Frame-Sequential
Stereoscopic Computer Graphics Displays
A frame-sequential stereo display presents alternating
left-eye and right-eye images on a CRT monitor. Shutter glasses are synchronized with the monitor to allow only the
appropriate eye to view the current image.
To avoid flicker, the monitor must be operated at twice the usual
frame-rate.
The advantages of a frame-sequential display are principally
that the use of a single monitor saves space and the cost of an extra monitor
and graphics controller, which are the most expensive components of a computer
graphics system. In addition, there are
never alignment problems such as result from the use of multiple CRTs on
optical channels. However, there are
some disadvantages. The liquid-crystal
shutters have imperfect performance.
Most importantly, most CRT phosphors have sufficiently long image
persistence that the image for each eye persists into the following frame,
which is viewed by the opposite eye.
These conditions produce crosstalk.
Our monochrome monitor and ferro-electric shutter glasses (described
below) largely eliminate these problems.
Our economy glasses are suitable for development and
testing. Our medium-priced shutter
glasses provide performance similar to other high quality shutter glasses on
the market. Switching times are about 2
msec., which is longer than the vertical blanking period and causes cross talk
at the top of the frame. Contrast
ratios are typically 150:1 measured with tungsten. This ratio does not describe performance with a CRT.
The persistence of standard CRT phosphors--either RGB or monochrome--are the major factor
limiting performance. Phosphors usually
have 2 or 3 components, each having a different luminance decay characteristic. One of the components, present in low
quantity, has very long persistence and limits performance over the entire
screen. This afterglow is called
“tailing.” The major phosphor
components have more rapid decay. A
standard RGB monitor, used with the Pi-Cell glasses, will produce extinction
ratios of roughly 40:1 for red, 20:1 for blue, and 8:1 for green, in the center
of the screen. Standard P-4 monochrome
phosphors are not significantly better.
In both cases, performance is worst at the bottom of the screen, because
the pixels there have little time to decay before the next frame. Performance is worse at the top because of
the switching time of the Pi-Cell glasses.
To improve performance, Vision Research Graphics (VRG) has
introduced two products:
1) a
monochrome monitor with a custom phosphor compounded to eliminate tailing,
2) Ferro-Electric
shutter glasses with extremely fast (50 msec.) switching
times. Used together, these products
result in an excellent monochrome display that virtually eliminates cross
talk. The cross talk is so low that we
have been unable to measure it, although it is visible in a darkened room. The monochrome phosphor is yellowish in
color. The Ferro-Electric glasses are
essentially optically perfect but have relatively small apertures (1”) and are
designed to be held close to the eyes.
The shutter positions are fully adjustable, and can be used by children.