Test Scores
• Mean 69.53%
• Median 70.00%
• C+ / B-
• To go over your test, make an
appointment with your TA Karla
(karla.ponjavic@uleth.ca)
Upcoming:
Read article by Gregory for February 10th
Read chapter by Pinker for February 17th
Read article by Land for March 3rd
Seeing Depth
• The world is 3D, the retina is 2D !
Depth Cues
•
Four categories:
1.Pictorial
2.Physiological
3.Motion
4.Stereoscopic
Pictorial Depth Cues
– Shadows and
Shading:
visual system
assumes light
from above
Pictorial Depth Cues
– Shadows and
Shading:
visual system
assumes light
from above
Pictorial Depth Cues
– Shadows and
Shading
Pictorial Depth Cues
– Shadows and
Shading
Pictorial Depth Cues
– Retinal image size
– far objects smaller than near objects
Pictorial Depth Cues
• Retinal image size
– problem: big far things same as close
small things
– “solved” by size constancy: Perceived size
is adjusted according to perceived
distance (based on other cues)
– forms the basis for several visual illusions
Pictorial Depth Cues
• Retinal image size
Pictorial Depth Cues
• Retinal image size
Pictorial Depth Cues
• Linear perspective
Pictorial Depth Cues
• Texture gradient
Pictorial Depth Cues
• Height in the plane
More Depth Cues
•
•
•
•
Pictorial Depth Cues
Physiological Depth Cues
Motion Parallax
Stereoscopic Depth Cues
Physiological Depth Cues
–
Two Physiological Depth Cues
1. accommodation
2. convergence
Physiological Depth Cues
–
Accommodation
Physiological Depth Cues
–
Convergence
Physiological Depth Cues
–
Convergence
– small angle of convergence = far away
– large angle of convergence = near
–
–
What two sensory systems is the brain
integrating?
What happens to images closer or
farther away from fixation point?
Physiological Depth Cues
–
Convergence and accommodation are
reflexively linked
Under what circumstances might this be a problem?
Motion Depth Cues
–
Motion
1. Parallax
Motion Depth Cues
–
Parallax
Motion Depth Cues
–
Parallax
– points at different locations in the visual
field move at different speeds depending
on their distance from fixation
Depth Cues

Pictorial Depth Cues: aspects of 2D images that imply depth

Physiological Depth Cues: Proprioception in ocular muscles indicates
accommodation and convergence

