Re: Ortho Seat

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

Paul T writes:

> As usual, I can't begin to keep up with John's insightful
> analysis.  :(  But I want to ask a question anyway.  ;)

If you can't, I screwed up and I apologize.  8-(  I'm certainly
willing to expand on any section you'd like.

> As Bob Aldridge noted (but disagreed with) most projectionists
> adjust the projector so that the window is at the screen.
> But in the 4200mm image height example, you seem to be saying
> that the window is 4200mm in front of the screen.  (6300mm
> viewer to screen distance, minus 2100mm viewer to window distance).

I think, to be more exact, Bob wants to put the window at the proper 
distance regardless of where the screen is.  You can do this if the 
room is dark.  What Bob specifically doesn't want to do is put the 
window at the screen when the screen is 50 feet square!  That makes 
you have to go walleyed if you look at objects placed at infinity.  
Of course if you're far enough from the screen, a 70 mm spacing will
have your audience diverge only a slight amount and you'll get away 
with it but there's no reason to push it.  Bob's point is, put your
infinity points 65 mm apart (some say 63.5 for more comfort for 
those with closer-spaced eyes and to ~match the infinity spacing on 
the slides).  Really there's no problem with putting them down at 
50 mm apart other than it pulls the window closer to you.

If you have a 4' high image, the window distance and the screen
distance match.  At all other image sizes (which cause different
ortho seat distances) the window will be off the screen.  A smaller 
screen will have the window farther away than the screen and a
larger screen will have the window closer than the screen.  The
window is always 7' away because that is the chosen standard for 
the system and the way the normal masks are to be set.

> And then there's the whole issue of the reconstructed distance
> to the window changing with chip placement in the mask...

Well, sure, you can put the most distant objects in the slide 
farther apart than normal infinity setting but that would be 
considered an error because the projector would have to be
reconverged to bring them back to 63.5 on screen.  Then the 
window would come off the screen at you if you're you happened
to be using a 4' image but that's not a problem if the room is 
dark enough.

I'll try to think of how to say this stuff more clearly and post
again.  (What's that groaning sound?)

John B


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