How Far Away Is It?
Depth Perception
Depth Perception
• The ability to see objects in three dimensions
although the images that strike the retina are
two-dimensional, allows us to judge distance
• How do we see a 3-D world using only the
2-D retinal images?
• We are able to see in 3-D because the visual
system can utilize depth cues that appear in
the retinal images
Gibson & Walk’s Visual Cliff
• A laboratory device for testing depth perception in
infants and young animals
• Infants that have been crawling for about a month
are reluctant to crawl past the “edge” of the visual
cliff
• Other animals had similar results.
• Suggests that depth perception, to some extend, is
inborn but needs experience of crawling
Visual Cliff
Visual Cliff Experiment
Click on link above to view
the visual cliff experiment
Depth Perception:
Monocular Depth
Cues
Monocular Cues
• Depth cues that require the use of only one
eye
• Monocular depth cues include:
– relative size
– relative motion
– interposition
– relative height
– texture gradient
– relative clarity
– linear perspective.
Monocular Depth Cues – Relative Size
• Using the perceived size of a familiar object
to determine depth
• The larger the object appears, the closer the
object is to the viewer
• The smaller the object appears, the farther
away it is to the viewer.
Monocular Depth Cues – Relative
Motion (Motional Parallax)
• A person who is moving can determine depth by
focusing on a distant object.
• As we move, objects that are actually stable may
appear to move
• Objects further away than the object of focus will
appear to move slowly in the same direction as
the subject is moving.
• Objects closer than the object of focus will
appear to move quickly in the opposite direction.
Relative Motion
In this example,
the passenger is
moving past a
stable world. If she
fixes her gaze on
the bridge, objects
behind it will
appear to move
forward. The
farther away the
object is, the more
slowly it will appear
to move. Objects in
front of the fixation
point appear to
move backward.
Monocular Depth Cues –
Interposition
• Method of determining depth by noting that
closer objects partially obstruct/block the
more distant objects
• Also called “overlap”
Monocular Depth Cues – Relative Height
• Method of determining depth by noting that distant
objects appear higher in your field of vision than do
closer objects
• This reversed above the horizon where higher
objects seem closer.
You know that the trees
and houses are farther
away than the lake because
they are higher up in the
drawing than the lake is.
Monocular Depth Cues –
Texture Gradient
• Method of determining depth by noting that
distant objects have a smoother texture than
nearby objects
Individual flowers
are visible in the
foreground, but in
the distance they
look like a smooth
carpet.
Monocular Depth Cues –
Relative Clarity (Aerial Perspective)
• Method of determining depth by noting that distant
objects are less clear than nearby objects
• Tends to work outdoors because light from distant
objects passes through more atmosphere, we
perceive hazy objects as farther away than sharp,
clear objects
The distant mountains
look blue & hazy
because of dust &
moisture in the
atmosphere.
Monocular Depth Cues–
Linear Perspective
• Method of determining depth by noting that
parallel lines appear to converge in the
distance
• The lines appears to eventually merge on the
horizon.
The sides of the road
seem to converge in
the distance.
Monocular Depth Cues–
Light and Shadow
• Nearby objects reflect more light into our eyes.
• Given two identical objects, the dimmer one seems
farther away.
• Brain assumes that light comes from above.
Watch as the
picture flips
upside down.
Can you apply these principles to
a painting?
On a piece of paper explain the following:
–
–
–
–
–
–
–
Relative Size
Interposition
Relative Height
Texture Gradient
Relative Clarity/Aerial Perspective
Linear Perspective
Light & Shadow
Explain how those terms are
being used in this picture.
Accomodation
• Accommodation is also used by the brain to
estimate distance.
• Brain takes into account how the muscles in
your eye affect your lens - thickens (for
nearby objects) or flattens (for far away
objects).
• However, we rely on pictorial cues more
than this.
Depth Perception:
Binocular Depth Cues
Binocular Cues
• Depth cues that require the use of both eyes
• There are two types: Retinal Disparity &
Convergence
Retinal Disparity
• A binocular depth cue resulting from slightly different
images produced by the separation of the retinas in the left
and right eye
• Because our eyes are about 2 ½ inches apart, the retina
receives slightly different images of the world.
– The greater the difference between the two images, the
closer the object is to the viewer.
– The more alike the two images, the further away the
object is perceived.
• Is most effective when the item is quite close to the person
• Example: hold a pencil just in front of your nose and close
one eye, then the other. The pencil should move showing
the disparity. Now focus on something far away and do the
same. What happens?
Some People’s Eyes are Further
Apart than 2 ½ inches
Binocular Depth Cues: Finger Sausage
Hold your two index
fingers about 5
inches in front of
your eyes, with
their tips half an
inch apart. Now
look beyond them
and note the weird
result. Move your
fingers out farther
and the retinal
disparity—and the
finger sausage—will
shrink.
Autostereogram
• Another way to create the illusion of
depth through binocular stereopsis is
with an Autostereogram
• An autostereogram is formed by
superimposing two repeating patterns
• The two patterns are slightly offset, and
when viewed properly, this offset is seen
as a binocular disparity
Autostereogram
To see the 3-D image, first view the picture closely. Focus your eyes as if you are
looking at some far off distant object (look through the picture). Try relaxing your eyes
so they swing outward. Without changing your focus, slowly move away from the
picture. The image will begin to to come into focus. Do you see a Face? If you have
trouble, it just means your eyes are difficult to trick, they want to focus at the proper
depth!
Convergence
• A binocular depth cue related to the tension in
the eye muscles when the eyes track inward to
focus on objects close to the viewer
• The brain notes the angle of the convergence
and then computes the distance of what you
are focusing at.
• The more tension in the eye muscle, the closer
the object is
• Works best at close distances
Depth Illusions
• Sometimes our perception of depth can be tricked.
Julian Beever’s Sidewalk Chalk Art
Want to see more? Goto http://www.etntalk.com/sidewalk/
Julian Beever’s Sidewalk Chalk Art
Want to see more? Goto http://users.skynet.be/J.Beever/pave.htm