Depth perception is the ability to see the world in three dimensions and to perceive distance. Although this ability may seem simple, depth perception is remarkable when you consider that the images projected on each retina are two-dimensional. From these flat images, we construct a vivid three dimensional world. To perceive depth, we depend on two main sources of information: binocular disparity, a depth cue that requires both eyes; and monocular cues, which allow us to perceive depth with just one eye.
Binocular Disparity
Perhaps the most important perceptual cues of distance and depth depend on so-called binocular disparity. Because our eyes are spaced apart, the left and right retinas receive slightly different images. This difference in the left and right images is called binocular disparity. The brain integrates these two images into a single three-dimensional image, allowing us to perceive depth and distance. The phenomenon of binocular disparity functions primarily in near space because with objects at considerable distances from the viewer the angular difference between the two retinal images diminishes.
Monocular Cues
Monocular cues are cues to depth that are effective when viewed with only one eye. Although there are many kinds of monocular cues, the most important are
Interposition
The most important monocular cue is interposition, or overlap. When one object overlaps or partly blocks our view of another object, we judge the covered object as being farther away from us.
Atmospheric Perspective
The air contains microscopic particles of dust and moisture that make distant objects look hazy or blurry. This effect is called atmospheric perspective, and we use it to judge distance.
Texture Gradient
A texture gradient arises whenever we view a surface from a slant, rather than directly from above. The texture becomes denser and less detailed as the surface recedes into the background, and this information helps us to judge depth.
Linear Perspective
Linear perspective refers to the fact that parallel lines, such as railroad tracks, appear to converge with distance, eventually reaching a vanishing point at the horizon. The more the lines converge, the farther away they appear.
Size Cues
Another visual cue to apparent depth is closely related to size constancy. If we assume that two objects are the same size, we perceive the object that casts a smaller retinal image as farther away than the object that casts a larger retinal image. This depth cue is known as relative size, because we consider the size of an object's retinal image relative to other objects when estimating its distance. Another depth cue involves the familiar size of objects. Through experience, we become familiar with the standard size of certain objects. Knowing the size of these objects helps us judge our distance from them and from objects around them.
Height Cues
We perceive points nearer to the horizon as more distant than points that are farther away fom the horizon. This means that below the horizon, objects higher in the visual field appear farther away than those that are lower. Above the horizon, objects lower in the visual field appear farther away than those that are higher. This depth cue is called relative height, because when judging an object's distance, we consider its height in our visual field relative to other objects.
Motion Parallax
Motion parallax appears when objects at different distances from you appear to move at different rates when you are in motion. The rate of an object's movement provides a cue to its distance. The more distant objects appear to move in a more slower pace.
Ambiguous Depth Cues
The methods of depth perception discussed above rely on different views from different eyes. However there are other more subtle ways to make a two dimensional image "feel" three dimensional. These methods are called ambiguous depth cues and are used by artists to make their pictures appear more realistic. These techniques are called ambiguous because there is more than one way to interpret them and the brain prefers to see them as three dimensional. While the other methods of depth perception occur in the eye, these methods occur in the brain. The primary forms of ambiguous depth cues are size, brightness, shadows, perspective, the use of the straight line and obstruction.
Size
When an object gets farther away it focuses on a smaller area on the retina. As a result, we generally associate a smaller object with being further away.
Brightness
If a light source illuminates closer objects more than those further away, we see different shades of brightness and assume different distances from a light source. Colour can have a similar effect as through changing colours artists can create the illusion of fog and make a piece of a picture appear further away.
Shadows
When a light source illuminates an object, it will not illuminate objects behind it. Shadows make one object appear to be in front of another and the shadow size determines the depth between them.
Perspective
As we look at the horizon, straight lines appear to converge. By converging lines in a picture, an effect of making them appear to move further away can be created.
A good example of perspective is the Ames room or the distorted room constructed by the ophthalmologist Adelbert Ames. The room from the front appears to be an ordinary rectangular room with a back wall and two perpendicular parallel side walls. However, in reality this rectangular appearance is a trick of perspective. The true shape of the room is trapezoidal with slanted walls and the ceiling and floor at an incline. The room is observed through a peephole in order to create the best viewpoint and remove any sense of depth which would otherwise be created if viewed through both eyes. The perspective illusion is often enhanced by adding additional clues such as a checkered floor and rectangular windows on the back wall.
There is nothing surprising in our perception of the empty room as normal, for the image we see through the peephole is identical to that which would be received from a normal rectangular room. However, when people stand in the room there is a conflict. The person in the further corner has a smaller image, due to their greater distance from the observer as compared to a person in the nearer corner. What is surprising is that observers see the people distorted in size and the room retains its rectangular shape, presumably because we are used to seeing rectangular not trapezoidal rooms. Thus, an adult in the further corner will appear smaller than a child in the nearer corner. Ames and subsequent researchers used this phenomenon to demonstrate the importance of experience in perception.
The Straight Line
By curving a "straight" line, a two dimensional picture may appear to be bending into three dimensions.
Obstructions
Because we can not see around corners, an object will block your view of everything behind it. In a picture, the object that blocks your line of site must be in front.
Reference:
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http://www.newworldencyclopedia.org/entry/Ames_room. [Accessed: 21 May 2010]
Depth Perception [online]. Available at:
http://library.thinkquest.org/27066/depth/nlindex.html. [Accessed: 21 May 2011]
Introduction to Perception [online]. Available at:
http://www.sapdesignguild.org/resources/optical_illusions/intro_depth.html. [Accessed: 21 May 2011]
Depth Cues [online]. Available at:
http://psych.hanover.edu/krantz/art/cues.html. [Accessed: 21 May 2011]
Ames Room. 1997. Available at: http://www.psychologie.tu-dresden.de/i1/kaw/diverses%20Material/www.illusionworks.com/html/ames_room.html. [Accessed: 21 May 2011]
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