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Using Optical Illusions to Convert 2D into 3D

The Pulfrich Effect

Probably the first 3D effect of a straight surface was described in 1922 by Carl Pulfrich. Ironically, the good doctor himself only had one eye and could rely only on what his assistant Fertsch was telling him.

The reaction time of the human eye is inversely correlated to the brightness of the environment. Therefore, using sunglasses or neutral density filters in front of one eye will slow down the signal processing in this eye. A pendulum swinging in a 2-dimensional plane will cause some strange optical illusions. That is, the eye that is slowed down, will signal the position of the pendulum with a time lag to the brain. If the other eye is functioning normally, this will result in the effect that the two eyes will signal different positions of the pendulum to the brain. If optical axes of both eyes for the virtual image are drawn out, they will cross over either in front or behind the plane of the pendulum, depending on whether the pendulum goes back or forth. Therefore, dimming down one eye will create the illusion of the pendulum moving in an ellipsoid (3D) path rather than swinging in a strictly 2D plane. I am using this example simply because it can be repeated at home to show how relative depth perception is.

Later came the isotopic 3D elevation maps with red and blue lines as a means to provide a stereoscopic image. The problem with this technique is that the blue and read glasses used to blank out the unwanted information to each eye make it difficult to achieve reasonable color. Instead of this kind of chromatic interleaving, temporal interleaving of frames can be used. That is, alternating images for the left and right eye, respectively, are displayed and the "unwanted" eye is occluded via shutter glasses synchronized to the frequency of the display.

Shutter Glasses

In theory, they work and, in practice, they work, too even if it is to a limited degree, only. However, there is the drawback, that each eye can see only ½ of the refresh rate of the monitor. In real life, that means that, at 60 Hz refresh rate of the monitor, each eye will see images flickering at 30 Hz, and this is intolerable, even in a dim environment (the critical flicker fusion frequency (CFF) is dependent on light / dark adaptation of the eye). In order to see a flicker-free image, both images must run at a minimum of at least 60 Hz, meaning that the monitor refresh rate must be set to at least 120 Hz. Even midrange monitors are, in most cases, unable to sustain this vertical refresh rate. In addition, most users complain about nausea, headaches and migraines after prolonged use of 3D shutter glasses.

To recapitulate the situation, the requirement is that, at a fixed distance, one eye does not see what the other sees. It is not possible to use color filtering, and we don't want to use any glasses, particularly not shutter glasses. The fixed distance is a negligible factor since, as mentioned, depth perception relies mostly on the relative cue of binocular disparity, that is the lateral offset of image points on the retina.

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