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Lecture 11

Lecture 11 Depth Perception and Motion.docx

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McMaster University
Joe Kim

1 Lecture 11: Depth, Perception and Motion Introduction to Depth and Distance Perception  We need to make split-second decisions about depth and distance everyday  We can make these split-second decisions because we can create a mental image of a three-dimensional world, based on a combination of top-down and bottom-up processing Monocular and Binocular Cues  How do we transform flat, two-dimensional visual images projected onto our retina into an accurate 3-D mental image of the environment  Psychologists have identified two main classes of cues that we use to perceive depth in our environment o Binocular cues, which are depth cues that require two eyes o Monocular cues, which are depth cues that you can get using one eye  Stereopsis: ability to see depth using only binocular disparity  Two types of binocular cues to depth perception o Convergence  Results from the way our eyes turn inwards to fixate on a specific point  Ex. If you look at your finger at arm’s length and then slowly bring it towards you while focusing on your finger  As your finger approaches your face, you’ll feel your eye muscles training  The feedback that we receive from these eye movements gives us information about depth  Convergence as a depth cue only works for objects that are relatively close because with objects that are far away, the eyes don’t have to turn in at all to fixate on the same point  Path of sight for two eyes becomes parallel  Retinal Disparity  Caused by the fact that our eyes, which are located about 6 cm apart, will each see slightly different visual scenes  Ex. If you point to an object in the distance and then open and close one eye at a time  You will see your finger jumping around and pointing to a different object when you look with each eye  These are the two different scenes that each eye is receiving  When these two scenes are combined in the brain, the resultant perception is depth  Our visual systems are equipped with a class of neurons that fire maximally only when each retinal image is slightly different  Three types of monocular cues 2 o Accommodation  Involves changes in the shape of the lens as you focus on objects at different distances  When objects are near, we make a different accommodating response than when objects are farther away  However, the lens can only change so much in shape and because of this, accommodation is only an effective cue for depth up to about 2 metres  For all points beyond this distance, the lens is a constant shape and so we have to rely on other depth cues o Motion  Motion Parallax  Refers to the fact that when we pass by a scene, objects in the scene pass by us at different speeds, depending on how far away these objects are relative to us  Objects that are close to us appear to speed by much faster than objects that are farther away  Optic Flow  Refers to the changing optical projection of a scene that is caused by the motion of the observer, as well as motion of objects within the scene  As you get closer, an object will get bigger and it will get smaller as you move away from it  Not only does the size of the object that you’re focusing on change as you move toward or away from it, so does the entire visual scene  Objects that are close to you will seem to move more in the visual scene and change more in size than objects that are farther away and these changes in size and motion can give cues to depth ** The object moves in motion parallax, doesn’t move in optic flow** o Pictorial  Interposition  When you have an object that partially blocks another, it is perceived as being in front of the other object  Most effective when the objects are familiar and you know what their shapes should be  Can still provide information about depth if objects are unfamiliar because of Gestalt principle of closure  Ex. If you have two circles that are partially overlapping, you will tend to see the partially blocked shape as a circle and not as a crescent because of the principle of closure  As a result, you would perceive the full circle being in front of the partially blocked circle instead of seeing a full circle beside a crescent shape 3  Relative Size  If you have two objects that are the same shape but different sizes, then the larger shape will be perceived as closer  If you know what size a car should be, if you see an image of a tiny car, you will know that the car is far away  Relates to the concept of optic flow  We avoid collisions with other cars by monitoring the rate of expansion of the approaching car on our retina and keeping this rate constant  Linear Perspective  Ex. A railroad track  Even though you know the tracks are parallel, they appear to converge at a single point on the horizon  This provides a cue to depth because objects that are farther away decrease in size and spacing between objects  Texture and Haze  Sometimes called aerial perspective  Ex. If you’re looking at a gravel road, then you will easily be able to see the texture of the rocks under your feet, but as you look in the distance, you will just see the same rocks becoming a uniform grey colour with little texture  Ex. When we’re looking at objects that are farther away, it will be harder to see the outline and texture of objects compared to objects that are closer to you  Shading  Cannot give us information about how far away the object is from us, but can tell us what part of the object is close to us and what is farther away  We are used to light coming from above, like the sun, so we automatically use the pattern of light striking an object to tell us whether the object surface is coming toward us or receding away from us  If the light is striking the bottom of the object, then it is receding from us and if the light is hitting the top of the object then it is coming towards us  Elevation  Objects that are higher up in a picture or closer to the horizon are seen as farther away than objects that are lower in a picture o Aerial perspective: the visual effect of light when passing through the atmosphere. o Monocular depth perception is learned in infancy at around 6 months of age. o Binocular depth perception exists in innate, but also through maturation or learning. 4 Evolution of Depth Perception  The effect of eye placement on depth perception o The types of cues that an animal can use to perceive depth depends a lot on where the animal’s eyes are placed on its head o Prey animals typically have eyes that are on the side of their heads, like rabbits and fish  These animals have very limited depth perception from binocular cues and must rely on monocular cues o Predator animals typically have both eyes facing front, like cats and primates  These animals are able to use both binocular and monocular cues  In addition to providing information about distance between predators and prey, the binocular cue of retinal disparity is excellent at detecting camouflage o Retinal disparity allows you to group together features or objects that are at the same distance  The necessity of multiple depth cues o The reason we have so many different depth and distance cues is because we needed a visual system that was flexible and capable of processing distance and depth in many different situations o Ex. Motion parallax and optic flow only work as cues for depth if you or the scene around you is moving o Texture only works as a good depth cue if the surface has a consistent texture and if none of that textured scene is blocked from view o Accommodation, convergence and retinal disparity only work for objects that are less than approx. 30 feet from you o Elevation only works if
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