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

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

3/23/2005 © Dr. Zachary Wartell 4 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”

3/23/2005 © Dr. Zachary Wartell 5 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]

3/23/2005 © Dr. Zachary Wartell 6 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

3/23/2005 © Dr. Zachary Wartell 7 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)

3/23/2005 © Dr. Zachary Wartell 8 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)

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

3/23/2005 © Dr. Zachary Wartell 10 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

3/23/2005 © Dr. Zachary Wartell 11 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)

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

3/23/2005 © Dr. Zachary Wartell 13 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

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

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

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

3/23/2005 © Dr. Zachary Wartell 17 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

3/23/2005 © Dr. Zachary Wartell 18 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.

3/23/2005 © Dr. Zachary Wartell 19 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

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

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

3/23/2005 © Dr. Zachary Wartell 22 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

3/23/2005 © Dr. Zachary Wartell 23 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.

3/23/2005 © Dr. Zachary Wartell 24 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.