3D Vision

1
3D Vision "Monocular Cues"
How do we extract information about the 3rd
dimension from our 2 dimensional retinal images?
2
Chapter 8 Seeing the third dimension from a two
dimensional retinal image
3
Although we associate depth perception with
binocular vision, we clearly perceive the 3rd
dimension even monocularly. What are the
monocular cue to depth?
Monocular depth cues are the same cues that
artists use to create the impression of the 3rd
dimension in a 2D picture.
First what information is lost?
Size and shape ambiguity. Does this mean that
size and shape cannot be used to ascertain object
distance? No, known shapes and predictable sizes
can be used.
4
There are four general cues to target depth
and/or distance.
1. Overlay or superposition cue
Behind
In front

Since most objects are familiar and obey simple
rules, and since the alternative objects are
unlikely, the overlay cue is very valuable and
highly visible.

5
Figure ground illusions employ stimuli in which
the bran cannot establish which part of the
stimulus overlays the other. That is, which is
figure (in front) and which is ground
(behind). Here are two classic examples of how
the brain deals with this ambiguity.
6
Count the legs and then count the feet of this
elephant. Anything odd here? The artist has
employed figure ground ambiguity to generate this
impossible creature.
This is a perfect example showing that the
retinal image may not be sufficient to identify
the object. In this case we realize that two
rectangular pieces of wood generate the same
retinal image as three circular pieces, and from
this the artist generates an impossible figure.
7
Example View of highway from above.
2. Geometric Cues or Geometric Perspective
This retinal image is interpreted as three
parallel lines of varying distance, and not three
converging lines at a single distance (although
both objects would generate the same retinal
images)
Angle
Distance
Sometimes referred to as aerial perspective
because we see this commonly from above
8
What makes the perceived depth more salient in
the image on the left?
In the above example, some geometric cues are in
opposition. Which ones?
9
Ambiguous Figures the 3D cues are not sufficient
to provide a unique perceptual solution
Is the green dot in the front or back of this
cube?
Check out this web site http//dogfeathers.com/ja
va/necker.html
10
In this example, the artist has employed three
monocular depth cues, but made an impossible
figure by using the ambiguity of our two
dimensional representation of three dimensional
space.
11
Can you figure out how this illusion works? It
is a nice version of the type of illusion made
famous by the Dutch artist Escher.
12
Example
This rectangular table-top produces a trapezoidal
retinal image. Do we see trapezoidal tables?
No, we always see rectangular tables with their
fronts and backs at different distances.
Geometric perspective can be employed by the
visual system to generate an accurate perception
of distance because the angular subtense and thus
retinal image size are proportional to target
distance, AND many objects have familiar,
regular, and fixed shapes. Thus, the only way
that the table can generate a trapezoidal retinal
image is if its front and back are at different
distances.
13
Shape constancy and depth perception example
Here we do not see the door remaining in the
plane of the wall and distorting from a rectangle
to a trapezoid. Instead, we see a rectangle that
is tilted in depth (the door is open). This is a
good example of perceptual learning.
14
3. Texture Gradients
Again, due to relationship between object
distance and retinal image size, objects with
uniform textures (as many have) produce texture
gradients when the object spans a range of
distances. We can use this cue to see depth in
an object surface.
Distance Angular size Texture Density
15
4. Depth from Shading
Which half avocado contains the pit?
Because almost all light sources are high
(above), protruding objects appear brighter on
top and darker due to shadow on the bottom, and
the reverse is true for cavity objects. We can
use this cue. In this example, we have two
directions of shading, and we see one protrude
and one recede.
16
All four of the depth cues are employed in the
cube illusion demonstration at the following web
site. Can you construct the illusion and
identify the role of each?
http//www.psych.upenn.edu/backuslab/cubeillusion.
pdf
17
5. Motion Parallax
Commonly observed while travelling by train, or
as a passenger in a car.
View from right
View from left
When in motion we see that ojects at different
distances change their relative locations. In
the above example, as we move from left to right,
the near object (car) appears to move from right
to left relative to the tunnel which appears to
move in the same direction as us. This relative
motion cue is called motion parallax and the
direction and speed of the relative motion
provides us with information about the relative
distances of targets.
18
The Size/Distance interrelationship.
Gradients of size within objects (geometric and
texture cues) provide cues of varying distance.
However, when objects are seen to be at greater
distances, they are perceive as larger than might
be expected from their (small) retinal images.
1. Size Constancy (objects do not appear to
shrink as they become more distant). Emmerts
Law Perceived size is proportional to perceived
distance 2. Shape Constancy (objects do not
appear distorted when they span a range of
distances). We do not see trapezoidal tables,
doors, windows.
19
1. Demonstrate Emmerts Law. Generate
after-image and view at different distances.
How does perceived distance affect perceived
size? The after-image has a fixed neural size.
.
20
Is the man behind the same size as the larger man
in front or the smaller man in front?
21
Is the small monster running from a large monster?
Use rectangle to check sizes.
22
The Ames Room
These two girld are the same size and no trick
photography was used to make this illusion, in
fact, if you were located at the same place as
the camera, you would have seen the girl on the
right to be very large.
23
Ames Room explanation
Explanation Using the size and shape ambiguity
pointed out at the beginning of this lecture, a
room has been constructed that, from one point of
perspective it has the same angular dimensions as
a regular rectangular room. However, on the
right the real room is close and small, and on
the left the room is distant and large. Thus,
not only does the far wall of the room appear
equally far away, so do the two girls, but of
course, the girl on the right is much closer and
thus subtends a much larger visual angle and thus
generates a much larger retinal image. Since wwe
perceive them to be equidistant, the only
perceptual interpretation is that the right hand
girl is simply much larger than the one on the
left..
24
Size Constancy is basis for many classic size
illusions
Muller-Lyer Illusion
Ponzo Illusion