Chapter 10: Perceiving Depth and Size

Oculomotor Cues Oculomotor cues are based on sensing the position of the eyes and muscle tension Convergence - inward movement of the eyes when we focus on nearby objects Accommodation - change in the shape of the lens when we focus on objects at different distances 2

Monocular Cues Monocular cues come from one eye Pictorial cues - sources of depth information that come from 2-D images, such as pictures Occlusion - when one object partially covers another Relative height - objects below the horizon that are higher in the field of vision are more distant Objects above the horizon lower in the visual field are more distant 3

Monocular Cues - continued Relative size - when objects are equal size, the closer one will take up more of your visual field Perspective convergence - parallel lines appear to come together in the distance Familiar size - distance information based on our knowledge of object size 4

Figure 10.3 (a) A scene in Tucson, Arizona, containing a number of depth cues: occlusion (the cactus on the right occludes the hill, which occludes the mountain); relative height (the far motorcycle is higher in the field of view than the closer motorcycle); relative size (the far motorcycle and telephone pole are smaller than the near ones); and perspective convergence (the sides of the road converge in the distance). (b) 1, 2, and 3 indicate the increasing height in the field of view of the bases of the motorcycles and the far telephone pole, which reveals that being higher in the field of view causes objects on the ground to appear farther away; 4 and 5 reveal that being lower in the field of view causes objects in the sky to appear farther away. Figure 10-3 p229

Figure 10. 4 Pietro Perugino. Christ Handing the Keys to St Figure 10.4 Pietro Perugino. Christ Handing the Keys to St. Peter (Sistine Chapel). The convergence of lines on the plaza illustrates perspective convergence. The sizes of the people in the foreground and middle ground illustrate relative size. Figure 10-4 p230

Monocular Cues - continued Atmospheric perspective - distance objects are fuzzy and have a blue tint Texture gradient - equally spaced elements are more closely packed as distance increases Shadows - indicate where objects are located Enhance 3-D of objects 7

Figure 10.6 A scene on the coast of Maine showing the effect of atmospheric perspective. Figure 10-6 p231

Figure 10. 7 Texture gradients created by marathon runners and flowers Figure 10.7 Texture gradients created by marathon runners and flowers. The increasing fineness of texture as distance increases enhances the perception of depth. Figure 10-7 p231

Figure 10.8 (a) Where are the spheres located in relation to the checkerboard? (b) Adding shadows makes their location clearer. Figure 10-8 p231

Figure 10.9 (a) Early morning shadows emphasize the mountain’s contours. (b) When the sun is overhead, the shadows vanish, and it becomes more difficult to see the mountain’s contours. Figure 10-9 p232

Covering an object is deletion Uncovering an object is accretion Motion-Produced Cues Motion parallax - close objects in direction of movement glide rapidly past but objects in the distance appear to move slowly Deletion and accretion - objects are covered or uncovered as we move relative to them Covering an object is deletion Uncovering an object is accretion 12

Binocular Depth Information Stereoscopic depth perception Differences between 2D and 3D movies “Stereo Sue” Strabismus 13

Binocular disparity - difference in images from two eyes Difference can be described by examining corresponding points on the two retinas 14

Disparity (Geometrical) Created Stereopsis (Perceptual) Stereopsis - depth information provided by binocular disparity Stereoscope uses two pictures from slightly different viewpoints. Random-dot stereogram has two identical patterns with one shifted in position. 15

Figure 10. 19 The two images of a stereoscopic photograph Figure 10.19 The two images of a stereoscopic photograph. The difference between the two images, such as the distances between the front cactus and the window in the two views, creates retinal disparity. This creates a perception of depth when the left image is viewed by the left eye and the right image is viewed by the right eye. Figure 10-19 p239

Disparity (Geometrical) Created Stereopsis (Perceptual) - continued Three types of 3D TV Passive – polarized glasses Active – electronic shutter glasses Lenticular – mini lenses on screen, no glasses needed 17

Figure 10.20 Three types of 3-D TV. See text for details. Figure 10-20 p239

Figure 10. 21 (a) A random-dot stereogram Figure 10.21 (a) A random-dot stereogram. (b) The principle for constructing the stereogram. See text for an explanation. Figure 10-21 p240

The Correspondence Problem How does the visual system match images from the two eyes? Matches may be made by specific features of objects. This may not work for objects like random-dot stereograms. A satisfactory answer has not yet been proposed. 20

The Physiology of Binocular Depth Perception Neurons have been found that respond best to binocular disparity. These are called binocular depth cells or disparity selective cells. These cells respond best to a specific degree of absolute disparity between images on the right and left retinas. Disparity tuning curve 21

The Physiology of Binocular Depth Perception - continued Experiment by Blake and Hirsch Cats were reared by alternating vision between two eyes. Results showed that they: had few binocular neurons. were unable to use binocular disparity to perceive depth. 22

The Physiology of Binocular Depth Perception - continued Experiment by DeAngelis et al. Monkey trained to indicate depth from disparate images. Disparity-selective neurons were activated by this process. Experimenter used microstimulation to activate different disparity-selective neurons. Monkey shifted judgment to the artificially stimulated disparity. 23

