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Calculating for screen size...I am missing something

Philip Powell

Philip Powell

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Hello all! And thanks in advance for helping to enlighten the unenlightened (that would be me.)

I've become quite fascinated with stereo since I did some stereo CGI output for an IMAX film last year. (Fortunately I didn't make too many mistakes and it worked well for the most part). Since then I've been reading all I can get my hands on and doing still tests, but I'm yet to do any "live" shooting.


Here's my question, how do you practically calculate for screen size?

I understand the reasons you NEED to do this, (parallax effects change relative to screen size, potentially unfuseable images etc. etc.)

And I see how I could calculate "proper" stereo for a given very specific screen size, but given that movies being shot these days are likely to wind up any number of places, on screens and displays of an almost infinite of sizes and shapes. How is this done?

My only guess is that a film is shot with an intended "average" screen size for a given outlet. (IMAX, traditional theatrical, HD etc) and filmmakers just live with the potential poor stereo consequences if it winds up somewhere else.

Either that or the very unlikely option that everything is shot and posted multiple times for a wide range of potential screen sizes that it could wind up on.

Thanks for helping me to understand what I am sure is very obvious point that I am missing. (it wouldn't be the first time)
 
To be honest, I don't think everyone has things quite nailed down yet. I'd be interested to hear what calculations the "every day" 3D guys do.

I think the most important thing is to view your footage on your target display size as soon as possible. In the BTS scenes for Avatar, it seems they mostly didn't bother with small 3D monitors - they would overlay the images for convergence checking and watch actual stereo footage on a big plasma in a tent on set.

Also try to train your eye and brain so you understand what translates from small to large screen.

We've done a bunch of stereo CG finishes for theatrical... and it's still a bit nail-biting when we go into the DI theatre. Obviously, one way is just to build a nice 3D projection setup for yourself. Personally I'm just going to get the biggest 3D plasma I can - like that 65" Panasonic - and do tons of tests (live action and CG).

I figure if everything looks "good but a little understated" then it might not be too exaggerated if projected on something larger. I'm also going for the "put the focal point at screen plane most of the time" method. So hopefully that will help.

The other alternative is to shoot on 3 cameras - so you can make a "wide stereo" and "close stereo" view :) Maybe a good idea if you're targeting tiny autostereoscopic screens - eg 4.3" Nintendo 3DS and cellphones - as well as theatrical output!

I'm quite interested in that technique for other reasons - eg seeing if it's possible to re-synthesize things a bit to get rid of some of the cardboard cutout effect, kinda like an HDR, but in Z-space. Sorry going OT.

Bruce Allen
www.boacinema.com
 
Hi Philip,

The only safe option is to calculate for the largest screen your footage will ever be shown on.
This might seem overly conservative but it's not as bad as you might first think.

When you look at the footage on your fancy new 3D TV at home it generally occupies a smaller fraction of your field of view than the screen in a cinema and by a quirk of illusion your brain will actually amplify the perceived depth in the scene when you watch it on the smaller screen!
Try this for yourself - play some material you are used to on a monitor and then go stand way further back and observe the depth - it will appear to have deepened.
It's all an illusion anyway :-)

In any case - compromising the apparent depth is a lot better than making the image unwatchable (or hurtful) on larger screens.
 
I am a beginner at 3D, that's for sure. I'm just practicing in my spare time, so far, and I'm far from having any authority on the subject, but the IOD Calc Iphone app (which it appears is Leonard's baby) is pretty sweet, and has been a great help in figuring out what setup delivers what kind of look. You can easily program in the screen size, and then compare how the differences between the best setups and screen size differ.
 
You have to compromise somewhere...

A good and very safe way is to calculate for the biggest possible screen size.

One way is to calculate for a screen size in the middle, and use floating windows whenever required on the big screen version (utilizing more negative parallax to reduce maximum positive parallax)...

I'm quite certain everyone has a different take on this one...
 
Calculate for the biggest screen size.

When working in 3D, I find that if it looks good in the theatre, it will look good on the 24" monitor on my desk. The converse does not hold true, however - it is easy to have things look good (i.e. comfortable to view) on the small monitor that will cause strain in the theatre.
 
