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 distancesPurdue 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)Purdue 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 Francis2PSY 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?Purdue 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 aheadPurdue 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 Francis3PSY 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 directlyPurdue 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 textbookPurdue 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 Francis4PSY 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 hillPurdue 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.movPurdue 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 Francis5PSY 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 indistinctPurdue 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 depthPurdue UniversityLinear perspective Parallel lines appear to converge in depthPurdue UniversityLinear perspective Parallel lines appear to converge in depthProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityLinear perspective Parallel lines appear to converge in depthPurdue UniversityLinear perspective It’s not just long pathwaysPurdue UniversityLinear perspective It’s not just long pathwaysPurdue UniversityLinear perspective Easier to see convergence without the picture!Purdue UniversityTexture gradient Textures become more dense (packed) as distanceincreasesPurdue UniversityTexture gradient Textures become more dense (packed) as distanceincreasesNotice,almost allmonocularcues arepresent here!Prof. Greg Francis7PSY 310: Sensory and Perceptual ProcessesPurdue UniversityConclusions Depth perception Oculomotor cues Convergence Accommodation Monocular