Distance and size perception are interrelated Perceiving Size Distance and size perception are interrelated Experiment by Holway and Boring Observer was at the intersection of two hallways. A luminous test circle was in the right hallway placed from 10 to 120 feet away. A luminous comparison circle was in the left hallway at 10 feet away. 24

Perceiving Size - continued Experiment by Holway and Boring On each trial the observer was to adjust the diameter of the test circle to match the comparison. Test stimuli all had same visual angle (angle of object relative to the observer’s eye). Visual angle depends on both the size of the object and the distance from the observer. 25

Figure 10. 26 Setup of Holway and Boring’s (1941) experiment Figure 10.26 Setup of Holway and Boring’s (1941) experiment. The observer changes the diameter of the comparison circle in the left corridor to match his or her perception of the size of test circles presented in the right corridor. Each test circle has a visual angle of 1 degree and is presented separately. This diagram is not drawn to scale. The actual distance of the far test circle was 100 feet. Figure 10-26 p243

Perceiving Size - continued Part 1 of the experiment provided observers with depth cues. Judgments of size were based on physical size. Part 2 of the experiment provided no depth information. Judgments of size were based on size of the retinal images. 27

Perception of an object’s size remains relatively constant. Size Constancy Perception of an object’s size remains relatively constant. This effect remains even if the size of the retinal image changes. The changes in distance and retinal size balance each other 28

Nonveridical perception occurs during visual illusions. Müller-Lyer illusion: Straight lines with inward fins appear shorter than straight lines with outward fins. Lines are actually the same length. 29

Why does this illusion occur? Misapplied size-constancy scaling: Müller-Lyer Illusion Why does this illusion occur? Misapplied size-constancy scaling: Size constancy scaling that works in 3-D is misapplied for 2-D objects. Observers unconsciously perceive the fins as belonging to outside and inside corners. Outside corners would be closer and inside corners would be further away. 30

Figure 10. 36 The Müller-Lyer illusion Figure 10.36 The Müller-Lyer illusion. Both lines are actually the same length. Figure 10-36 p249

Figure 10.37 According to Gregory (1966), the Müller-Lyer line on the left corresponds to an outside corner, and the line on the right corresponds to an inside corner. Note that the two vertical lines are the same length (measure them!). Figure 10-37 p249

Müller-Lyer Illusion - continued Since the retinal images are the same, the lines must be different sizes. Problems with this explanation: The “dumbbell” version shows the same perception even though there are no “corners.” The illusion also occurs for some 3-D displays. 33

Figure 10. 38 The “dumbbell” version of the Müller-Lyer illusion Figure 10.38 The “dumbbell” version of the Müller-Lyer illusion. As in the original Müller-Lyer illusion, the two straight lines are actually the same length. Figure 10-38 p250

One possible explanation is misapplied size-constancy scaling. Ponzo Illusion Horizontal rectangular objects are placed over railroad tracks in a picture. The far rectangle appears larger than the closer rectangle but both are the same size. One possible explanation is misapplied size-constancy scaling. 35

Figure 10. 41 The Ponzo (or railroad track) illusion Figure 10.41 The Ponzo (or railroad track) illusion. The two animals are the same length on the page (measure them), but the upper one appears larger. Figure 10-41 p251

Two people of equal size appear very different in size in this room. The Ames Room Two people of equal size appear very different in size in this room. The room is constructed so that: The shape looks like a normal room when viewed with one eye. The actual shape has the left corner twice as far away as the right corner. 37

Figure 10.42 The Ames room. Both women are actually the same height, but the woman on the right appears taller because of the distorted shape of the room. (The Exploratorium/S. Schwartzenberg.) Figure 10-42 p251

Figure 10. 43 The Ames room, showing its true shape Figure 10.43 The Ames room, showing its true shape. The person on the left is actually almost twice as far away from the observer as the person on the right; however, when the room is viewed through the peephole, this difference in distance is not seen. In order for the room to look normal when viewed through the peephole, it is necessary to enlarge the left side of the room. Figure 10-43 p251

The Ames Room - continued One possible explanation - size-distance scaling Observer thinks the room is normal. Women would be at same distance. Woman on the left has smaller visual angle (R). Due to the perceived distance (D) being the same her perceived size (S) is smaller 40

The Ames Room - continued Another possible explanation - relative size Perception of size depends on size relative to other objects. One woman fills the distance between the top and bottom of the room. The other woman only fills part of the distance Thus, the woman on the right appears taller 41

One possible explanation: Moon Illusion The moon appears larger on the horizon than when it is higher in the sky. One possible explanation: Apparent-distance theory - horizon moon is surrounded by depth cues while moon higher in the sky has none. Horizon is perceived as further away than the sky - called “flattened heavens”. 42

Moon Illusion - continued Since the moon in both cases has the same visual angle, it must appear larger at the horizon. Another possible explanation: Angular size-contrast theory - the moon appears smaller when surrounded by larger objects Thus, the large expanse of the sky makes it appear smaller Actual explanation may be a combination of a number of cues. Other factors are: atmospheric perspective, color, and oculomotor cues. 43