Depth PerceptionDepth Perception

By; IMRAN ALI

Perception Perception

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

Depth perception Depth perception is the is the visual ability to perceive ability to perceive the world in three the world in three dimensions ( (3D) and ) and the distance of an object.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.

Binocular depth cuesBinocular depth cuesConvergence

◦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)

Binocular visual fieldsBinocular visual fieldsTo make use of binocular depth cues, an organism

must have a binocular visual field -- a region of overlapping visibility for the two eyes.

Retinal disparityAlthough 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

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.

Monocular depth cuesMonocular depth cuesOcclusion: 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.

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....

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.

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.

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).

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.

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).

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.

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.