From: dstamp@watserv1.waterloo.edu (Dave Stampe-Psy+Eng)
Subject: Re: half silvered lenses (was Re: Direct Neural Input (Was Re:
Date: Tue, 5 Nov 1991 03:30:24 GMT
Message-ID: <1991Nov5.033024.17522@watserv1.waterloo.edu>
Organization: University of Waterloo



uselton@nas.nasa.gov (Samuel P. Uselton) writes:

>>>        The first Chapel Hill system also used temple bar mounted CRT's
>>>        and 1/2 silvered lenses, superimposing wireframe images on the
>>>        real world.
>>>
>>>        The images look "ghostly" because you do see the world "through"
>>>        things rendered.  I don't know how satisfactory that would be,
>>>        or how to avoid it.  (The other option, of course, is to digitize
>>>        the "real world" and include it in the virtual environment.)
>>
>>I thought about that kind of system a few years ago, but I had some doubts
>>about it.  The problems I felt it had were:
>>
>>- Any motion of the headset translates into BIG shifts in picture
>>location because of the mirror's magnification
>
>Who said anything about magnification?  If you are doing head tracked
>rendering, the change in what you see should be determined by the motion.
>Whether it is wireframe or raster shouldn't change apparent position.
>The CRT's and half-silvered lenses are all part of the same head mounted
>rig, so their *relative* positions don't change.  Therefore, whatever
>distortion is required is constant.

There HAS to be magnification, unless the display element (CRT, LCD panel,
etc.) is as big as the mirror.  You are increasing the apparent size of the
display to put it at an apparent distance of 100 cm or more, while increasing
the field of view.

In optical systems like this, motions of the viewpoint (the pupils of the
eyes) in relation to the mirror will cause shifts in image position much
as if the display element moved in relation to the mirror.  The pupils of
eyes move through about 12 mm during normal eye movements, but this can
be compensated for in the optical design (just have the viewpoint be at
the eye's center of rotation, 12 mm behind the cornea).  However, shifts
in the helmet or headmount with relation to the eyes are more difficult
to fix.  Having just finished a head-mounted gaze tracking system, I can
say that these shifts DO exist, and the problem increases with the mass
(rotational inertia) of the headmount.  Movements of facial muscles are
a problem even if the headmount is tightly fastened to the head.

Perhaps I'm overrating the problem here, but I do beleive that this problem
will limit the magnification usable for the mirror, setting a minimum size
for the CRT or display element.

>>- delay between the video and the real world meant that if, say, you
>>were moving your hands to manipulate virtual objects, the objects would
>>lag behind your hand movements: VERY disconcerting!
>
>This is a problem with (1) the tracking technology and (2) the compute
>power to re-render your scene.  This method is independent of (1) and 
>(2) should make (2) easier than raster systems.  

Time delays ARE a problem, even in complete VR worlds, including represent-
ations of the user's hands, etc.  The reason is that the human motor system
does NOT use negative feedback directly, due to long neural delays (>100mS).
Instead, it relies on Kalman-type filters and task-specific motor programs
learned through practice.  Any large change in motion-to-visual feedback
delays requires a recalibration of the system at best, and relearning of
fine control for a task at worst.  To say nothing of motion sickness 
brought on by changing head-motion-to-scene-motion delays, which has NOT
been covered well in VR literature, but exists nonetheless.

>>If these objections can be overcome, this system of half-silvered glasses
>>has BIG advanteges for the user, as eye-hand coordination is unaffected
>>by system delays, and real as well as virtual objects can be worked with.
>>Needs a lot less computer power, too.
>
>Of course it depends on the application.  They even tried filled polygon 
>rendering and half silvered lenses.  Then the objects *really* look ghostly.
>
>If you want a substitute reality, this doesn't work too well.  If you 
>want to add some ghosts to this reality, then it will work.

Yes, exactly.  What I was proposing involved being able to interact with
virtual *objects*, while letting the user see his hands, the computer
keyboard, etc.  This solves a LOT of the psychophysical and kinesthetic
problems.  But the time delay problem kills it, for now at least. (sigh).
Again, this approach should not be discarded, but problems should be kept
in mind.

--------------------------------------------------------------------------
| My life is Hardware,                    |                              | 
| my destiny is Software,                 |         Dave Stampe          |
| my CPU is Wetware...                    |                              | 
| Anybody got a SDB I can borrow?         | dstamp@watserv1.uwaterloo.ca |
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