P3D Re: IMAX 3D projection

[Paul Talbot <ptww@xxxxxxxxx>]

Thanks for the additional comments, Boris.  I mentioned I'm pretty
dense on the technical side, and shall now likely prove it to any
remaining doubters.  :)

(heavily snipped)
> "Into the Deep" objects within four or five feet of the camera
> a few scenes with very small creatures only 10 inches from my
> eyes, based on sitting 6 rows back

So you suppose these objects were even a lot closer than 4-5 ft
from the camera?  Is that "where they went wrong" in that film,
in your estimation?  How would you want them to keep from presenting
that close to your eyes?  (Which, I agree, is too close.)

> Now, to elaborate on my comments regarding "mounting to the screen."
> Assuming you sit 50 ft. from the screen, and the perceived stereo
> image does not "pierce" this screen

This still sounds like it is taking my comments to a greater extreme
than I intended.

> AND the image is orthoscopic

I think you might mean to say "ortho-stereoscopic."  According to
Ferwerda, "orthoscopic" means not-pseudoscopic. (p.24).  Assuming
you do mean "ortho-stereoscopic," however, I'm still confused.  The
perceived image will only be ortho-stereoscopic for the few people
fortunate enough to sit in the "ortho-seat(s)," according to my
(limited) understanding of the technical aspects of projection.
So, unfortunately, I still do not understand the assumptions you
use to reach your conclusion.

> looking as if through a window six stories tall, 80 ft. wide

Is this the actual screen dimensions, or numbers derived from
some input variables?

> at subject matter that cannot have very much depth, because it
> is so far away to begin with.

Another part I don't really understand.  When viewing projected
slides, as I move back away from the screen, the perceived image
stretches on the z-axis.  Objects, and the air spaces between them,
are both stretched.  This is very different from what happens when
viewing a real scene, of course.  So that is where I am stumbling
on your analysis, which seems to say that because real objects 50
feet from our eyes don't show much perceptible stereoscopic depth,
the same automatically applies to projected images 50 feet away.
I have not (knowingly) suggested any requirement that the objects
50 feet away have the same separation as they have in the real world.
(That would be impossible to control from everyone in a theatre,
anyway, because everyone is viewing from different distances.)

> mounting to this screen

See above comment about taking my notes to an extreme I did not
intend.  If by "mounting to this screen" you are saying to mount
so all objects are at or behind the screen, this is not what I
am suggesting.

> would mean limiting your disparity to 2.5 inches max "on-screen,"
> That's why Imax wouldn't bother with anything other than infinity
> at the screen.  they are happy to have "infinite" points fall within
> five inches of each other, one way or the other.

If five inches disparity is acceptable with infinity at the screen,
why impose the 2.5 inch limit when placing infinity somewhere behind
the screen?
 
> doesn't mean you can't have a stereo window preceding the close-up

My concern is not with the window, but with the problem of watching
"ghost-like" images that dance in thin air and pass through other
patrons in the theatre.

Thanks for the patience.

Paul Talbot


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