P3D Re: IMAX 3D projection

[aifxtony@xxxxxxxxxxxxx (Tony Alderson)]

>Boris Starosta wrote (digest 2763):
>Correcting for extreme close ups seems to me easiest at the camera. But I
>am unfamiliar with the technological constraints that the camera designers
>face for this format. Perhaps having shift lenses is just one too many
>variables to add. I did get the impression, that the camera has no
>adjustments of any kind for stereo base, vertical alignment, toe in etc.<

The first IMAX 3D films were shot with a twin-camera rig, similar to the
Disney camera.  IMAX still uses these rigs, when they need a variable base.
However, a few years ago, they designed a unitary twin-strip stereo camera.
It has interchangeable lenses, but the interaxial is fixed at 72mm.  This
lens spacing was chosen as a compromise between the average interocular and
the engineering problem of fitting everything in a reasonably compact
package.  Since the IMAX philosophy is to place infinity at the screen,
there is no need for shift lenses.  (Although the twin rig can toe-in, this
is apparently not done, except by mistake.) The lenses point into angled
mirrors, that reflect onto horizontally travelling film.  The camera can
run at up to 48 fps.  It might be possible to make an adjustable macro
camera, but I think it more likely they would use the twin rig for such
shots. Anyway, to control close-ups with the stereocam, they can only (1)
not get so close, (2) use a shorter focal length (3) adjust the shot after
the fact, either by optical printing or in projection.  (While I can buy
that the vertical alignment is adjusted "on the fly", I still find it hard
to believe IMAX tries to ride horizontals during projection.  If for no
other reason, it contradicts their position of placing infinity at the
screen.  Part of the whole point of that is to avoid image translations!)

If you want to know more about the design of the IMAX camera, go check out
the October 1994 issue of the SMPTE Journal.

>On the vertical alignment question and projector adjustability: That film
>was obviously shot and projected running horizontally - so I do not see
>where vertical misalignment can even enter into the system!? Horizontal
>alignment (disparity) seems more of a troublesome variable for this
>system. <

Nothing is ever perfect. I'm sure there are lots of places for vertical
errors to creep in. Keep in mind IMAX uses separate L and R film strips.

>>Boris Starosta (digest 2758) Reviewed: Into the Deep
>>>I did not perceive any flicker at first, though later I saw a little
>>>"jumpiness" in bright objects moving rapidly across screen.<

>>I suspect this was strobing from the frame rate, not stereo ghosting. You
>>can see this artifact in horizontally (rapidly) moving objects in most

>>>>I thought that in regular cinema, motion blur hides strobing effects
>>>>like this. I am a skeptic here, and propose that either the shutter
>>>>glasses or the horizontally running film format (or both) have
>>>>something to do with it.<<<<

For motion blur to hide strobing, the blurs have to "overlap" from frame to
frame.  If the object is moving too fast, or the shutter angle is too small
(i.e., if the exposure time is too short), there will still be strobing.
If the exposure was instantaneous, about everything would strobe. A good
example of this is the strobing in stop-motion puppet animation. (OK, I
know animation is not instantaneous, but the effect is the same).  While I
don't see how the horizontal path of the film would affect this, the
comments in this thread make me reconsider that the shutter glasses could
aggravate the situation.  Maybe it is an artifact of the phase difference.

>With the tremendous image pair isolation apparently achieved by the system
>(i.e. perfect lack of crosstalk), projecting a "virtual" window close to
>the viewer should be no problem. It should be easy to do - maybe someone
>there will try it.<

It could be done, I kinda like the idea, but I doubt IMAX is interested.
Their claim is that the stereo window is irrelevant in IMAX 3-D, so why
would they go to the expense of introducing a floating window?  But P3Ders
should consider this concept for their stereo videos.

>At 3:54 AM -0000 6/3/98, Paul Talbot wrote: [deleted]

>I did not experience any significant eyestrain viewing the film, but it
>was definitely a workout for the eye muscles, switching between viewing
>objects appearing almost in your lap, and objects at the screen. This
>makes me wonder about someone's recent comment that sitting at the back is
>easier on the eyes. With the stretch effect, the objects at the lap of a
>viewer in the first row will also appear to be at the lap of the viewer in
>the last row, will they not? So the viewers closer to the screen have much
>less changing of accommodation (?) to do, as best as I can figure it.<

The stretch effect doesn't work quite that way.  X amount of screen
parallax does not mean an image will be Y distance away from the viewer, it
means the image will be the proportion Y of the distance from the viewer to
the screen.  Consider this: Assume a viewer with an eye spacing of 2.5
inches.  An image with 2.5 inches of positive screen parallax will appear
to be at infinity, because the viewer's eyes will be parallel.  With zero
parallax, of course, the image will appear to be at the screen.  With -2.5
inches of screen parallax, they converge at a point midway to the screen.
So, no matter where the viewer moves, the image will appear halfway between
himself and the screen. If the viewer is 30 feet from the screen, and the
image appears 3 feet in front of him, when he moves to a seat 60 feet away,
the image will be more like 6 feet in front of him.  The convergence of his
eyes will be less, and the image less difficult to view. (As was noted in
another reply, for any given seat, the accomodation, i.e., focus, is fixed,
the vergence varies.) It follows that the difference in effort between
focussing at 30 feet vs. 60 feet is less than the difference in converging
at 3 ft vs. 6 ft. You should try to set up an experiment and test this.

That's enough for now. Hope I haven't made any ridiculous mistakes.

Tony Alderson
aifxtony@xxxxxxxxxxxxx



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