Chapter 10: Perceiving Depth and Size
- Cue approach to depth perception , focuses on identifying information in the
retinal image that is correlated with depth in the scene. For example, when one
object partially covers another object, as the tree in the foreground covers part of
the house, the object that is partially covered must be farther than the object that is
- This situation, which is called occlusion , is a cue that one object is in front of
- After this learning has occurred, the association between particular cues and
depth becomes automatic, and when these depth cues are present, we experience
the world in three dimensions.
- There are three major cue groups:
o Oculomotor. Cues based on our ability to sense the position of our eyes
and the tension in our eye muscles.
o Monocular. Cues that work with one eye.
o Binocular. Cues that depend on two eyes.
- The oculomotor cues are created by:
o convergence, the inward movement of the eyes that
occurs when we look at nearby objects, and
o accommodation, the change in the shape of the lens
that occurs when we focus on objects at various distances.
- The idea behind these cues is that we can feel the inward movement of the eyes
that occurs when the eyes converge to look at nearby objects, and we feel the
tightening of eye muscles that change the shape of the lens to focus on a nearby
- The feelings you experience as you move your finger closer are caused by:
o the change in convergence angle as your eye muscles cause your eyes to
look inward, and
o the change in the shape of the lens as the eye accommodates to focus on a
- If you move your finger farther away, the lens flattens, and your eyes move away
from the nose until they are both looking straight ahead
- Convergence and accommodation indicate when an object is close and are useful
up to a distance of about arm’s length, with convergence being the more effective
of the two
- Monocular cues work with only one eye. They include accommodation, which
we have described under oculomotor cues; pictorial cues, which are sources of depth information in a twodimensional picture; and movementbased cues, which
are sources of depth information created by movement.
- Pictorial cues are sources of depth information that can be depicted in a picture,
such as the illustrations in this book or an image on the retina
- Occlusion occurs when one object hides or partially hides another from view.
- It indicates that the object that is partially covered is farther away than another
object, but from occlusion alone we can’t tell how much farther.
- Relative height , objects with their bases closer to the horizon are usually seen as
being more distant.
- This means that being higher in the field of view causes objects on the ground to
appear farther away, whereas being lower in the field of view causes objects in the
sky to appear farther away
- Relative size , when two objects are of equal size, the one that is farther away will
take up less of your field of view than the one that is closer.
- This cue depends, to some extent, on a person’s knowledge of physical sizes
- When you look down parallel railroad tracks that appear to converge in the
distance, you are experiencing perspective convergence .
- This cue was often used by Renaissance artists to add to the impression of depth
in their paintings
- We use the cue of familiar size when we judge distance based on our prior
knowledge of the sizes of objects.
- If you are influenced by your knowledge of the actual size of dimes, quarters, and
halfdollars, you would probably say that the dime is closer than the quarter.
- An experiment by William Epstein (1965) shows that under certain conditions,
our knowledge of an object’s size influences our perception of that object’s
distance. The stimuli in Epstein’s experiment were equalsized photographs of a
dime, a quarter, and a halfdollar, which were positioned the same distance from
an observer. By placing these photographs in a darkened room, illuminating them
with a spot of light, and having subjects view them with one eye, Epstein created
the illusion that these pictures were real coins.
- This result does not occur, however, when observers view the scene with both
eyes, because, as we will see when we discuss binocular (twoeyed) vision, the
use of two eyes provides information indicating the coins are at the same distance.
- The cue of familiar size is therefore most effective when other information about
depth is absent
- Atmospheric perspective occurs when distant objects appear less sharp than
nearer objects and often have a slight blue tint - The details in the foreground are sharp and well defined, but details become less
and less visible as we look farther into the distance.
- The farther away an object is, the more air and particles (dust, water droplets,
airborne pollution) we have to look through, making objects that are farther away
look less sharp and bluer than close objects.
