How we perceive 3D?

How  do we able to perceive 3 dimensional world around us?

Depth Perception is the only answer for that. Depth Perception is the ability to see the world in three dimension and to perceive distance. So, if we are able to see 3D, that is because of our Depth Perception and even if we are able to see the distance between the objects, that is also because of Depth Perception.

For instance, if a person is paying attention to a building which is on the other side of the road, he will be able to perceive the distance between him and the building. The reason for his perceiving distance is Depth Perception. So basically if we are knowing how we get to know about distances then it is important for us to know about the types of distance.

There are two types of distance – Absolute distance and Relative distance.

Absolute distance is the distance between a person and an object. For Instance, in the picture given below a man is watch television, then the distance between him and television is absolute distance.

Relative distance is the distance between two objects and even the distance between the parts of same object. For instance, in the picture given below there are two computers on a table, so the distance between one computer and another computer is relative distance. Even distance between the parts of one computer is relative distance i.e. distance between monitor and CPU of the same computer.

Concept of 2D and 3D.

Whatever be the distance, our ability to perceive these, depends on depth perception.

Now we need to come on the concepts of 2D (2 dimensional) and 3D (3 dimensional). We all are concerned with these two names. We watch movies in the theaters in 2D and 3D now a days. We all know the basic difference between these two. Even if you do not know that, let us have a look on the basic information about 2D and 3D. Have a look on the picture given below.

There are two images in this picture above. Both the images are same but looks so different from each other. The reason behind this is that the image on the left is 2D and the image on right is 3D. We can easily get the idea that 2D images are flat but 3D images are not flat, it shows the depth of distances in the picture.We all have eyes and there is a part of eye called Retina, on which image is projected whenever we see anything. The image projected on retina is always 2D, yet the world is 3 dimensional and we are able to perceive it that way.

Now the question arises if image forms on retina is 2D then how we perceive 3D? So, psychologists give some cues i.e. Signals which help us to perceive distance or 3D. These cues are of two types – Monocular cues and Binocular cues.

Monocular cues of depth perception works from only one eye of a person.

Binocular cues of depth perception works from both the eyes of a person.

Monocular cues

1. Size of Image: If we look at an object and it seems small in size but we know that the actual size is not small, we perceive that the object is at some distance from us and if we are able to see the same object of its actual size then we perceive that the object is near. For instance, when we look outside the window the trees far away from us seems small but the tress just near the window seems big as its actual size. So if the size of object is small, there is more distance and on the other hand if the size of object is big, there is less distance.

2. Interposition: If one object obstructs the view of another object, it means the object which is obstructing the view is near and the object whose view is being obstructed is far or placed at some distance behind the object which obstructed its view. The picture given below is the accurate example.

3. Linear Perspective: when the object converges two parallel lines at some point of horizon, that converging point shows the distance. When we travel in a car and look at the trees very far from the front seat, it seems likes the two parallel lines of trees are getting joint.That joining shows that those trees are very far.

 4. Aerial Perspective: When an object seems clear and its detailed information is delivered to us it seems near and when the object we see at some foggy or smoky environment it seems at some distance because details are not well shown. For instance, if a person see a building or mountain at a sunny or clear environment it seems near and if he sees the building or mountain at some foggy view it seems at distance.

5.Texture:  The more clear the design of texture is shown, the more it is near to us and the lesser clear its design is shown to us, at more distance it is. For instance, when we travel in a train, there are small stones on the railway lines and we can watch them so clear sitting at our seat, but if we watch the same stones of railway track at some distance we will not be able to see the detailed shape and size of the stones.

 6.Motion Parallax: It is a kind of cue which we can see in motion. when we  travel in car, we see the trees at very far and it seems moving very slow, but if we see the buildings or houses which are at lesser distance than tress, they seem moving little fast than the trees and when we see the electricity pols just near our car it seems moving faster than tress and houses. So if the object moves fast it is near, but if the object moves slow it is at some distance. One more example for this is flying helicopter, it seems moving very slowly when we see it from ground  because we are standing very far away from it.

Binocular Cues

1.Retinal Disparity: The object we see from both eyes, the vision is not similar. If we see a bottle from right eye, we are able to see the right side of the bottle more than left eye and if we see the same bottle from the left eye, we are able to see the left side of the bottle more than right eye. There is always be some dissimilarity between the vision and this dissimilarity is known as Retinal Disparity. For instance, if we place the thumb just near the center of both the eyes it will overlap because of the dissimilarity of  vision between both the eyes. The more dissimilarity there will be, the more near the object is and if the object is placed at distance the position of the object will not vary whether we see from right or left eye.

2.Convergence: Vision of both the eyes is in parallel lines. If we focus on an object placed far away the lines will be first parallel but it will converge at the point of focus i.e. at object. The more the convergence, the more near the object is and on the other hand , the lesser the convergence, at more distance the object is.

3. Accomodation: Accomodation is a cue which effects the lens of eyes. Ciliary muscles are responsible for the contraction and expansion of the lens. At more distance the object is placed, the more flat the lens is and on the other hand, the more near the object is placed, the more convex the lens is.