Visual Cues for Perceiving Distances from Objects to

“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 © Dr. Zachary Wartell 3/23/2005 1

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 objectsurface 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 © Dr. Zachary Wartell 3/23/2005 2

Introduction (cont. ) © Dr. Zachary Wartell 3/23/2005 3

Prior Work: Does stereo help? It depends… • stereo not better than mono , [Kim 87, Reinhart 90, Barfield 95] • stereo → learn task more quickly [Drascic 91] • stereo →performing task more quickly [Spain 90, Drascic 91, Yeh 92, Hsu 93, Ware 96] • stereo →performing task more quickly (Cole 90, Barfield 95] • generally stereo increases in effectiveness: – as task is more difficult – as visual scenes has fewer other depth cues – for “personal space” © Dr. Zachary Wartell 3/23/2005 4

Prior Work: Shadows • shadows important – [Yonas 78, Kjelldahl 95, Kersten 97, Madison 2001] • shadow effectiveness – varies widely between tasks [Wanger 92] – somewhat between individuals [Hu 00] • interaction of shadow & other depths cues – shadows sometimes override conflicting cues [Bolj 99] – shadows sometimes degrade task accuracy and speed when added to stereo [Hubona 99] © Dr. Zachary Wartell 3/23/2005 5

Prior Work: Interreflections • in real-world they’re often visual indistinct (but in VR we can do anything!) • evidence that they’re used perceptually [Kersten 96] • perhaps as spatial cues [Madison 2001] – for contact perception interrefl. ’s strong as shadows © Dr. Zachary Wartell 3/23/2005 6

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 (1280 x 1024), 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) © Dr. Zachary Wartell 3/23/2005 7

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 • 5 x 5 cm block with infinite height • Table disappear/reappear between trials (no jumping) © Dr. Zachary Wartell 3/23/2005 8

General Method (cont. ) © Dr. Zachary Wartell 3/23/2005 9

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 difficult switching between stereo and biocular so: – turn-off HMD between sessions – vary combination of shadow & interrefl. in session © Dr. Zachary Wartell 3/23/2005 10

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) © Dr. Zachary Wartell 3/23/2005 11

Exp. 1 Results © Dr. Zachary Wartell 3/23/2005 12

Relative vs Absolute Distance and Slope vs R 2 • 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 R 2 © Dr. Zachary Wartell 3/23/2005 13

Results Table © Dr. Zachary Wartell 3/23/2005 14

Exp. 1 Result (cont. ) © Dr. Zachary Wartell 3/23/2005 15

Result Table (cont. ) • all sub’s perform better with stereo than mono (higher R 2) • 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 © Dr. Zachary Wartell 3/23/2005 16

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 © Dr. Zachary Wartell 3/23/2005 17

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. © Dr. Zachary Wartell 3/23/2005 18

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 © Dr. Zachary Wartell 3/23/2005 19

Exp. 2 Results © Dr. Zachary Wartell 3/23/2005 20

Results: R 2 and Cue Combinations © Dr. Zachary Wartell 3/23/2005 21

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 © Dr. Zachary Wartell 3/23/2005 22

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. © Dr. Zachary Wartell 3/23/2005 23

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. © Dr. Zachary Wartell 3/23/2005 24