Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityBinocular cues to depthPSY 310Greg FrancisLecture 21How to find the hidden word.Purdue UniversityDepth perception You can see depth in static images with just oneeye (monocular) Pictorial cues However, motion and binocular cues generallyplay an important role in depth perception Motion parallax StereopsisProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMotion and depth Consider a point far awayPurdue UniversityMotion and depth When it moves in the world, the point also moves on theretinaProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMotion and depth Consider a point that is closerPurdue UniversityMotion and depth Have it move the same amount in the world Notice how much it moves on the retinaProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMotion and depth Compare the movements for near and far dotsIn the worldOn the retinaFarNearIt’s avariation ofthe retinalsize withdistancepropertyPurdue UniversityMotion and depth If you move, then the projection of light from nearby objects movesmore quickly on the retina than for far objects Motionparallax.gifProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMotion and depth Motion parallax is a property of physics Motion and projection on to a surface The retina A cameraPurdue UniversityMotion parallax Many organisms use motion parallax to identify depth of objects Rabbits, prairie dogs, insects, humans, robots…Prof. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTwo views We noted last time that with a single view there is no way toidentify the depth of a point Most of the monocular cues work with objects, not points Motion parallax can identify the depth of a point because itincludes several different views For motion to exist, there must be more than one view Either you move Or the object moves We can get depth of a point from different kinds of views Two eyes Binocular visionPurdue UniversityBinocular vision Suppose you look at a scene with two shapes at differentdepths You focus (converge your eyes) on object BProf. Greg Francis7PSY 310: Sensory and Perceptual ProcessesPurdue UniversityBinocular vision Suppose you look at a scene with two shapes at differentdepths You focus (converge your eyes) on object B B is in the same place on the retina for the left eye and theright eyePurdue UniversityBinocular vision Suppose you look at a scene with two shapes at differentdepths You focus (converge your eyes) on object B A is in a different place on the retina for the left eye and theright eyeProf. Greg Francis8PSY 310: Sensory and Perceptual ProcessesPurdue UniversityBinocular vision The difference in the position of A across the twoeyes is called disparity Measured in terms of visual anglePurdue UniversityTwo views Similarly, if you look at thetree, the policeman is indifferent places for thefoveae of the left and righteyeProf. Greg Francis9PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTwo views Even though the eyes are not far apart, the views can be quitedifferentPurdue UniversityTwo views Even though the eyes are not far apart, the views can be quitedifferentMake them alternateto give a goodimpression of depth.Motion parallax.TwoViews.gifProf. Greg Francis10PSY 310: Sensory and Perceptual ProcessesPurdue UniversityHoropter Differencesbetween theview of the twoeyes can beused to identifydepth What aboutpoints that fallon the samerelative positionof the tworetinas?Purdue UniversityHoropter Suppose you stareat point D The point D falls onthe central part ofthe fovea of eacheyeProf. Greg Francis11PSY 310: Sensory and Perceptual ProcessesPurdue UniversityHoropter Suppose you stareat point D The point D falls onthe central part ofthe fovea of eacheye Then point C willalso fall on the samerelative positionPurdue UniversityHoropter Suppose you stareat point D The point D falls onthe central part ofthe fovea of eacheye Then point B willalso fall on the samerelative position The angle betweenD and B is the samefor both eyesProf. Greg Francis12PSY 310: Sensory and Perceptual ProcessesPurdue UniversityHoropter The horopter is theset of points that fallon the same relativepositions of the twoeyes No disparity forthese points Consider a point W The angles in thetwo eyes aredifferentWPurdue UniversityHoropter Easier to see ifwe get rid of thebackgroundWDProf. Greg Francis13PSY 310: Sensory and Perceptual ProcessesPurdue UniversityHoropter Easier to see if weget rid of thebackground Easier to see if were-position thelinesWPurdue UniversityHoropter Consider a pointbehind D The projection of Wis on opposite sidesof the projection of D Both on the nasal(nose) side of theretina Uncrossed disparityWProf. Greg Francis14PSY 310: Sensory and Perceptual ProcessesPurdue UniversityHoropter Consider a point infront of D The projection of Wis on opposite sidesof the projection of D Both on the temporal(temples) side of theretina Crossed disparityWPurdue UniversityLots of depth cues How do we know the two views of the eye are used tocompute a depth percept? What about all the monocular cues? Does disparity do anything? Need two views that have no monocular cues Random dot stereogramProf. Greg Francis15PSY 310: Sensory and Perceptual ProcessesPurdue UniversityRandom dot stereogram Random dots contain no monocular cues to depthPurdue UniversityRandom dot stereogram Dots in the middle are in the same relative positionProf. Greg Francis16PSY 310: Sensory and Perceptual ProcessesPurdue UniversityRandom dot stereogram Dots in the middle are in the same relative positionPurdue UniversityRandom dot stereogram Dots in the surround are shiftedProf. Greg Francis17PSY 310: Sensory and Perceptual ProcessesPurdue UniversityRandom dot stereogram Dots in the surround are shiftedPurdue UniversityRandom dot stereogram The points in the different eyes introduce disparity You have to look at the images with a special device(stereoscope) Or learn how to cross or uncross your eyes to get the images toconverge together And you see depth!Prof. Greg Francis18PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySingle Image