Charles has it. Making a 3D film that is "optimized" for all screen sizes is not feasible. You should make it work for the biggest screen you can realistically see you work going on to (IMAX, RealD, Dolby, some other theatrical format) and just live with the knowledge that the film and its stereoscopic effect will have less dramatic impact on smaller screens. This sounds bad when written and read, but it is true of all films 2D or 3D. I saw Dark Knight at the Sony IMAX theater in NYC and was completely blown away by one of the most successful theatrical cinematic presentation I've experienced. Also saw it on a 17" screen on an international flight and still enjoyed the film. I would focus less on calculating numbers and more on familiarizing yourself with what a given IO value does for you when considered in terms of the overall screen size of any given of screen. Also bear in mind that on most shows you will not have the luxury to go back and forth from camera to large screen tweaking settings. Get used to setting up your shots on a 6" Transvideo monitor and making them look good on a 40 foot screen.
 
So, what is the formula for a large screen size? I've seen formulas that suggest the closest object be no closer than 30x the interaxial, but for large screens that number becomes 60:1 or even 100:1. What calculations do you use?

Also, is this number whether it is 30:1 or 60:1 represent even the close up "gag" elements or should it represent the screen plane (point of convergence)?

Thanks,
Phil
 
There is no set formula, but generally the parallax of the images should be no more than 1.5% of the screen width for cinema screens. Imax should be even more conservative at 1%. Having said that, there are no defined rules so you could have brief moments of pushing the parallax distance further for occasional extreme 3D. It seems to be easier to work out based on the parallax width rater than checking up the formula software. This more fluid way of working also allows you to be more creative with the stereo effects.

Our stereobrain calculator is pretty comprehensive software which lets you set the screen size and shows the viewing cone as you change the various parameters. You can download a free trial from our website.

http://www.inition.co.uk/inition/di...tID_=81&model=products&action=get&tab=summary

FrameForge is also very useful for checking 3D shots as you can set a full scene including actors, props and sets.

http://www.inition.co.uk/inition/di...tID_=81&model=products&action=get&tab=summary
 
Leonard Coster said:
Hi Philip,

The only safe option is to calculate for the largest screen your footage will ever be shown on.
This might seem overly conservative but it's not as bad as you might first think.

When you look at the footage on your fancy new 3D TV at home it generally occupies a smaller fraction of your field of view than the screen in a cinema and by a quirk of illusion your brain will actually amplify the perceived depth in the scene when you watch it on the smaller screen!
Try this for yourself - play some material you are used to on a monitor and then go stand way further back and observe the depth - it will appear to have deepened.
It's all an illusion anyway :-)

In any case - compromising the apparent depth is a lot better than making the image unwatchable (or hurtful) on larger screens.
Click to expand...

Thanks for the info. Does this necessarily imply that there will be a reduced effect on the viewer the smaller the screen size becomes once calculated in this way?
 
Ivan Kovax said:
Thanks for the info. Does this necessarily imply that there will be a reduced effect on the viewer the smaller the screen size becomes once calculated in this way?
Click to expand...

Yes. Just as if you calculate stereo for a 20" LCD monitor and then project it in a theatre, the stereo effect will be amplified. (and potentially hard to fuse.)
The opposite is also true, if calculate your IO for "giant screen" projection and then play it back on a 20" LCD the stereo effect will be reduced.

Which brings me back to my original question when I started this thread.
It seems to me that the sad truth is you have to live with less than optimal stereo eventually, on some delivery front or another. Unless you're using some post tool to "change" your IO for each .

Assuming 3D is going to continue to be a trend, and a given 3D film might be viewed in theatres, on televisions, computer screens, mobile devices of all different shapes and sizes, I'm sure somebody will figure out a production or post production workflow that will make each of those different delievery points work "optimally" with stereo.
 
IOD CALC on the app store does all the screen size math as well. I don't think a 3d movie shot with a IMAX screen size in mind will look very 3d when viewed on a iphone size screen, even with a re-convergence session, mainly because you can not change IA in post.
 
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