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» Film-Tech Forum   » Operations   » Large Format Forum   » IMAX Film 3D Weirdness (Page 1)

 
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Author Topic: IMAX Film 3D Weirdness
Mark J. Marshall
Film God

Posts: 3184
From: New Castle, DE, USA
Registered: Aug 2002


 - posted 03-08-2010 08:02 PM      Profile for Mark J. Marshall     Send New Private Message       Edit/Delete Post 
I posted a while back that I thought the IMAX at the King of Prussia theater in Philly had a very slight out of sync problem, but I wasn't sure what it was I was seeing. Some folks here said that it was impossible on certain IMAX projectors, and possible but very unlikely on the other kinds of IMAX projectors. I'm not an IMAX guy, so I don't know for sure one way or the other.

This past weekend we saw Alice in Wonderland there, and it looked pretty good (other than the 3D effect being of varying degrees of View-Masterish compared to real 3D). But every once in a while - usually during cuts between scenes - things looked like they weren't in sync. At this point, I've come to just think "whatever" when ever I see that.

But then at the end of the show, while leaving the theater, I looked at the scrolling credits on the screen WITHOUT my glasses on. Instead of seeing something like the top three lines in the image below - which is what I would have expected to see, I saw what looked like the bottom three rows in the image below.

The two images were not aligned on the screen horizontally - which I thought was quite odd. Can anyone explain what would have caused that? Doesn't seem normal to me.

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Edit: I used Trajan just to piss Bobby off. [Razz]

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Julio Roberto
Jedi Master Film Handler

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From: Madrid, Madrid, Spain
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 - posted 03-08-2010 08:57 PM      Profile for Julio Roberto     Send New Private Message       Edit/Delete Post 
Because of the use of Trajan, it's not possible to have but an exact diagnostic. [Razz]

Possible explanations, besides "shooting" errors, are obviously, lens distortion and not perfectly centered or aligned projectors.

Most lenses will have some uncorrected barrel distortion as they are spherical. When the screen is flat, this distortion is assymetrical when you try to hit two images in the same spot through convergence of two separate lenses.

Easier explained with a picture. To indulge Bobby, I have tried hard to stay away from Trajan ...

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... but failed miserably. [Razz]

This leads to vertical parallax in parts of the screen. The more as you go more off-center. Then the projector position etc comes into play. As does the separation between the projectors/lenses. Etc.

This is one area where a single-projector solution (RealD, Dolby, Master Image) outperforms dual projection or dual-beam-of-light (Sony, Imax, RealD XL, dual-projectors, Technicolor 3D).

Single projectors with single beams of light have symetrical distortions on both 3D views, so no problems there.

With digital, you could perhaps adjust a bit better the images, but with film-based stuff like regular Imax, it's just not possible since each theater has a certain screen size/focal length and you can't practically made a compensated print just for that theater.

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Dave Macaulay
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 - posted 03-08-2010 09:21 PM      Profile for Dave Macaulay   Email Dave Macaulay   Send New Private Message       Edit/Delete Post 
King of Prusia is a "classic" 15/70 film projection system. I don't know if it's a large rotor machine or the newer "SR" smaller rotor machine. It's almost certain to be the dual-rotor system, with both film strips running through one projector head. There are (were?) a few 3D systems with two projectors.
Anyway there is an adjustment for the vertical misalignment you show. Some projectors have a servo drive adjustment system and can apply a shot by shot correction. There is either a lack of adjustment or a failure of the adjustment system there.
The vertical misalignment is actually not a big deal: it causes a slight feeling of oddness if the misalignment is changing during viewing, but if slight and static it doesn't affect 3D viewing for most people.
The worst 3D misalignment problem is when you try and make people's eyes spread outwards (walleyed). This is an unnatural alignment of the eyes. Some people will do OK with a bit of walleyeing, most will get a dull headache, some will puke. The farthest objects in a 3D scene should have left and right images superimposed, closer objects crossing eyes.

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Julio Roberto
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 - posted 03-08-2010 09:52 PM      Profile for Julio Roberto     Send New Private Message       Edit/Delete Post 
The keystoning when coming from two off-the-shelf lenses occupaying different parts of the space can not be corrected to a high degree of perfection, specially w/o digital compensation, as the distortion becomes assymetrical.

quote: Dave Macaulay
The farthest objects in a 3D scene should have left and right images superimposed, closer objects crossing eyes.
Although that's the way Imax does it to avoid having to worry about the different screen sizes on their different theaters, the farthest objects in a 3D scene should have left and right images at a possitive distance of around 6cm. Objects that you want to appear on the screen: superimposed. Objects that you want to be between the screen and the viewer, negative values of parallax.

