3D Display Methods
The first of them (the motion parallax) does not even require binocular vision and consists in a change of relative object positions within the field of view as the observer moves relative to the scene.
The occlusion effect is blocking a part of one object by another, located in front of the first.
The accommodation effect is the result of the physical effort needed to bring the viewed object into sharp focus.
The toe-in effect comes from our eyes’ optical axes intersecting at the viewed object rather than being parallel. The angle between them changes depending on the distance to the observed object.
All these visual cues (together with others, such as perspective, lighting, and shading) work in concert and are integrated by the observer’s brain into a single image, which has the quality of depth.
Any technology aimed at recreation of spatial perception must simulate these effects and cues, however many of these technologies essentially rely on stereoscopic effect alone to produce an illusion of 3D view.
The existing types of 3D display devices can be broken into several categories depending on their principles of operation summarized in the following diagram:
Both glasses-based and glasses-free 3D displays are being developed by the industry and independent research groups and several specific types of such displays listed above are available as commercial products.
Glasses-dependent 3D Technologies
Glasses-based 3D systems produce a 3D effect by offering two views simultaneously, with each view captured or digitally created to show a slightly different angle or perspective and destined to be visible by the left and the right eye correspondingly. In direct-view or projection displays, the glasses worn by observers use either polarization or a shutter synchronized with the display unit to make visible to each eye only its corresponding view. In head-mounted displays, these two views are generated by two independent display units visible to each eye. The brain processes these images similarly to real-life scenery, combing the two views to give an illusion of depth. Although it is theoretically possible to supply other visual cues in such systems (at least in those intended for a single viewer), all contemporary glasses-based systems rely solely on the stereoscopic effect, or stereopsis, for an illusion of volume, which reduces the quality of perception and raises concerns over health- and safety-related implications. For more information, please, refer to a separate page explaining glasses-based 3D technologies.
Glasses-free 3D Technologies
In order to eliminate the need of a headgear to perceive spatial quality of images, it is necessary to direct appropriate images to each eye of an observer by using other means, which must provide these images irrespective of the observer’s position. This is naturally achieved in so-called volumetric displays where image is created within a physical volume and each pixel occupies a specific element of a physical 3D space. Holography is another, rather extreme, example of glasses-free technologies, which fully recreates the light field emitted by a real object by using ultra-high-density recording media, and thus leads to a viewing experience virtually identical to that of real objects.
More practical approaches to glasses-free 3D display use techniques commonly known as auto-stereoscopic to project various numbers of directional views, of which two different ones are visible by each eye of the observer. This is currently the most popular way of achieving 3D viewing perception without the use of glasses. More details of glasses-free technologies can be found at this link.