Stereoscopic displays: Glasses-free processes
2023-02-19
Overview
In addition to the dominant glasses-based methods, there are also glasses-free approaches under the name of autostereoscopy, which are very suitable for prints such as those on postcards due to the absence of external aids 21 , but cannot be used for projections such as those in the cinema 5 .
Besides the physical separation of the eyes into separate optical devices, their natural distance from each other and thus their slightly different perspectives are utilized to select the correct partial image for each eye with the help of so-called parallax barriers or lenticular grids 5 / 30 / 31 .
Functioning
In general, the image information is again divided column by column alternately between the two eyes by interleaving, i.e. in the form of two horizontally interlaced partial images, which inevitably results in at least a halving of the image quality compared with regular images on the same display 5 / 21 / 30 / 31 (see Fig. 15 ).
A parallax barrier is a striped mask that is placed in front of the image of the display and, by correct selection of spacing as well as width of the alternating transparent and opaque strips, makes an image column visible to the corresponding eye while it remains invisible to the other eye 5 / 21 / 30 / 31 (see Fig. 19 left part). If the parallax barrier is implemented by an additional LC display that can be controlled electronically, the stereoscopic effect can be activated or deactivated as desired 5 / 21 / 30 / 31 . However, the physical blocking of image parts always results in a substantial reduction of the perceived brightness of the display 5 / 30 / 31 .
This problem can be avoided by using a so-called lenticular grid, which can be understood as a string of many small, specially tuned converging lenses that refract the light in such a way that within a certain viewing angle range only the image part assigned to each eye becomes visible 5 / 31 / 32 (see Fig. 19 right part).
Similar to the shutter process, the quality of the stereoscopic effect is unsatisfactory in reality when using only two partial images despite theoretical fulfillment of all necessary criteria. However, this cannot be attributed to too slow changes of the partial images, but to only a very narrow viewing angle range in which the effect can be optimally perceived 5 / 21 / 30 / 31 / 32 .
A possible solution to extend the available viewing angle range is to use a larger number of interlaced sub-images, but this further reduces the effectively usable resolution 5 / 21 / 30 / 31 / 32 . Alternatively, the viewer's eye movement can also be tracked by the playback system in real time and the image content can be adjusted accordingly, but this is only suitable for so-called single-view applications, where a single viewer is served, due to the complexity involved and targeted tuning to a single pair of eyes 5 / 30 / 32 .
Benefits and drawbacks
The obvious advantage of autostereoscopy is the elimination of the glasses otherwise required to perceive the stereoscopic effect 5 / 21 / 31 , which tends to promise wider acceptance by users.
However, this method is not suitable for projections, which make up the majority of all stereoscopic displays, and there are significant quality or comfort limitations depending on the specific implementation 5 / 21 / 30 / 31 . Despite the possibility of color reproduction, these aspects can be such major drawbacks to the perceptual experience that even the outdated anaglyph method is preferable in most situations.
Appendix
Fig. 19: Comparison of parallax barrier and lenticular grid
Sources
Text
- Grasnick, Armin: 3D ohne 3D-Brille. Handbuch der Autostereoskopie, Berlin 2016, p. 97 – 124.
- Grasnick, Armin: Grundlagen der virtuellen Realität. Von der Entdeckung der Perspektive bis zur VR-Brille, Berlin 2020, p. 260 – 282.
- Grasnick, Armin: 3D ohne 3D-Brille. Handbuch der Autostereoskopie, Berlin 2016, p. 124 – 142.
Appendix
- Based on: Cmglee: Parallax barrier vs lenticular screen.svg, https://commons.wikimedia.org/wiki/File:Parallax_barrier_vs_lenticular_screen.svg , 14.02.2023.