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Mark J. Marshall
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From: New Castle, DE, USA
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 - posted 03-08-2010 10:14 PM      Profile for Mark J. Marshall     Send New Private Message       Edit/Delete Post 
This misalignment seemed fairly consistent across the entire screen during the credits though. I have to believe that there must be some kind of an adjustment that would just move the image down (or up) a bit on the screen similar to lens eccentrics adjustments on 35mm projectors.

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John Wilson
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 - posted 03-08-2010 10:52 PM      Profile for John Wilson   Email John Wilson   Send New Private Message       Edit/Delete Post 
IMAX Sydney had an up/down lens adjustment option. Two different prints could have two different settings so it would have to be fixed.

The sideways shift would be a 3D 'location' thing.

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Julio Roberto
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 - posted 03-08-2010 11:05 PM      Profile for Julio Roberto     Send New Private Message       Edit/Delete Post 
quote: Mark J. Marshall
This misalignment seemed fairly consistent across the entire screen during the credits though. I have to believe that there must be some kind of an adjustment that would just move the image down (or up) a bit on the screen similar to lens eccentrics adjustments on 35mm projectors.
My missunderstanding. If one of the whole images was slightly above the other, then sure, you can adjust one of the lenses/gates/projectors/whatever and move the whole thing up/down a bit.

I thought you were talking about the vertical parallax that results from the assymetric keystoning when two lenses try to shoot for the same plane. That's "never" perfect and not correctable mechanically with off-the-shelf optics.

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Hillary Charles
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 - posted 03-09-2010 07:37 AM      Profile for Hillary Charles   Email Hillary Charles   Send New Private Message       Edit/Delete Post 
Vertical misalgnment, if it's big enough, is extremely uncomfortable. I've never seen anything like that at our IMAX, but as mentioned, it can be corrected.

Based on those credits, perhaps Buckaroo Banzai can help. [Big Grin]

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Gordon McLeod
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 - posted 03-09-2010 08:49 AM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
If it is the twin machine SR system it is possible that it is in the tracks that the machine slides on to the threading position not returning to the exact location
If it is the twin rotor then it is a misalignement of the lens mount
I would phone the theatre and ask to talk to the chief projectionist

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Kurt Zupin
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 - posted 03-13-2010 12:44 PM      Profile for Kurt Zupin   Email Kurt Zupin   Send New Private Message       Edit/Delete Post 
Our credits have the same issue, and I can assure you every thing is aligned correctly. This seems to be an issue at the printer. The feature and trailers are aligned perfect then the scrolling credits hit screen and look like crap.

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Mark J. Marshall
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From: New Castle, DE, USA
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 - posted 03-13-2010 02:31 PM      Profile for Mark J. Marshall     Send New Private Message       Edit/Delete Post 
Thanks guys. And thanks Kurt. Interesting to know. I wonder if the DLP 3D shows have that problem.

I was wondering who would be the first to notice that, Hillary! [thumbsup]

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Adam Martin
I'm not even gonna point out the irony.

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From: Dallas, TX
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 - posted 03-13-2010 02:54 PM      Profile for Adam Martin   Author's Homepage   Email Adam Martin       Edit/Delete Post 
Both the GT and SR projectors have right-eye lens shift adjustments. The GT is on the "Adjust" screen and the SR is on the "Lens Adjust" screen.

The PLC can even memorize what the correct setting is for each different show and return the lens mount to the appropriate position when the show is initiated.

It shouldn't be that difficult for the operator to change the lens shift when the credits hit the screen, assuming there's even an operator in the room.

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Kurt Zupin
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 - posted 03-15-2010 11:08 AM      Profile for Kurt Zupin   Email Kurt Zupin   Send New Private Message       Edit/Delete Post 
This is true Adam, but with this feature that does not help. I fiddled with it during the credits to no avail. Where I have it set at as the "Normal placement" is the best the credits looks. It has something to do with the scrolling credits on this feature. The credits that are in the picture frame look sharp and have crisp lines.

I do agree though, the operator should be adjusting as needed to make the show look the best it can.

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Tom Petrov
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 - posted 02-06-2011 04:27 AM      Profile for Tom Petrov     Send New Private Message       Edit/Delete Post 
quote: Mark J. Marshall
The two images were not aligned on the screen horizontally
I did notice this during some of tonights sub-titles of Sanctum. Very slight but I was sitting off-centre

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Mark J. Marshall
Film God

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From: New Castle, DE, USA
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 - posted 02-08-2011 01:50 PM      Profile for Mark J. Marshall     Send New Private Message       Edit/Delete Post 
Is Sanctum digital only?

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