Perception, CSE 2017

Q. How are we able to perceive the world in three dimensions when our eyes are able of sensing only two dimensional images? 15 marks.

8 thoughts on “Perception, CSE 2017”

  1. From the 2 dimensional images that fall on our retina, we construct 3 Dimensional perception of objects. This ability, called depth perception, is due to different sources of information or cues.

    Some of these cues are available to each eye singly, called monocular cues, and others are available to the 2 eyes together, called binocular cues.


    1. ACCOMODATION & CONVERGENCE: The lens bulges to focus on near objects and flattens to focus on far objects, called accommodation. It’s a physiological (monocular) cue.

    Eyes pivot inwards to focus on near objects (and outwards for distant objects), called Convergence. It’s a physiological binocular cue.

    Brain translates movement of the eyes into distance of the object.

    2. BINOCULAR DISPARITY (BD): The 2 eyes form slightly different retinal images due to their distance from each other. The difference between the 2 images is called BD. If the difference is more, we perceive the object as far away.


    1. LINEAR PERSPECTIVE: Parallel lines tend to appear to merge in distance. So we perceive converging lines to be far away.

    2. RELATIVE SIZE: When objects appear to be smaller than what we expect them to be, we perceive them to be far away.

    3. OVERLAP/ INTERPOSITION: When one object is blocking part of another, we perceive the blocking object to be nearer than the blocked object.

    4. AERIAL/ ATMOSPHERIC PERSPECTIVE: We perceive hazier objects to be further away than clearer objects.

    5. TEXTURE GRADIENT: We perceive objects with textured surfaces as nearer.

    6. MOTION PARALLAX: While in motion, observer perceives nearer objects to be moving quickly in opposite direction and distant objects to be moving slowly. This discrepancy in relative motion of near and far objects is called motion parallax.

    7. BRIGHTNESS AND SHADOW: We also perceive depth based on pattern of light and darkness.

    The brain combines monocular cues into binocular cues to form a single perception out of 2 retinal images. This process, called STEREOPSIS, creates 3D perception out of 2D sensations.

  2. Our eyes are capable of sensing only two dimensional images. Yet we are able to perceive the world in three dimemsions. This is due to our potential for depth perception which allows us to transform 2D retinal images into 3D perceptions using cues either from the environment or from our own eyes. It also allows us to estimate an objects’s distance from us.
    While some of the cues require both eyes to work together (known as Binocular cues), others are available to each eye independently (known as Monocular cues).

    Depth perception resulting from binocular cues rests on 4 processes:

    1. Convergence: The process whereby eye points more and more inwards (convergence) as an object gets closer. The brain interprets this process to judge the distance of an object form the body.

    2. Retinal Disparity: It refers to the fact that because the eyes are at slightly different locations, a slightly disparate view of an object is cast on each retina. The closer the object the greater the disparity. The degree of this disparity helps the brain to translate 2D images into 3D objects by providing an estimate of its distance from the body.

    3. Accommodation: It is the process by which the eye focusses an image on the retina by adjusting the contraction and expansion of ciliary muscle. The closer the object, the more the contraction. Brain makes use of this degree of contraction for depth perception.

    4. Steropsis: It is the process whereby the brain combines the two images cast by two eyes into one to give us a 3D perception.

    Apart from these binocular cues there are a number of monocular cues that help us in 3D perception of the world such as Relative size ( smaller objects perceived as more distant), Relative brightness (brighter objects appear nearer), Superimposition (object that blocks the view of others appears nearer), Texture gradiet (smooth texture signals distant objects), Linear & Aeriel perspective (parallel lines appear to converge at a distance and distant objects appear hazy), Variation in light and shadow and Motion Parallex (nearer objects seem to move faster than distant objects).

    Thus, with the help of these and many such cues, we are able to perceive the world in 3D even though our eyes sense it in 2D.

  3. 2d images fall on our retina and we are able to organise them into 3d perception through DEPTH AND DISTANCE PERCEPTION with the aid of BINOCULAR CUES.

    Binocular cues are available when we are able to perceive with the aid of both working eyes. It consist of 4 components which are stated below :

    1. Retinal disparity – Since both of our eyes are at slightly distant location so they form different view of same object which are kept near to us but when the same object is kept far away from us they form same view. for eg : A finger kept close to us at nose and then the same finger taken aback from us

    2. Stereopsis – Though we have two working eyes we don’t see double images else we see single image which resembles that our brain transform them into single image which gives us a 3d sensation.

    3. Accomodation – Our ciliary muscles expand more when object gets far away from us whereas it gets contracted when object gets near to us and it sends this information to the brain which help us in determining the distance at which the object is placed.

    4. Convergence – Our eyes come more closer as the distance of object from our eyes start decreasing whereas they start reaching their normal state when distance of object from our eyes increases and it sends this information to the brain stating the distance of object from us.

    Moreover people with one eye vision are also able to form 3d perception because for objects kept at far distance the image formed by eyes are same and hence they get monocular cues through single working eye.

  4. The image formed on retina is 2-dimentional but our brain can analyse it as a 3-dimentional image because of our ability called Depth perception.
    This ability seems to be the result of our experience mainly which includes the awareness of visual cues which assists the brain in estimating the factors like depth and distance in flat retinal images.
    These visual cues are of 2 types-monocular, involving just one eye and binocular, which involve both the eyes.
    Monocular cues include-
    Linear perspective-here two straight lines seem to converge but instead assumed to be indicating a distant point.
    Overlap-the object which seems to partially cover another is assumed to be closer and the other one as farther.
    Relative size-the smaller object is assumed to be farther than larger one.
    Texture gradient-seemingly finer texture is assumed to be farther while coarser texture as nearer.
    Aerial perspective-the vivid and clearer objects are assumed to be nearer while blurrier objects as farther.
    Motion parallax- when seen from a moving vehicle, the faster moving objects are perceived to be nearer while slow moving one as farther.
    The Binocular cues include-
    Convergence-the brain takes notes note of degree of rotation of eye balls in their respective sockets to focus on an object(eg. When we hold a pen and bring it closer towards eyes the convergence is more and when we move it away, convergence is lesser.)
    Binocular disparity- since two different images are obtained corresponding to each retina, the brain analyses both. If the difference is more, the object is perceived closer else farther away.

  5. Despite sensing 2D image on retina we are able to percieve the things in 3D because of the concept of DEPTH PERCEPTION .
    In which eyes with the help of some cues which is broadly catogerised into monoculor cues i.e primary cues and binocular cues i.e secondary cues eyes are able to percieve in 3D.
    Monocular cues further classifed as-
    1)size cue- larger the object is, will appear closer
    2)texture cue-farther the object is ,smoother it will appear
    3)linear perception-if parallel lines seems converge at distant point ,object will appeae more far away
    4)atmospheric perception-external factors like fog, smog,haze makes object image unclear for far view
    5)motion parallex- while the observer is in motion the farther objects seems to travel in same direction and closer objects seems travel in opposite direction
    BINOCULAR cues are further classfied as
    1)Retinal disparity-images formed on both retina are distinct .we can observe this as closing right eye and observing the finger at a close distance around 6 cm with the left eye and repeating this with left eye closing and observing from right eye there will be a difference in botb the images . All this helps in geeting cue about depth and distance.
    2)Convergence-for observing the objects at different distance our eyes used to adjust , for convergence eyes move towards inward , greater this effect the object will appear closer to us.
    With the help of both the primary and secondary cues we are able to judge depth and distance to percieve in 3Dimension

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