3/23/2005 © dr. zachary wartell – [scarfe2006] 1 “disparity-defined objects moving in depth do...

3/23/2005 © Dr. Zachary Wartell – [Scarfe2006] 1

“Disparity-defined objects moving in depth do not elicit three-dimensional

shape constancy”

P. Scarfe, P.B. Hibbard

Vision Resions, Vol 46., 2006, 1599-1610.

Presentation: Revision 1.0

Copywrite Zachary Wartell University of North Carolina at Charlotte 2006


Introduction: Recall horopter, etc..

AB

bl

br

cl

cr

C

horopteral

bl

cl

crar

br

retinal disparity:

θb= θbl-θbr < 0 θc= θcl-θcr > 0

θ<0 θ>0


Introduction: Recall “metric distance”

• retinal disparity + X = (theoretically) absolute/metric distance, where X is:– vergence angle– vertical disparities– other cues….

• “scaled horizontal disparity” - process of combining horizontal retinal disparity with other info. to yield metric distance. This is theoretical possibility; generally human vision system (HVS) seems unable to do this


Previous Work

• typical results: overestimate relative depth at close distance, underestimate at far distance; i.e. no shape constancy

• artificial stereo displays (stimuli < 1.5m) and real-world objects exhibit distortion

• Johnston [1991]:– demonstrated lack of shape constancy


Johnston [1991]

ab


Previous Work

• does combining structure from motion with stereopsis lead to shape constancy?– theoretically possible– numerous studies show SFM + stereo are not

always combined to proper perception– some studies indicate improvement towards

veridical shape judgement with SFM + stereo• perhaps in these instances, HVS uses non-

generic, context-specific strategies for recovering veridical shape


Previous Work

• does HVS need to recover Euclidean shape?– maybe numerous non-generic processing

strategies allow for accurate responses to experimental task without veridical perception

Ex 1) matching task between two shapes at different distances can be performed even if HVS only extracts distance ratios to 2 shapes

Ex 2) prehension task – perhaps HVS uses ‘on-line nulling of disparities’ approach to guide hand to object


Current Study• Investigate extent to which binocularly defined

objects moving in depth exhibit shape constancy – perceptually constant cylinder =

physically constant cylinder

OR

– perceptually constant cylinder =cylinder whose physical shapemodulated by results of Johnston 1991


Theory

• authors show that given certain approximations for constant shape:

• this indicates that theoretically at least cylinder motion + stereopsis could yield enough information for HVS to determine if shape is changing

/2

/

rateof changeof disparity disparity

rateof changeof retinal image size retinal image size


Experimental Tasks Overview

• apparently circular cylinder (ACC) task – sub. adjusts shape of hemi-cylinder so it appear circular

• matching task – sub. adjust shape of hemi-cylinder at one distance to match the shape of another hemi-cyl. at second distance

• forced-choice judgement – sub. responds whether approaching or receding hemi-cyl. is expanding or contracting in depth


Method

• hemi-cylinder shown on CRT w/ LCD glass– cylinder surface rendered w/ Gaussian blob’s

• 3 cm high, length roughly 8 cm

– blob size & luminance provide cue to object distance but not shape

– assume m.e.s. of 6.5cm– use red color (low persistence)– 32.5 cm screen distance, chin & headrest

• (Johnston1991 appears to have varied screen dist.!)

– cylinders appear at 40,50,60,70cm hence there is acc/verg. conflict

– no-perspective size change for diff. distances!• 7 subjects

– complete ACC or matching task first, then motion task


Apparently Circular Cylinder Task


Apparently Circular Cylinder Task

• adjust depth of hemi-cylinder to make it appear cylindrical– depth range {0,9}cm (recall height is 3cm)– distances {40,50,60,70} cm– diagonal depth profile (left/right) per trial is random– keyboard press increase/decrease depth– initial depth randomly selected– no time limit– 15 settings per distance, 60 total


Results: ACC Task

• mean depth-to-width ratio calculated as mean of the set shape for each of two positions (1 per diagonal depth profile)

• across observers weak effect of distance: – marginally more depth

at far distance


Results: ACC Task


Results: ACC Task

• Individual sub.’s data:– Only D & PS show effect of distance; if

removed from group overall all effect no longer stat. sign. but could be due to loss of stat. power


Discussion: ACC Task

• ACC task produced only weak effect in limited # of sub.’s; unexpected:– same dist. range yielded effects in matching &

motion tasks in this study– previous similar studies found otherwise

• this study didn’t use handheld reference• this study’s eccentricity of view angle on diagonal

path could effect stereopsis


Matching Task


Matching Task

• match shape of 2 cylinders, the “reference” and the “match” at 40 and 70cm

• ref. and match shown side by side but at diff. distances (dot appears above match)

• initial match depth random in {0,9} cm• ref. cyl. always physically circular• sub. adjust match depth w/ keyboard and 3rd key

used to indicate he is done• match presented 15 times at each distance in

random order, match and ref. alternate left/right over trials


Results: Matching Task



• sub.’s set significantly more depth for far cylinders than near ones– true individually for all but sub. F & PBH

• dist. effect not accompanied by reduced variability in sub.’s setting (i.e. std. dev.) compared to ACC; this contrasts w/ majority of sub.’s impression that matching task was easier than ACC task

• matching task results consistent with hypothesis that sub.’s set matching cyl. depth based on incorrect estimated of viewing distance—just like in ACC task

• suggests sub.’s do NOT use alternative strategy to set relative shapes of 2 objects using dist. ratios alone (i.e. w/out needing veridical distance scaling)


Discussion: Matching Task & Dist. Ratio Strategy

• matching task – sub.’s didn’t use theoretical strategy that only needs dist. ratios to get shape matching– where to get dist. ratio info?

