Space Perception and Binocular Vision

6

Figure 6.1 The Euclidean geometry of the three-dimensional world turns into something quite different on the curved, two-dimensional retina

Parallel lines remain parallel as they are extended in space

Objects maintain the same size and shape as they move around in space

Internal angles of a triangle always add to 180 degrees

Figure 6.2 The two retinal images of a three-dimensional world are not the same

Two eyes are better than one…

but retinal images not identical = Binocular disparity

basis for stereopsis, perception of the 3D

Introduction

Monocular depth cue: A depth cue that is available even when the world is viewed with one eye alone

Binocular depth cue: A depth cue that relies on information from both eyes

• Binocular depth cues provide: Convergence Stereopsis Ability of two eyes to see more of an object

than one eye

Monocular Cues to Three-Dimensional Space

Occlusion: A cue to relative depth order in which, for example, one object obstructs the view of part of another object

Figure 6.5 Figure 6.4 could be an “accidental” view of the shapes shown in (a), but it is much more likely that it is a generic view of the shapes shown in (b)

Unconscious inference: the most likely interpretation chosen

Monocular Cues to Three-Dimensional Space

Metrical depth cue: A depth cue that provides quantitative information about distance in the third dimension

Nonmetrical depth cue: A depth cue that provides information about the depth order (relative depth) but not depth magnitude

Monocular Cues to Three-Dimensional Space

Relative size: A comparison of size between items without knowing the absolute size of either one

• All things being equal, we assume that smaller objects are farther away from us than larger objects

Figure 6.6 These balls are resting on the same surface; the small ones appear to be farther away

Monocular Cues to Three-Dimensional Space

Relative height: Below the horizon, objects higher in the visual field appear to be farther away. Above the horizon, objects lower in the visual field appear to be farther away

Texture gradient: A depth cue based on the geometric fact that items of the same size form smaller images when they are farther away

• Texture gradients result from a combination of the cues of relative size and relative height

Figure 6.7 The size cue is more effective when size changes systematically

Figure 6.8 A size cue is most effective when it is consistent with an object arranged on the ground, not on a wall

Figure 6.9 Relative height as a cue to depth

Figure 6.10 The two smaller rabbits in the image are the same size. Why don’t they appear to be?

Monocular Cues to Three-Dimensional Space

Familiar size: A cue based on knowledge of the typical size of objects

Monocular Cues to Three-Dimensional Space

Aerial perspective: A depth cue based on the implicit understanding that light is scattered by the atmosphere

• More light is scattered when we look through more atmosphere

• Thus, more distant objects are subject to more scatter and appear fainter, bluer, and less distinct

Figure 6.13 The depth cue here is called “haze” or “aerial perspective”

Figure 6.14 A real-world example of aerial perspective

Monocular Cues to Three-Dimensional Space

Linear perspective: Lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image as they extend into the distance

Vanishing point: The apparent point at which parallel lines receding in depth converge

Figure 6.15 Linear perspective

Figure 6.16 Parallel lines in the image plane versus other planes

Figure 6.17 Picture in a picture

Monocular Cues to Three-Dimensional Space

Pictorial depth cue: A cue to distance or depth used by artists to depict three-dimensional depth in two-dimensional pictures

Anamorphosis (or anamorphic projection): Use of the rules of linear perspective to create a two-dimensional image so distorted that it looks correct only when viewed from a special angle or with a mirror that counters the distortion

Figure 6.18 A 1533 portrait by Hans Holbein, illustrating anamorphic projection

Monocular Cues to Three-Dimensional Space

Motion parallax: Images closer to the observer appear to move faster across the visual field than images farther away

• The brain uses this information to calculate the distances of objects in the environment

Figure 6.20 Motion parallax

Monocular Cues to Three-Dimensional SpaceAccommodation: The process by which the eye changes its focus

(in which the lens gets fatter as gaze is directed toward nearer objects)

Convergence: The ability of the two eyes to turn inward, often used to focus on nearer objects

Divergence: The ability of the two eyes to turn outward, often used to focus on farther objects

Next: Exam#2