Subject: Re: why 30hz flickers and movies don't

From: Dave Stampe-Psy+Eng <[email-masked]>
Date: Fri, 25 Oct 91 14:47:37 -0400

Coyt Watters sent me some mail.  I tried to get back to him, but the address 
didn't work.  So I'll post my reply here, as I think it's of general interest:

>Been following the thread on Sega LCD glasses.
>
>Why does 30hz to each eye cause a flicker, when 24hz (Film) does not?
>

I believe the reason is that the Sega glsses give you 50% duty cycle
on the images to each eye, and a movie projector gives over 90%.
This results in 5 times the 24 Hz/30Hz flicker (at least).  You can
see the flicker in movies if you have a fast-moving white object on
a dark background-- just look at the edges.

>
>Would it be possible to use two camcorder viewfinders, mounted at
>the sides of the head and projecting onto curved screens before the
>eyes?
>
>The center of vision would not be a problem, but there would be distortion
>at the edjes of vision due to the curving screen.  This could be overcome
>by the lens used to project the image.
>
>Attempt at picture
>               __   __
>              /  \ /  \  <-- screen
>             /    |    \               not to scale (of course!)
>             |   o^o   |
>               U(   )U <-----projector
>                 """
>                 ^
>               User
>Provided the screens were made of a tough, lightweight plastic, the unit would
>not weigh very much, and the weight of the video units would be centered
>of either side of the head, so their weight would balance.
>
>-Coyt D. Watters ([email-masked])

Not that bad an idea.  There are a few implementation problems for
garage VR, though.  First, the mirrors must be a fairly special shape,
and making such a mirror is not trivial.  You'd have to have some way
to turn computer-generated shape data into a form to mold plastic around,
then get it coated.  Not my field, but someone else might know how.

The second, more serious problem has to do with the high mangification.
A slight shift of the mirrors on the head will cause motion in the image
proportional to the same shift in the progector CRT position.  For example,
a 1mm motion with a CRT 12mm wide (standard viewfinder CRT) shifts the
image by 1/12 of its width.  And unless the mirrors were REALLY light, it
is difficult to eliminate shifts entirely.

Still, I've seen military helmet-mounted displays that use this technique.
So perhaps these problems are more easily solved than I think.

BTW, sci.virtual-worlds is back up.  I guess we should take the eyephone
stuff back there (B-{))

--------------------------------------------------------------------------
| My life is Hardware,                    |                              | 
| my destiny is Software,                 |         Dave Stampe          |
| my CPU is Wetware...                    |                              | 
| Anybody got a SDB I can borrow?         | [email-masked] |
__________________________________________________________________________

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From: [email-masked] (Christian Ludwig - CATS)
Date: Fri, 25 Oct 91 16:02:43 EDT

> a dark background-- just look at the edges.

Also...  movies are filmed at 24fps, but when projected, each frame is shown
TWICE in that 1/24sec bit of time.  This approximates the effect of 48fps,
hence, no (or at least far less) flicker.

Chris Ludwig