Hubona99]
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Prior Work: Interreflections
• in real-world they’re often visual indistinct (but in VR we can do anything!)
• evidence that they’re used perceptually [Kersten96]
• perhaps as spatial cues [Madison2001]– for contact perception interrefl.’s strong as
shadows
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General Method
• shadows, interrefl.’s and stereo all computable at a cost, but stereo needs special hardware: So how do these compare in task performance?
• Equipment:– HMD: hi-res (1280x1024), 40.5 HOV– no head position tracking (no motion parallax)– no head orientation tracking (force sub to look
down in virtual world)– fixed IPD (6.5)
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General Method (cont.)
• virtual environment: textured block approaching textured table
• IV: stereo vs biocular, shadows (on/off), interrefl. (on/off): 8 combinations
• table height and light position randomly varied between trials
• table texture adjusted for distance, so texture size doesn’t give distance info.!
• table height [46,60] cm below sub’s viewpoint• 5x5 cm block with infinite height• Table disappear/reappear between trials (no
jumping)
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General Method (cont.)
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Experiment I
• 6 sub’s• control block height with
physical block tracked via mech. tracker
• per sub.: 480 trials over 6 sessions
• pilot study indicates difficultswitching between stereo and biocular so:– turn-off HMD between sessions– vary combination of shadow & interrefl. in
session
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Experiment 1 (cont.)
• sub’s have 1 s to bring block down and start back up (“bring to pt. just before contact”)– if contact, then give negative feedback &
discard trial (in exp. 22% trials discarded)
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Exp. 1 Results
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Relative vs Absolute Distance and Slope vs R2
• shadows and interrefl. are “scale-invariant”– only give relative dist., i.e. comparison of pair
of distances• stereo cable of:
– relative dist. – retinal disparity (relative to horopter)
– absolute dist. – if above combined with vergence
• human vis. system often assigns absolute dist. even given relative dist. cue
• implication: bad to compare slopes; rather compare R2
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Results Table
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Exp. 1 Result (cont.)
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Result Table (cont.)
• all sub’s perform better with stereo than mono (higher R2)
• 2 x 2 x 2 (stereo x shadow x interrefl.) ANOVA indicates statistically significant effect of stereo but no other effects
• nonparametric test indicated statistically significant effect of shadows as well as stereo
• some sub’s show stat. sign. effect for shadows– 1 sub effect by shadow & not stereo
• across all sub’s interrefl. has no stat. sign. effect
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Bias from negative feedback
• perhaps sub’s learned range of table heights from motor memory and stopped block at some average dist. above table even when visually they found dist. Ambiguous
• statistical analysis of data between sessions yields some stat. sign. learning effects– case BSI improves between session 1 & 2– case B improves between session 1 & 3
• bias may have been higher in cases with only rel. dist. cues
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Experiment 2
• 6 sub’s: no glasses, tested for fusion, not from exp. 1
• block falls toward table and stops automatically at some distance above table
• sub: indicate the block-table dist. by sliding index finger & thumb on scale to match finger-thumb dist. to block-table dist.
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Experiment 2
• IV: table-surface distance, DV: thumb-finger dist.
• sub’s perform task 48 times in biocular display – shadow and interrefl. combinations vary
• 1-3 weeks later: sub’s perform task 48 times in stereo display
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Exp. 2 Results
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Results: R2 and Cue Combinations
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Discussion• Exp. 2 appears to have eliminated some biases
of exp. 1:– no cue case:
• Exp 1: sub’s perform better than random• Exp 2: sub’s perform closer to expected
• individually each sub.:– stat. sign. effect of stereo– stat. sign. effect of shadow under non-stereo
• some sub’s:– stat. sign. effect of interrefl. for non-stereo
• in pooled data all 3 occurred • interesting that abs. dist. cue (S) and rel. dist.
cue (BSI) yield similar performance
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Conclusions
• two experiments over 8 combinations of stereo, shadow and interrefl.– stereo is strong cue for object-surface dist.– nonparametric statistics indicate shadows are
sign. in exp 1– exp 2. shows:
• case BSI similar performance to case S• shadows alone are effective but there are greater
individual differences
• individual diff’s:– perceived scaling ratios vary widely– some use shadows better; other’s interrefl.
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Future Work
• generality of results needs further confirmation• study effect of task viewing and motion:
– exp 1. = dynamic, visual-motor task, closed-loop– exp 2. = open-loop matching task with dynamic and
stationary views• study effect of distance judgements along line-of-sight
versus perpendicular to LOS• study effect of varying geometry, surface markings, and
materials on effectiveness of shadow and interrefl.