P3D Re: Large screen 3D alignment

[Larry Berlin <lberlin@xxxxxxxxx>]

>Date: Tue, 30 Dec 1997
>From: Andrew Woods writes:
>.................
>I visited the new IMAX 3D theatre in Darling Harbour, Sydney the week
>before last.  They are currently screening two 3D films: Across the
>Sea of Time and Nutcracker.
>
>I saw both films and also noticed problems with the infinity setting 
>of the projected 3D image.  In this case the infinity point was actually 
>in front of the screen - and by quite a substantial amount.  Both films
>seemed to have the same setting.
>
>Although 3D has more impact when it comes out of the screen, 3D is easier
>on the eyes if you pay attention to the accommodation/vergence conflict
>and project images +/- from the screen.  (In this case all the images were
>projected in front of the screen).

****  It sounds like their cameras aren't built to provide windowing
control. The situation you describe is what happens if you use an ordinary
camera to capture the L and R views. When they are viewed in stereo before
setting the window, the image is totally projected in front of the *screen*
which is the same thing as the stereo window for LCS applications. It would
seem that they could control that somewhat at projection time though.

>
>On a slight tangent from this topic - I keep in trying to think of a
>way of measuring visual angle easily while I'm watching 3D movies.
>I'd like to be able to measure the amount of vertical parallax on the
>screen and where the infinity point is set to.  Any ideas?  Perhaps
>a toilet roll tube with a piece of clear plastic stuck to one end onto
>which is printed a scale...

****  That's an easy one. Take the LCS glasses off and look at the screen
directly. The parallax will be directly measurable by seeing the distance
between any two obviously similar image features. Take note of which
features you are watching, as to whether it's a foreground or background
feature. Features at the stereo window level will have NO parallax. Features
rojected in front or behind the window will have parallax, but in different
directions from each other. 

You could estimate visual angles by knowing your distance from the screen
and estimating the parallax distances.

>From your description I would expect to see the infinity points with NO
parallax. For best results it should be somewhere in the middle of the total
depth.

>
>I saw both movies.  I though Across the Sea of Time was great.  On the
>other hand I thought Nutcracker was wierd.  My wife liked it although
>she did say that she was familiar with the Nutcracker story.  I certainly
>don't think it benefits any from being in 3D.

****  Now that's pretty subjective. What exactly does it take for a movie to
*benefit* from being in 3D? I'd prefer all movies to be in 3D regardless of
some limited, subjective determination of whether or not it benefits
thereby. In my mind being in 3D is a benefit all to itself and has nothing
to do with whether or not the movie benefits by the 3D. Of course it
benefits, and just maybe some story factor is enhanced too. That would be a
desirable extra. The prime benefit of 3D movies is that my eyes are wired
for 3D and I appreciate a movie that provides for the human sensory input
system. ANY movie not in 3D therefore fails to be fully what it could be. I
don't need the 3D aspects to be super spectacular just for purposes of
justification of the medium. Biology is sufficient justification. Special
effects that are well done are frosting on the cake.

>
>I wonder whether IMAX has recommendations of the correct (horizontal) 
>alignment of the projected image.  I presume they would but two
>cases (London and Sydney) indicate that maybe they don't or possibly
>alignment procedures are not being followed.
>

****  Maybe the projectionists don't understand the process of adjusting for
these factors? One can often read and follow every letter of instructions
for something that is new or not understood, only to have the results not
work and there may be no clue why not. Maybe they need a better method of
instruction for the operators of the many new IMAX theaters being built? Or
maybe this observed alignment is the officially desired result for some
reason? In which case IMAX needs the education...

Larry Berlin

Email: lberlin@xxxxxxxxx
http://www.sonic.net/~lberlin/
http://3dzine.simplenet.com/


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