- The reason that farther objects look bluer is related to the reason the sky appears
blue. Sunlight contains a distribution of all of the wavelengths in the spectrum,
but the atmosphere preferentially scatters shortwavelength light, which appears
- Texture gradient : Elements that are equally spaced in a scene appear to be more
closely packed as distance increases
- Shadows—decreases in light intensity caused by the blockage of light—can
provide information regarding the locations of these objects.
- Shadows also enhance the threedimensionality of objects.
Motion Produced Cues
- There are two motion produced cues:
o Motion parallax
o Deletion and accretion
- Motion parallax occurs when, as we move, nearby objects appear to glide rapidly
past us, but more distant objects appear to move more slowly.
- Thus, when you look out the side window of a moving car or train, nearby objects
appear to speed by in a blur, whereas objects that are farther away may appear to
be moving only slightly
Deletion and Accretion
- As you move your head, your left hand appears to cover your right hand. This
covering of the farther right hand is deletion . If you then move your head back to
the right, the nearer hand moves back and uncovers the right hand. This
uncovering of the far hand is accretion .
Binocular Depth Information
- Twoeyed depth perception, called stereoscopic vision , involves mechanisms
that take into account differences in the images formed on the left and right eyes.
- When the left eye was open, the images of the near and far fingers were lined up
with the same place on the retina. This occurred because you were looking
directly at both objects, so both images would fall on the foveas of the left eye.
- When the right eye was open, the image of the far finger still fell on the fovea
because you were looking at it, but the image of the near finger was now off to the
- These different viewpoints for the two eyes is the basis of stereoscopic depth
perception —depth perception created by input from both eyes.
- Stereoscopic depth perception is qualitatively different from monocular depth
perception. Seeing Depth With Two Eyes
- When we view a twodimensional movie, the left and right eyes receive
essentially the same images, so depth is indicated only by monocular pictorial
- When viewing a 3D movie, the left and right eyes receive different images, so
stereoscopic depth perception occurs
- “Walleye” condition in which the eyes look out, are forms of strabismus , or
misalignment of the eyes. When this occurs, the visual system suppresses vision
in one of the eyes to avoid double vision, so the person sees the world with only
one eye at a time.
- Binocular disparity , the differences in the images on the left and right retinas, is
the basis of the stereoscopic vision Susan experienced.
Corresponding Retinal Points
- Corresponding retinal points —points on the retina that overlap if the eyes are
superimposed on each other
- Thus, whatever a person is looking at directly (like Julie) falls on corresponding
points, and some other objects (like the tree) fall on corresponding points as well.
Julie, the tree, and any other objects that fall on corresponding points are located
on a surface called the horopter .
Absolute Disparity Indicates Distance from the Horopter
- The images of objects that are not on the horopter fall on noncorresponding
- The degree to which these objects deviate from falling on corresponding points is
called absolute disparity .
- The amount of absolute disparity, which is called the angle of disparity , is
indicated by the red arrow, which shows the angle between the corresponding
point for the lefteye image of Bill (red dot) and where the image is actually
- The angle of disparity therefore provides information about an object’s distance
from the horopter, with greater angles of disparity indicating greater distances
from the horopter.
Relative Disparity is Related to Objects’ Positions Relative to Each Other
- The difference in absolute disparities of objects in a scene, called relative
disparity , remains the same as an observer looks around a scene. Relative
disparity helps indicate where objects in a scene are located relative to one
Disparity (Geometrical) Creates Stereopsis (Perceptual)
- Disparity is related to geometry—the locations of images on the retina. Stereopsis
is related to perception—the experience of depth created by disparity.
- Stereopsis —the impression of depth that results from information provided by
binocular disparity. An example of stereopsis is provided by the depth effect achieved by the stereoscope, which produces a convincing illusion of depth by
using two slightly different pictures.
- The passive method works according to the principles we have described for 3D
movies, with two superimposed polarized images viewed through polarizing