How to understand stereopsis and stereoscopy

Firstly we should analysize the difinition between the stereopsis and stereoscopybefore understanding them clearly.Below are about the basic meaning related: Stereopsis (from stereo- meaning "solid" or "three-dimensional", and opsis meaning appearance or sight) is the impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision. Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due to the eyes' different positions on the head. These differences, referred to as binocular disparity, provide information that the brain can use to calculate depth in the visual scene, providing a major means of depth perception. The term stereopsis is often used as short hand for 'binocular vision', 'binocular depth perception' or 'stereoscopic depth perception', though strictly speaking, the impression of depth associated with stereopsis can also be obtained under other conditions, such as when an observer views a scene with only one eye while moving. Observer motion creates differences in the single retinal image over time similar to binocular disparity; this is referred to as motion parallax. Importantly, stereopsis is not usually present when viewing a scene with one eye, when viewing a picture of a scene with both eyes, or when someone with abnormal binocular vision (strabismus) views a scene with both eyes. This is despite the fact that in all these three cases humans can still perceive depth relations. To understand what stereoscopic 3D is, it's necessary to understand perceived depth. There are many cues that help us perceive depth.

Stereoscopy (also called stereoscopics or 3D imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word stereoscopy derives from the Greek "στερεός" (stereos), "firm, solid"[2] + "σκοπέω" (skopeō), "to look", "to see".[3]

Most stereoscopic methods present two offset images separately to the left and right eye of the viewer. These two-dimensional images are then combined in the brain to give the perception of 3D depth. This technique is distinguished from 3D displays that display an image in three full dimensions, allowing the observer to increase information about the 3-dimensional objects being displayed by head and eye movements.

Objects in perspective, occlusion, and relative size are good indicators of depth.

An object that is farther away is interpreted as such by our brains if it is much smaller than another object next to it. Our brain already knows how big those objects should be in relationship to one another. If two objects are roughly the same size in our field of view, and one is occluded by or is occluding another object, our brain infers that one of those objects is in front of the other. (Occlusion mean one object is laid on top of the other and obscures the other.) Paintings or games can appear 3D because they obey these rules. After Effects also obeys these rules when you create a 3D composition with a camera.
Another important depth cue is lens blur. If our eyes (or a camera lens) focus on a specific object, and another object appears blurred next to it, our brain knows that the other object is either in front of or behind the object. If there is no blur, our brain thinks that the two are at a similar distance. One can clearly see this phenomenon as our eyes focus on different objects and our retinas blur the out-of-focus objects in the background. Our brain interprets this as a depth cue without us realizing it. This phenomenon is subtle as our brain filters it seamlessly into our perception. It is usually unnoticed to the average person. However, it is possible to train our eyes and brain to experience and be conscious of the depth of field by relaxing the eye muscles and using the following (or similar) technique. Look through a windshield with water droplets on it at night. When you focus outside the windshield, the water droplets turn into little halos of color called bokeh. Similarly, when you focus on the droplets, the streetlights in the background turn into bokeh. This effect can be accomplished with one eye closed. Therefore, it has nothing to do with stereopsis, but instead has to do with our eye’s lens focusing, similar to how a camera lenses focus. Understanding how depth of field is related is important when attempting to create realistic images and works hand-in-hand with stereoscopic 3D in After Effects. Especially with the new and improved Camera Lens Blur effect and related features in After Effects CS5.5.
Finally, arguably the most powerful depth cue is stereopsis. Stereopis is the ability of our brain to take two input images from different perspectives and gain an understanding of how far away two different objects are in relationship to each other. The key point to understand is that since our eyes are spaced apart on our heads, each eye can view a slightly different perspective of the world in front of us. Look at an object nearby and close one eye, then switch eyes back and forth several times. Then try this same exercise on an object that is far away.  You notice that the object that that is nearby jumps from side to side in your field of view a lot more drastically than the object far away. If the close object is in the same general direction as the far away object, the close object switches sides of the far away object. This is the basis of how stereopsis works. Your brain takes the relative horizontal distance between objects in your field of view and compares them to gain an understanding of where those objects are in relationship to each other in terms of depth. It is theorized that pigeons bob their head in order to gain depth perception (since their eyes are on opposite sides of their head and they can’t see depth otherwise). If you look through only one eye, you lose your stereopsis depth cue. However if you bob your head from side to side with that eye still closed, you can get a sense of depth again. This separation between eyes that provides different perspectives is the key to stereopsis.

How to understand stereopsis and stereoscopy

It is important to keep all these depth cues in mind when constructing a stereoscopic 3D composition in After Effects. In the real world, it is possible to give contrary information to the brain and trick it. Optical illusions like the Ames Room, the Infinite Staircase, or tilt-shift photography are all examples of how depth cues can be manipulated and our brains tricked. (Tilt-shift photographiy is a method in which a post-process depth-of-field blur is added to an image to give a broad landscape the feeling of a miniature.) Since After Effects gives you control of all of these depth cues, it's important to maintain control over their interaction and make sure that they are not giving our brains too many contrary depth cues. In real life, one can mess around with our surroundings in intelligent ways to create optical illusions. But more often than not, inconsistencies in the digital realm are considered unnatural and can even cause eyestrain or brain pain. Stereopsis, being the most powerful depth cue, is no exception. It's important to make sure that it is not painful to look at the stereoscopic result on different screens. Ones viewing experience can change depending on how big the screen is and how far away the viewer is from the screen.

Stereoscopy

is a digital technique for allowing our brain to see stereopsis by tricking it. This technique is done is by presenting each eye with a different image. The left eye is presented a view of a scene from some virtual or real camera that shows the left perspective. Similarly, the right eye is presented with an image of the right perspective. In this way, each eye is presented with a different image independently and our brain puts them together, and we perceive depth. When viewing a stereoscopic 3D scene on a monitor, the elements in the scene have a tendency to pop out or sink into the screen. Stereopsis is telling us that the object is closer or farther away from us than how far away the monitor actually is.
Many different devices and systems exist for delivering stereopsis to our brains. But in general the principle behind all of them is the same; get one eye to see one view, and the other to see a different perspective of the same scene. Anaglyph glasses are the oldest method, and by far the cheapest. Different colored lenses color filter each eye’s view differently. Red-blue glasses filter out blue on the left eye and red on the right eye. On the display side, the left image is colored red, and the right is colored blue. Then the images are overlapped. Each eye sees only the associated image. Because of the inherent color distortion, it is difficult to see all the colors accurately using anaglyph. But the setup is very easy and works accurately for judging depth and convergence. Polarized glasses work on a simple principle. Two images are displayed on a screen, one image emits horizontally polarized light only, and one emits vertically polarized light only. The glasses have polarized lens such that each only lets through light polarized in one direction. Active shutter glasses work by blocking one eye at a time at a high rate (usually 60fps) and switching the left and right images every frame while synchronized with the monitor. Some TVs use no glasses at all, such as those from Alioscopy. Aliscopy uses lenticular technology, in which the lens on the monitor itself actually refracts the lights in different directions so that each eye gets a different perspective simply by being in a different location in relationship to the TV. There are many more methods for stereoscopy.
When dealing with stereopsis in the real world, the only things that can vary are the positions of objects in front of you, and the perspective from each eye can only change based on that. The only way to make an object look closer throughstereopsis is to actually place it closer. You can’t easily change the distance between your eyes, your field of view, or the aperture of your eyes (at least not voluntarily) to modify the depth of field you perceive. However, in the digital realm, there are many more variables since all of these aforementioned things can be changed. Therefore, there is a high likeliness to introduce confusing depth cues that are contradictory and cause pain when viewing.