A Tale of Cylinders and Ghosts

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When designing CAVE2, one of the first questions the team had to answer was: how is it going to look like? Very different form factors were taken into consideration, each with advantages and disadvantages. One big favorite (and final winner) was the cylinder. Building a VR system using a purely cylindrical form factor was desirable for many reasons. We could get good immersion and peripheral vision levels, a big active enclosed space, simplify  assembly, and get a nice, seamless aesthetic design.

A big problem we had to solve with this design was limiting crosstalk levels on our passive stereo displays. When a user stands much above or below the display center, the polarization overlay will not match the correct display pixels anymore. The result is: you see ghosts. Well, ok, not really. But some of the light targeted for the left eye goes to the right eye, and vice versa, resulting in a blurry stereo image. We measured ghost levels on our test setups and concluded they were very consistent and low (<2%) throughout the viewing angles to approximately +/- 20 degrees. At that point, ghost level increased dramatically at a steep slope to uncomfortable and unusable levels (>10%).

This could simply not be ignored in our design. For example, a setup based on an 8-foot high LCD wall would need to be viewed from 12 feet away in order for the top and bottom of the display to have acceptable ghost levels. We wanted users to get as close as 2 feet from the displays. At these distances the stereo image from the top and bottom rows of displays would turn into a complete mess.

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Our solution was to physically shift the display polarization overlay: this way the polarizer rows are centered with pixel rows relative to the off-axis line of sight, rather than relative to the normal direction of the screen. For example, for a positive off-axis view angle (viewer’s eye lower than target) the polarization overlay would be shifted downward. Conversely, the polarizer would be shifted upward for negative viewing angles. These fixed shifts in registration would allow the viewer to see more of the “correct” pixel row and less of the “incorrect” or ghost producing adjacent pixel row. The net result was increased low-ghost viewing range, allowing a user to move closer to the display before exceeding the critical crosstalk angle. 

Cylinders: 1 Ghosts: 0.

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