Motion Depth Cues: foreground and background move in opposite directions

Stereoscopic Depth Cues: disparity between two retinal images indicates
distance
Illusions related to depth
perception
• Misperceptions of size
• Misperceptions of location
Muller - Lyer Illusion
Muller - Lyer Illusion
Muller-Lyer Illusion
• What theory does Gregory take up?
Muller-Lyer Illusion
• What theory does Gregory take up?
– Thiery / Woodworth : Muller-Lyer figure
“suggests perspective”
Muller-Lyer Illusion
• What theory does Gregory take up?
– Thiery / Woodworth : Muller-Lyer figure
“suggests perspective”
– Is there something about the presence of
pictorial depth cues in an image that
distorts size?
Depth Cues and Size
Distortion
The Ponzo Illusion:
Depth Cues and Size
Distortion
The Ponzo Illusion:
Depth Cues and Size
Distortion
• The illusion of depth in an image seems
to distort the perception of size in the
image
• This is due to misapplication of Size Constancy
Pictures are Weird
• What does Gregory find paradoxical
about pictures?
Pictures are Weird
• What does Gregory find paradoxical
about pictures?
Pictures convey depth yet also appear flat
Why?
Pictures are Weird
• When you view a picture there are
some depth cues telling you to perceive
depth, but other cues telling you that the
image is on a flat surface
• …the surface matters!
Pictures are Weird
• Retinal image size
Pictures are Weird
• Retinal image size
The Apparatus
• What does he do to remove the paradox
so that the Muller-Lyer figure no longer
looks flat?
• Remove the background surface
The Apparatus
Removing the Background
• With background removed, Muller-Lyer
figure actually looks 3D
• What prediction can you make about
how the perceived size and the
perceived distance of the center line are
related?
Removing the Background
• With background removed, Muller-Lyer
figure actually looks 3D
• What prediction can you make about
how the perceived size and the
perceived distance of the center line are
related?
When the centre line appears close,it should also appear small
When the centre line appears far, it should also appear large
Removing the Background
• How did Gregory test that?
The Apparatus
Removing the Background
• Perceived size and perceived distance are tightly
related
• inward arrowheads make line look closer / outward
arrowheads make line look farther
Seeing
Seeing in
in
Stereo
Stereo
Seeing in Stereo
It’s
It’svery
veryhard
hardtotoread
readwords
wordsififthere
there
are
aremultiple
multipleimages
imageson
onyour
yourretina
retina
Seeing in Stereo
It’s
It’svery
veryhard
hardtotoread
readwords
wordsififthere
there
are
aremultiple
multipleimages
imageson
onyour
yourretina
retina
But how many images are there on your
retinae?
Binocular Disparity
• Your eyes have a different image on each
retina
– hold pen at arms length and fixate the spot
– how many pens do you see?
– which pen matches which eye?
Binocular Disparity
• Your eyes have a different image on each
retina
– now fixate the pen
– how many spots do you see?
– which spot matches which eye?
Binocular Disparity
• Binocular disparity is the difference
between the two images
Binocular Disparity
• Binocular disparity is the difference
between the two images
• Disparity depends on where the
object is relative to the fixation point:
– objects closer than fixation project
images that “cross”
– objects farther than fixation project
images that do not “cross”
Binocular Disparity
• Corresponding retinal points
Binocular Disparity
• Points in space that have corresponding
retinal points define a plane called the
horopter
The Horopter
Binocular Disparity
• Points not on the horopter will be
disparate on the retina (they project
images onto non-corresponding
points)
Binocular Disparity
• Points not on the horopter will be
disparate on the retina (they project
images onto non-corresponding
points)
• The nature of the disparity depends
on where they are relative to the
horopter
Binocular Disparity
• points nearer than
horopter have crossed
disparity
• points farther than
horopter have
uncrossed disparity
The Horopter
Binocular Disparity
• Why don’t we see double vision?
Binocular Disparity
• Why don’t we see double vision?
• Images with a small enough disparity
are fused into a single image
Now on to Magic Eye
Stereograms
Stereograms
Divider
Left Eye
Right Eye
•Right eye sees face to the
left; left eye sees face to the
right therefore:
faces present crossed
disparity to the brain
•Face appears in front of
square
Autostereograms
• Optically separate images aren’t
needed
Autostereograms
• Optically separate images aren’t
needed
• WARNING! Tricky stuff coming in the
next slides
Autostereograms
• Uneven spacing between identical
objects in a single picture can appear as
disparity if the angle of convergence is
inappropriate
• TRICK: Seeing depth in
autostereograms requires you to
suppress the reflexive coordination
between convergence and
accommodation
Autostereograms
Any repeating objects that have a spacing different from the background will
have either crossed or uncrossed disparity
RIGHT EYE
LEFT EYE
If you uncross convergence, your right eye gets these faces
shifted slightly to left, left eye gets them shifted to right =
CROSSED DISPARITY
What would you see?
Autostereograms
Any repeating objects that have a spacing difference from the background
will have either crossed or uncrossed disparity
RIGHT EYE
LEFT EYE
If you uncross convergence, right eye gets these faces shifted
slightly to right, left eye gets them shifted to left =
UNCROSSED DISPARITY
What would you see?
Autostereograms
• by adjusting the disparity at different parts of
the image (with a computer usually) one can
make shapes that emerge or recede in depth
“Magic Eye” Stereograms
• Usually viewed with uncrossed convergence
• Imagine gazing farther than the surface (let your eyes
“relax”)
• Now try to notice objects or forms in the blurriness
• As you become aware of shapes, try to focus
(accommodate) the plane of the image without
converging your eyes
Autostereograms
Autostereograms
Autostereograms