Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMonocular cues to depthPSY 310Greg FrancisLecture 20A second look at vacation photos.Purdue UniversityDepth perception Vision is a long-range filter It allows us to know some properties of objectsfrom a distance Close distancesProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityDepth perception Vision is a long-range filter It allows us to know some properties of objectsfrom a distance Far distancesPurdue UniversityThe problem The structure of light that falls on our retina does not have a 3-Darrangement It’s a projection on to a screen (the retina)Prof. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityThe problem A point in the real world projects to a point on the retinaPurdue UniversityThe problem A point in the real world projects to a point on the retina Physical distance has little to do with where the point projects on the retinaProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityThe problem A point in the real world projects to a point on the retina Physical distance has little to do with where the point projects on the retinaPurdue UniversityThe problem A point further way than a point closer simply projects to a different place onthe retina How do we know one point is further away than another?Prof. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityThe solution Identification of the depth of a single point is impossible withone view of the point However, we do a pretty good job of perceiving depth because We have two eyes, which can identify depth of a point (later) Oculomotor cues Monocular cues to depth Much of this works because we perceive objects rather thanpointsPurdue UniversityOcculomotor cues The muscles of the eye function differently for near andfar objects This provides information about the distances of objects For far objects, both eyes look straight aheadProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOcculomotor cues You can tell when people are looking at something far away Gaze detectionPurdue UniversityOcculomotor cues The muscles of the eye function differently for near and farobjects This provides information about the distances of objects For near objects, the eyes converge inwardProf. Greg Francis7PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOcculomotor cues You can tell when people are looking at something near Difficult to find a picture But easy to experiencePurdue UniversityMuscle patterns Different muscle combinations are needed to turn the eyesto focus on an object at different distances This happens automatically, without our consciousawareness The brain can use the muscle combinations as a method ofidentifying some information about the depth of the object weare looking at Doesn’t help identify the depth of objects we are not lookingat directlyProf. Greg Francis8PSY 310: Sensory and Perceptual ProcessesPurdue UniversityAccommodation The lens of the eye changes focus to image objects atdifferent distances Things far away The ciliary muscles relax and the fibers pulling on the lensbecome taut, which stretches the lensPurdue UniversityAccommodation The lens of the eye changes focus to image objects at differentdistances Things nearby The ciliary muscles tighten and the fibers pulling on the lens becomeslack, which allows the lens to be thickerNicedemonstrationfrom textbookProf. Greg Francis9PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMuscle patterns Depth from accommodation is similar to depth fromconvergence Muscle patterns can be known to the brain and used toestimate depth of an object you are looking at Not much use for other objects in a scene Both convergence and accommodation work for objectswithin about an arm’s length For depths farther away, the differences in convergence andaccommodation become very small There are many other cues to depth Monocular cuesPurdue UniversityMonocular cues Pictorial cues Occlusion Relative height Cast shadows Relative size Atmospheric perspective Linear perspective Texture gradient Many are related to each other They often work together to produce a percept of depthProf. Greg Francis10PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOcclusion An object in front hides part of an object behindPurdue UniversityRelative height Objects higher in the field of view (but below the horizon) arefarther away The castle is not on a hillProf. Greg Francis11PSY 310: Sensory and Perceptual ProcessesPurdue UniversityRelative height Objects higher in the field of view (but below the horizon) arefarther away The opposite for objects above the horizon (clouds)Purdue UniversityCast shadows We don’t usually notice them, but they make a big impact onperceived depth BallInABox.movProf. Greg Francis12PSY 310: Sensory and Perceptual ProcessesPurdue UniversityRelative size Objects that are further away take up less of the visual field(in terms of visual angle) Across a whole scene this is a strong cue to depthPurdue UniversityRelative size Compare the size of my daughter’s head and the head of astranger who is farther awayProf. Greg Francis13PSY 310: Sensory and Perceptual ProcessesPurdue UniversityRelative size Compare the size of my daughter’s head and the head of astranger who is farther away Familiarity can play a role here tooPurdue UniversityAtmospheric perspective Light travels through the atmosphere Particles in the air scatter the light Farther away objects are more indistinctProf. Greg Francis14PSY 310: Sensory and Perceptual ProcessesPurdue UniversityAtmospheric perspective Light travels through the atmosphere Particles in the air scatter the light Farther away objects are more indistinctPurdue UniversityLinear perspective Parallel lines appear to converge in depthProf. Greg Francis15PSY 310: Sensory and Perceptual ProcessesPurdue UniversityLinear perspective Parallel lines appear to converge in depthPurdue UniversityLinear perspective Parallel lines appear to converge in depthProf. Greg Francis16PSY 310: Sensory and Perceptual ProcessesPurdue UniversityLinear perspective Parallel lines appear to converge in depthPurdue
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