• ratio of heights of stimuli (assuming same size)• size of displays, experimental environment

– this study lacked these cues• but shape matching study [Bradshaw2000] with LED’s

yielded high accuracy; sparseness alone seems an insufficient explanation

• results may indicate the depth perception strategies used by HVS maybe more context dependent than originally thought


Discussion: Setting Similarity in ACC & Matching

• the dist. ratio strategy appears unused, yet this didn’t result all sub.’s making similar shape settings ACC and matching tasks– sub.‘s D & F are similar, but not C and PBH

• w/out dist. ratio strategy both reference and match cyl. are subject to mis-scaled dist., this could amplify the error and might explain why matching task showed stronger effect of dist. than ACC task


Motion Task


Motion Task

• cylinder diagonally translates toward or away from sub. along 1 of 2 diagonal depth profiles– [Johnson1991] prediction:

• away case - perception of round cylinder requires physical depth expansion

• towards case – perception of round cylinder requires physical depth compression

• contraction should be isotropic with respect movement direction

• motion direction, diagonal depth profile vary randomly (4 combinations total)

• physical cylinder shape either: constant, expand or contract in depth, or modulated by sub.’s settings in ACC task


Motion Task (cont.)

• for expand/contract case: – ratio range {0.25,0.5,1,2,4} based on overall

ACC results– depth change done smoothly over entire

motion• six conditions total: {e/c {0.25…4}, ACC

modulation, physically constant}• each presented in towards & away case (x2)

with 10 times per depth profile (x10) = 6 x 2 x 10 = 240 trials

• trials in random order, split in two 2 blocks to control for sub. fatigue


Motion Task (cont.)

• Sequence:– sub. press ‘start’ key – cylinder appears and is stationary for 1s– then move 17.8 cm/s and disappears– sub. press key to indicate if shape appeared

to ‘expand’ to ‘contract’ in depth extent• experimenter originally illustrates

expand/contract concept to sub.’s using cardboard model


Results: Motion Task (Single Sub.)

PSE


Results: Motion Task (Single Sub.)

• for expansion/contraction case {1/4, ½, 1,2,4}, effect of motion direction is significant

• Point of Subjective Equality (PSE) - 50% point of fitted function is the amount cylinder must expand/contract when moving toward/away so that sub. is equally like to judge it expand/contract (i.e. perceived it as physically constant)

• for sub. E point of (PSE) for movement toward sub. is 0.89 and movement away is 1.35; diff. is stat. sign.


Results: Motion Task (Combined)



• direction of motion has sign. effect consistent across 5 or 7 sub.’s; exceptions:– F no clear effect, PSE’s 1– PBH both PSE’s > 1 (so cyl. must always expands)

• 66% of physically constant cylinders moving towards sub. perceived as expanding; 35% of physically constant cylinders moving aways perceived as expanding

• individual differences in level of expansion/contraction needed with toward/away motion are large, but differences in PSE’s are not stat. sign.; indicates level of expansion for away motion and level of contraction for toward motion is isotropic.



• skipped analyzing ACC modulated depth case since only 2 or 7 sub.’s had sign. dist. effect in ACC task & effect was weak


Discussion: Motion Task

• sub.’s failed to accurately identify objects that exhibited physical shape constancy as they moved in depth

• a cylinder of constant perceived shape had to be physically expanded when moving away and contracted when moving forwards– predicted by [Johnston 1991]

• expansion and contraction appear isotropic



• Could retinal size mis-scaling explain motion task results?– possibly: approaching constant physical sized

object’s changing retinal image needs to be “scaled” by HVS dist. information to perceived a constant 3D object. What if dist. info. scaling for size constancy is incorrect?

– authors discount this• sub.’s shown physical cylinder model• cylinders in all trials same physical size but result

differ across experiment variables



• motion & matching tasks show greater consistency than with ACC task– sub.’s w/ effect of dist. in motion task also

show an effect of dist. in matching task– somewhat surprising since in ACC & motion

task there is 1 mis-scaling while in match task there are 2 mis-scaling

– but sub.’s report that ACC task is hardest of all three


Summary

• demonstration of failure of shape constancy• when stereopsis + motion is theoretically

sufficient to indicate a physically constant shaped stimuli, HVS perceives physically constant stimuli to expand and contract with forwards and backwards movement

• Big Question:– Why, given well-documented biases in

perception, humans perceive the world to be a relatively stable place?


Thoughts & Questions?

• “The blobs scaled appropriately with distance such that mean screen luminance increased with decreasing distance.” ???

• display not recalculated for diff. IPD’s– cylinders at 40,50,60,70cm– screen at 32.5 cm– what does false eye separation do? – would it confound results if some

cylinders crossed projection plane?


Underestimated Eye Separation

far space

near space


Are there point’s where space is locally expand as well as compressed


Appendix: Psychometrics

3/23/2005 © dr. zachary wartell – [scarfe2006] 1 “disparity-defined objects moving in depth do...

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