P3D Re: Stereo Window Demonstration

[abram klooswyk <abram.klooswyk@xxxxxx>]

David W. Kesner wrote (P3D 3738, 23 Feb 2000):

>I see the window as fixed no matter if it is in a viewer, projected,
>or prints. This window is at the surface in prints or the screen in 
>projection. The image moves through this window. If you have your 
>window placed in front of or behind the screen or surface then it is 
>wrong. How's them for fightin' words *{;-)

Gabriel Jacob (P3D 3739, 23 Feb 2000):
>the window can be off the screen in projection. 
>Example if you don't superimpose the windows,
>they can "float" off the screen or vice versa.

George Themelis (P3D 3739, 23 Feb 2000):
>As Gabriel pointed out, the window can be off the screen in
>projection.

The window not only can, but _should_ be floating off the
screen, 
when _large_ screens are used.

Suppose a "normal" scene from 7 feet to infinity, mounted in a
normal Realist mask. Suppose you project with the window on the 
screen (superimposing the images of left and right mask
apertures
on the screen).

The window deviation on the slide (infinity separation minus 
window separation) is some 1.2 mm. A screen infinity separation 
of 65 mm is achieved with a projection magnification of 65 /
1.2,
or about 50 times. Then the width of the projected image will
be 
about 50 * 21 mm = 105 cm, little more than 3.5 feet.

Keeping the window on the screen, any larger projected image
width 
will give a screen infinity separation in excess of 65 mm,
which 
requires divergence in viewing for the average spectator. 
Some divergence is acceptable, but obviously there are limits. 

Using a screen image width of 10 feet, with the window on the
screen,
gives a screen infinity separation of more than 20 cm, over 6
inches.
What is the limit? Hard to say exactly, but obviously there is
a 
magnification limit above which an off-screen stereo window is 
required.

In well darkened projection rooms or theaters it is hard to see 
at all where the window is, with respect to the screen. 
So there is no real argument to have the window on the screen,
other than that it makes life easier for the projectionist, who
just can superimpose the mask apertures. But stereo is not for 
the projectionist but for the spectator :-).

Abram Klooswyk