3D at an angle

My understanding of the 3D systems used by films like Avatar is that you need to keep your head vertical due to image alignment, polarisation, etc. Is this correct?

I ask because I’m disabled and my head is at an angle. I’d rather see Avatar in 3D, but I doubt it’s an option for me.

Modern 3D movies use electronic glasses that shutter back and forth between the eyes. First one eye is fed an image (timed to the screen) and then the other so that the brain can interpret the images. You should not have a problem with that aspect of it. Some people find it unpleasant to view for extended periods but that may be a function of motion sickness.

I suppose the only issue then is the fact that the images are projected with the eyes assumed to be along the horizontal (if you get what I mean).

My apologies, I may be way off here. The system I was shown years ago does not match what the internet sites describe for Imax 3D. It does sound like a polarized process with right angle lenses.

I call on anybody seeing the movie this weekend to tilt their head left or right and confirm if there is a loss of 3d effect and also tilted back as if in a headrest.

I think you’ll be ok if your head is tilted back because each seat puts the viewer at a different angle to the screen.

I don’t think that matters. It’s like a mirror appears to be a reflection along one axis only, but in fact it is along all axes.

I recently watched Final Destination 4 in 3D which uses RealD technology with polarized lens. Unlike the traditional blue and red glasses, the 3d effect was very pronounced and not tiring to the eyes at all. Also I didn’t notice any degradation of the 3d effect with different viewing angles.

There are a few things. Yes, the 3D effect is rendered assuming your eyes are level. However if you are not tilted too far, it doesn’t break the illusion too badly. Clearly the illusion fails if your head is on its side.

There are a number of ways of projecting a stereoscopic image. I know IMax have used two: passive polarised and active shutter glasses. Beyond that there are other techniques as well, including the ubiquitous red-green glasses. There is also the Infitec system, which uses interference filters in the glasses to provide narrow band transmission of colours - allowing two projectors to provide 6 colours - and the glasses to essentially split the colours to the eyes - allowing passive colour stereo. But the colours are still a bit wonky. I may turn out that, like with IMax, the exact projection technology depends upon the theatre.

Passive polarised has the additional problem that tilting your head means that the glasses no longer align with the polarisation of the projection, and you will get eye to eye bleed of the images, which will ruin the stereo effect. If the theatre uses passive polarisation, that will be the biggest problem.

What does this mean?

I’ve been in some 3D presentations where head tilt impacts image quality. No so much ruining the 3D nature of it but dimming the image. Sorry, I can’t remember which system that was.

As for Avatar I saw it last night in Real 3D (not IMAX) format and head motion was not at an issue at all.

IIRC, the Real 3D technology uses circularly polarized light (rather than plane polarized)*, which doesn’t depend on head orientation. This is an entirely passive setup; the provided glasses have a different circular polarization filter on each lens. My experience is that it’s pretty forgiving of smaller head movements, but I’ve never tried to see how sideways I can get before the illusion breaks down. But I’ve heard that extended head tilting can cause eye strain.

*once I knew this well enough to explain the difference, but now I’m probably better off pointing you to wiki rather than making up a half-assed explanation.

I assume the reference is to the fact that people often conceive of a mirror as flipping images horizontally but not vertically. When (and I imagine you realize this, but for anyone who doesn’t), in fact, a mirror treats the horizontal and vertical axes, and indeed all axes within its plane, identically: to wit, a mirror only really inverts depth (the axis perpendicular to its plane) and nothing else.

That text often appears in a mirror horizontally inverted but not vertically inverted is a function solely of the manner in which one rotates text to face the mirror, and not of the mirror itself; one could just as easily bring the text to face the mirror differently and achieve the opposite effect.

(I have a vague sense that there was some slightly contentious thread about this at some point in the past, or interlude within another thread, but I’m too lazy to hunt it down, particularly since I’m not sure it would add anything to this one)

I may see the 2D presentation first and then, if I’m feeling lucky, try the 3D version in a week or two. I’m actually intrigued by this experiment. I’ll report back if/when I get the results…

The Real 3D process mau use circular polariozers, but there are linear polarizers in front of them, because I see Brewster Angle Polarization of light reflected from seats armrests and other tilted surfaces when I wear the glasses – circular polarizers won’t attenuate that. Infact, they more likely use a linear polarizer and a quarter wave plate, but I haven’t studied it in detail.

