Depth perception by imran ali

2. Depth PerceptionDepth Perception
By; IMRAN ALI

3. Perception Perception
The process of organizing and
interpreting information, enabling us to
recognize meaningful objects and
events.

4. Depth perception Depth perception
is the visual ability to perceive the world in three is the visual ability to perceive the world in three
dimensions (3D) and the distance of an object.dimensions (3D) and the distance of an object.
Depth perception arises from a variety of depth cues.
These are typically classified into binocular cues that
are based on the receipt of sensory information in three
dimensions from both eyes.
Monocular cues that can be represented in just two
dimensions and observed with just one eye.

5. Binocular depth cuesBinocular depth cues
Convergence
◦You converge your eyes when looking at
close objects you diverge to look at
distant objects.
Accommodation
◦You accommodate more to focus on
closer objects
◦The amount of convergence and
accommodation can be used as cues for
absolute depth (for objects not too far
away from you)

7. Binocular visual fieldsBinocular visual fields
To make use of binocular depth cues, an organism
must have a binocular visual field -- a region of
overlapping visibility for the two eyes.
Retinal disparity
Although humans have large binocular visual fields,
each eye is getting a slightly different view of the
world because the two eyes are in slightly different
positions.
Retinal disparity is the difference between the
lateral position of object in the left and right eyes

8.  These images illustrate how the same scene might look to the left
and the right eyes. Notice that objects are different distances from
one another in the two pictures. The difference in the lateral
positions of an object in the left and right eyes' image defines the
retinal disparity for that object. The disparity in this scene is
particularly easy to see in the arrangement of the champagne
glasses and pointing hand on the right side of each scene.

9. Monocular depth cuesMonocular depth cues
Occlusion:
Closer objects partly block the view of more
distant objects.
Occlusion leads to a percept of depth (you see the
occluding object as closer than the occluded object).
Occlusion also enables us to complete and recognize
objects.
Occlusion is closely related to other perceptual
phenomena such as transparency and illusory
contours.

10.  Occlusion also is
related to illusory
contours. In each the
top two figures, you see
illusory shapes floating
in front of four
completed circles. Of
course, neither the
illusory shapes, nor the
circles are included in
the information that
reaches your retina....

11. Aerial perspective: More distant objects are
perceived less clearly than closer objects.
◦The further the light must travel to reach the eye, the
more likely that light will be interfered with in some
way by matter.
◦Aerial perspective sometimes is called atmospheric
perspective because the effect is due to the
atmosphere interfering with light.

12. Linear perspective: Lines that are
parallel in the real world appear to
converge in a drawing.
◦The greater the distance, the greater the
convergence.
◦At infinity, lines meet at the vanishing point.

13. Texture gradients: Texture is more
dense in a distant object than in an
identical closer object.
◦If there is no variation in texture density, no
depth will be perceived.
◦An abrupt change in texture implies a depth
discontinuity (a sharp bend).

14. Shading: Three-dimensional objects cast
shadows, so 3D objects tend to have luminance
gradients.
◦In perceiving depth from shading, we make certain
default assumptions about lighting
There is only one light source.
Light comes from above.
"Above" is defined retinaly, not environmentally
(i.e., the light seems to come from the same
direction as the top of your head, even when you
turn your head on its side).
◦Some animals have evolved a camouflage strategy
called counter-shading to counteract the effects of
shading. For example, fish are lighter on their undersides
and darker on their top sides. This way, when light hits
them from above, they'll minimize the luminance
gradient, and look as flat as possible.

15.  The "direction" of depth depends on the direction of
the luminance gradient. Because we assume (as the
default) that there is only one light source, and that
that light source is from above, if an object is brighter
on the top than on the bottom, we'll perceive that
object as convex (like the "eggs" that form an X in
this picture). In contrast, if the object is brighter on
the bottom than on the top, we'll perceive that object
as concave (like the "holes" in this picture).

16. Size: Smaller objects often appear to be
more distant than identical larger objects.
◦The link between size and perceived depth stems
from the fact that as an object moves further away
from you, it's retinal image size decreases (and
vice versa).
◦Two objects of different retinal image size are at
the same depth only if they really differ in size.

17. MOVINGMOVING
Motion Parallax
◦Objects at different distances from fixation move at
different rates and directions on your retina.
◦Motion parallax can be thought of a disparity across
time in contrast to the disparity across eyes seen in
stereovision.
◦Net result is the same: by integrating information about
slightly different views across time, you see depth.
◦Note that disparity from motion parallax is equivalent to
disparity from stereopsis when the head/eye is moved
the distance between the two eyes.