Re: Viewing System of Choice

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Sat, 11 Jan 1997 02:27:48 -0600
>From: P3D Oliver Dean  <3d-image@xxxxxxxxxxxxxx>
>Subject: Viewing System of Choice

I thought this was a very interesting post. I looked up some of the old posts
on IMAX 3D, and recent discussions, and I had some comments and questions:

>Bill Costa wrote:
>...
>>     Does anybody remember the frame rate used on an IMAX system and how
>>     that compares to a conventional 35MM feature film?

>I think I can say from a a position of experience that, for me, the LCD
>glasses system showed no sign of the annoying "watery ripple" that shows
>up with LCD videos taken with my Toshiba SK3D7 stereocam, and appears,
>for all practical purposes, to be as "solid" as 3D IMAX made for the
>Polaroid system. The "watery ripple" effect, similar to the effect seen
>when a stereo movie is about 1/2 frame out of sync, is most prevalent
>with objects that move rapidly in a vertical direction, and vertical
>water fountains video'd with the SK3D7 lose all sense of cohesion in the
>water droplets.  

I found that the high speed shutter setting on mine wasn't much use for
3D. For slow-moving objects you don't need it, and for fast-moving objects
it emphasizes the time difference between the left and right views. The part
of the brain that decodes stereo information in moving objects seems to work
like a pipeline processor, and if there are time or position discrepancies
between the two views, this part of the brain sets a "skeptical about the
validity of the image" that gets passed along to the subsequent processing
stages.

>However, in L5, there is a scene at the beginning in
>the "garden" that shows a very close waterfall, only about two or three
>feet away, that acts as a transparent "curtain" between you and the
>background garden, and there appears to be no problem with it.  Birds
>flapping their wings are another kind of shot that is especially
>susceptible to this effect, and the bright red lorikeet (the little
>girl's pet bird, "Torch") at the beginning of "L5" likewise seems to be
>free of such problems.

In addition to the faster shutter speed, the Toshiba system has a constraint 
that may not apply to the shuttered IMAX system. The left and right images in 
the Toshiba camera are displayed at different times, and also taken at 
different times, so there is no instant in time when the left and right
images being processed by the brain come from the same time frame, and
have the same positions of moving objects. (For instance, when the bird
flaps its wings, the wings will always be a little higher in one view than
the other, in what's being processed by the brain, and this discrepancy makes
it hard to fuse a stereo image.) For the Toshiba 3D, a little motion blurring
helps smooth over the discrepancies, and makes the stereo easier to view.

In the IMAX system, the old messages agree with your information, and
add that the information presented is 24 frames per second to each eye,
but each frame is flashed twice, so that if the left frames are numbered
L1, L2, L3..., and the right frames are numbered R1, R2, R3..., then what's
actually put on the screen is like:
   L1 R1 L1 R1 L2 R2 L2 R2 L3 R3 L3 R3..., 
at a field rate of at 96 Hz (48 Hz to each eye). If the movie was shot with
two synchronized cameras, then L1 could have been shot at the same instant
as R1, then L2 at the same time as R2, and so on. If the latter statement
is true (I don't know for sure), then about 3/4 of the time the brain would
be processing images that actually took place at the same time. Intuitively,
I'd think that would produce more realistic motion then the approach used
by the 3D camcorder.
  
>All that being said, what works for me may not work for someone else
>with a different physiological "threshold" for this kind of perception. 
>But it is clear that the greater the frame rate, the less apparent the
>effect.  If my information is correct, the IMAX 3D frame rate for each
>eye is a solid 48 frames per sec, which would be considerably better
>than the 30 fps for each eye with my flickery Toshiba.

See above comments. I agree that some people are much more sensitive to
discrepancies than others. Flicker sensitivity also varies among people.

>But both IMAX systems seem to have their advantages and disadvantages --

I agree. I like both, but they seem to work best with different kinds
of movies.

>The Polaroid glasses have an annoying tendency to "ghost" unexpectedly,
>especially with an unconscious head tilt or with viewing from a sharp
>angle from a side seat, although ghosting was minimal with the excellent
>quality of the IMAX polarizing materials and screen surface.  

>The Polaroid method also requires a metallic surfaced screen (more
>brightness fall-off from the side; also, more expensive?), which is not
>necessary with the LCD glasses.  Polaroid glasses are passive (no sync
>circuits and no LCD's needed), are MUCH lower in cost, and are lighter
>in weight -- and on, and on.

An item that confused me - on Friday, Noel wrote (in an explanation of
ghosting at the Tokyo theater) that the IMAX systems which use LC shutters
also tend to use supplementary polarizers, which would sort of imply a need
to use a screen surface that preserves the polarization. And an old message
[Andrew Woods, November 12, 1995] stated that the regular polarized system 
doesn't work well on the domed screen of a SOLIDO theater, and I had the 
impression that the LC shutter glasses are mainly used in SOLIDO theaters. 
So putting together this post, Noel's post from Friday, and the old post 
about polarization on domed screens, there seems to be an inconsistency. 
Can anyone straighten this out?

John R


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