Polarizers were used, by the way, in the first-run 3D movie theaters in the 1950s and 1980s. “Traditional” red and green glasses (anaglyphic 3D) was used in the smaller venues that couldn’t afford fancy and expensive equipment.

And, to answer the OP’s question – ANY two-image 3D process is going to be presenting the image you’re supposed to be seeing from a left eye and a right eye, regardless of whether it uses red and green filters, linear polarizers, circyular polarizers, or electro-optic shutters to separate the two. This means that if you tilt your head too far you won’t see the “correct” 3D effect. In some cases (like linear polarizers), tiltimng your head has the additional effect of no longer separating the two images, so you see douible images. But red and green glasses, for instance, or Real 3D will, I think, give you only one image per eye. The ptoblem is that you’re supposed to see one right and one left, so the effect won’t give you a proper image for one up and one down, or one at an extreme angle and its complement. I’m not sure how tolerant the glasdses and the mechanisms in your perceptive system are to angular detuning. Clearly, you don’t have to be perfectly vertical to within a millidegree.
The only sort of system that will give you completely correct 3D no matter how you tilt your head is holographic. They have made holographic movies – but they aren’t projected, and only a handful of people can watch them at one time, so as they exist right now there’s not a big market. I’ve seen one of bottles rotating. Whoopee.

As you realized later, this is incorrect. Active glasses are the least common of four basic 3D systems in multiplexes today.

The most common by far is RealD, which, as others have said, uses circularly polarized glasses. The angle of your head does not affect the perceived 3D effect. With something like 75% of the market, RealD is almost certainly what you will find in your local multiplex. Here’sa theater locator. (RealD is also known as Disney 3D.)

Next is linearly polarized, as used in IMAX theaters, both the traditional 70mm film theaters, and IMAX’s new digital theaters, the majority of which are in smaller multiplex auditoriums. These create some ghosting if you don’t maintain a vertical head position. (IMAX theaters formerly used big, heavy LCD active glasses, but they were cumbersome, expensive, and AFAIK are no longer being used by any IMAX theaters.)

Least common are the Infitec narrow-band color filter system (aka Dolby 3D) and active LCD glasses, provided by a number of vendors, including XpanD.

In light of the OP’s stated condition, I’d recommend he/she avoid IMAX theaters, but almost any other kind will be okay. CalMeacham is right in theory that an angled viewing position weakens the 3D effect, but unless your head is tilted nearly to vertical, you should see the 3D effect just fine.

Sorry for the continued hijack but, yes, there have been threads on this in the past. A really good one a couple (few?) years ago. I do have the time to search - but sadly not the Fu.

As I am responsible for the hijack, which in retrospect seems to be off a different track anyway, here’s a post that covers it.

Am I inferring correctly that the same movie is released in different formats? I’m trying to research the Harry Potter movie that I saw that had a 20 minute 3D scene in it. The glasses appeared to be electronic so I assumed they were of the shutter variety. If the films are released in different formats that means theaters maintain the format that reflects the equipment they installed (I think there’s a play on words in there somewhere).

On a side note, it would seem to me that the shutter style would work with a single projector running at a faster speed which would be Imax territory.

Yes, there are many different formats. IMAX is 70mm film run horizontally, each frame of which is 15 perforations wide. Regular film theaters run 35mm film vertically. Digital IMAX is a different file format from conventional digital, which comes in 2K and 4K varieties.

And there are the different 3D formats I outlined.

Both of the last two Harry Potter films included 3D sections in the IMAX release. The last one had 3D at the beginning, the one before that at the end. I’d be surprised if you saw the last one with active glasses. There may still have been a few theaters using them when the one before that came out.

Yes, they pretty much have to.

IMAX runs at 24 fps, just like regular 35mm. IMAX 3D uses two strips of 70mm film; some IMAX systems run both through a single projector, some use two smaller projectors. IMAX digital uses two separate projectors.

Well, I gave the RealD 3D version a shot today… and it worked! I don’t know if it was the fact that my tilt was under 45 degrees to the horizontal or what, but it worked. How about that!

Yay:) Good to hear and good to know. When you say 45 degress to the horizontal does that mean your head was leaning back or to the side?