Autostereoscopic Display Principles

In this article i will talk about stereoscopy and show in a very simple way, how most autostereoscopic 3D displays work nowadays.

 

What is Binocular Vision?

Usually the brain receives images from both eyes at the same time. These two images are slightly different from each other due to eye separation. The brain uses these small differences to combine these views into one and provide us with the notion of how far an object is (depth perception) and how fast the object is getting closer or away from us (movement perception).
In order to test how important binocular vision is to us, simply close one eye and at an arm length try bringing two pencil ends on top of each other. It is certainly more difficult than when both eyes are open. This happens because your skill in judging depth becomes poorer. By way of practice, these tasks may become easier and this is only possible since there are many other clues that help us when judging depth, the so called depth cues:

-    Size of objects (We know how big objects are).
-    Shadow
-    Interposition (the partial blocking of a more distant object by a nearer object)
-    Relative height (things near the horizon give the impression they are distant)
-    Perspective (parallel lines that recede into the distance appear to get closer together or converge)
-    Etc...

One thing that must be considered when creating stereo content is that not everybody has binocular vision. There are cases where it might be reduced or even completely lost, reasons include:

-    Reduced or lost vision on one eye
-    Loss of coordination of movement between both eyes
-    Issues with the brain comparing images from both eyes

Hence, when creating stereo content one has to make sure that binocular view alone is not enough for a perfect stereoscopic view, the other depth cues should also be used. 

Virtual Stereo

Using devices to generate stereo or binocular vision is a way to “trick” our brains in order to create a sense of depth; however this view will never be as correct as in the real world.
The problem with virtual stereo devices is that our eyes need to move in a way they are not used to.

Here is an example. Looking to an object close to the eyes; a person will see it double until the eyes converge to it so that only one object is seen. This process is called fusion.
The other task the eyes must do is to focus on an object so that it sharpens. This process is called Accommodation.

When using stereo displays, as expected, the eyes focus on one object but converge to another. You might be asking yourself how this could happen but the answer is quite simple. The eyes will try to accommodate to the display’s surface but the target object is actually a projection that is displayed on this monitor and therefore they will converge to the parallax of this object.
This might seem strange at first, but it’s important to mention it since some people will feel a certain discomfort when using such a display for the first time.

Autostereoscopic Displays

A stereoscopic display is one that differs from a planar display in only one respect: It is able to display parallax values of the image points. Parallax, as we shall see, produces disparity in the eyes, thus providing the stereoscopic cue.

The main difference between a stereoscopic display and a auto-stereoscopic display is the fact that the first is dependent on a additional device, and the last one not. In order to achieve that, these glasses-free 3D monitors display more than just 2 views like current 3D TVs.

The number of the views will also define the resolution per eye the user will perceive, a 5-view monitor offers 2/5 of the panel resolution per viewer. A 9 view display just 2/9, which would be less than our old CRT TVs.

Following I will show the basics of how a auto-stereoscopic display works, from acquiring the views up to the moment the display shows them.

Producing

The content is produced by taking “pictures” from 5 slightly different positions, making them look slightly different from each other (Parallax). In the example, the tree looks like it’s moving in front of the window.

 

Showing

Each of the 5 different pictures is shown at the same time on a modified LCD display and final output is compiled together using a special multiplex pattern where only some information from each picture is selected.

Splitting

A specially developed optical parallax barrier element is placed on top of the LCD display and is responsible for splitting the pictures from each other before different images are projected onto each eye.

3D Effect 

Due to the fact that your left eye sees a different picture to your right eye and therefore another perspective, the brain is capable of merging both perspectives into a three dimensional scene.
Parallax Barrier

Each pixel on the panel is based on 3 sub-pixels with different colors (red, green and blue).
a layer of an optical material with a series of precision slits is placed in front of the displays.
Each slit covers 3 sub-pixels in a slanted position, forming a new pixel which we call a view-pixel.

Each view-pixel contains information of a different perspective.
As both eyes are located in different places, each of them receives another image information.
The disparity between these images produces the sensation of depth.