Re: IMAX-3D

[Andrew Woods <RWOODSAJ@xxxxxxxxxxxxxxxx>]

Thanks very much to Brian Mason for the very interesting account of his
visit to the Sony studios.  Change that!  EXTREMELY INTERESTING!

Brian Mason (crhs@xxxxxxxxxxxxxx) asked:
>First of all, where does SONY come in? Did they buy out the rights to IMAX?
>I notice that the trademark is still held by the IMAX Corp. in Toronto,
>Canada.

I visited IMAX near Toronto last year, so I'll hopefully be correct
in these responses...

Sony commissioned IMAX (and presumably collaborated with) to produce what
is currently the most advanced IMAX projector currently in use.
It is a double decker 3D projector which is also capable of projecting
IMAX-HD 48fps.  I'm not sure whether it will do IMAX-HD and 3D at the same
time.  The technology I assume is still held by IMAX but with SONY owning
the theatre which the most advanced projector is in, a big bit of cross
promotion doesn't go astray.

>Second, why are they using liquid-crystal glasses in a film theatre
>setting? Isn't the frame speed too slow to prevent flicker, and wouldn't
>standard polarizing glasses do the job just as well? 

Firstly the frame rate for each eye is 24fps but each image is double
shuttered.  i.e. in 1/24th of a second four individual flashed of light
appear on the screen Left1,Right1,Left1,Right1 and then so on Left2,Right2,
Left2,Right2...

The reason they use shutter glasses instead of polarised glasses is a bit
of a funny one.  IMAX believe it is cheaper in the long term to use
shutter glasses than polarised glasses because the shutter glasses
last longer and have to be replaced less.  Seems a bit funny to me
but that's what their manager of film technology said...

They can use the polarised system in the IMAX theatres but not
so in the OMNIMAX (IMAX-DOME) theatres.  The domed screen does funny
things to the polarisation of the reflected light so shutter glasses
are a must in the domes theatres.  In 1993 the the IMAX theatre in
Vancouver Canada was using the polarised method to project IMAX-3D.
They were screening The Last Buffalo and it was great.  The 3D worked
wonderfully!  I assume they are still using the same system.

>Finally, in the Reel 3-D catalog, there was a refence to circular
>polarizers. How can these be used in 3-D projection, and is there any
>advantage? I am familiar with their use in killing specular reflections in
>photography, but can't see the 3-D advantage.

As other people have already mentioned, circular polarisation works great
for 3D.  Clockwise polarisation for one eye and anti-clockwise polarisation
for the other eye (just as you would use vertical and horizontal linear
polarisation).  I've seen it done with both the Tektronix 3D monitors
(I have one here which is used for teleoperation) and for projected 3D
video (at the Stereoscopic Displays and Applications Conference).

John Merritt (jomerritt@xxxxxxxxxxx) wrote:
>Shutter glasses are needed for IMAX because it is too difficult to have
>a screen that maintains polarization with such wide angles of projection
>and viewing.
>
>That's what I heard or read somewhere, but it makes sense.

As I mentioned earlier the IMAX-3D theatre in Vancouver Canada uses
polarisation and works great.  Maybe it was difficult to achieve and
is partly why they're going for shutter glasses these days.  

John Roberts writes:
>From previous posts, the Imax theaters that use
>the liquid crystal glasses have a dual projection system, so it's possible
>that the glasses are switching at 144Hz. (Anybody know for sure?) That would
>result in very low flicker, though field-sequential stereo systems inherently
>have problems with very fast motion.

And in reply Ray Hannisian (ray@xxxxxxxxx) wrote:
>Yes, Imax glasses shutter at 48 fps.  I was lucky enough to get a privete
>tour of the projection booth in NYC last July, from an IMAX executive who
>had been involved in both the development of the 3D system, and the actual
>production of the "Wings of Courage" film that I went there to see.  He
>said that the relatively low brightness of film (compared to TV ) prevented
>any visible flicker.  I was also give a 70mm frame of IMAX film as a
>souvenier.

It is my understanding that the IMAX shutter glasses switch at _96_fps 
(i.e. each eye receives 48fps).  See my explanation above.

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