P3D Re: IMAX Window & Strobing

["Peter Muyzers" <peter.muyzers@xxxxxxxxx>]

Derek,

Derek wrote:
> Peter, what do you mean by Imax only runs at 30 fps?  Only Imax HD and
> Showscan use higher frame rates.  Why exactly is there strobing during
> an Imax 3-D pan and why haven't I noticed strobing in regular 35mm &
> 70mm 3-D movies?  Thanks for all the insights into 3-D Imax!


The problem is that at 30 fps, strobing mostly occurs with computer
generated images (CGI) but also with live images, projected on a large
screen. The effect of the difference between a point on an image and the
same point in the following image of the sequence, is called strobing. The
difference is too big for our brain to perceive as "smooth" motion.
Live footage has less strobing because the images are motion blurred
(depending on the shutter speed) and therefor appear smoother to our brain.
Still, live images can show heavy strobing as well, if the shots are not
planned well by the director. Bright objects on dark backgrounds moving
across the screen will show more strobing than less contrasting scenes.
Also, the speed at which the objects are moving across the screen makes a
difference. While a jet fly-by has less strobing (our brain knows a jet is
fast and tends to correct any visual problems) , a ballerina might show
heavy strobing and a snail crawling across the screen hardly shows any
strobing.

Now that we know what strobing is and what causes it, we might do something
about it. For CGI, one of the solutions might be to use computer generated
motion blur. Although this is a really expensive cost (realistic CG
motionblur is a difficult algorithm for the computer to calculate),
sometimes it is absolutely needed to correct the strobing problems. Another
solution - for both CGI and live - could be the use of higher frame rates.
Instead of running at 30 fps, projectors might run at 60 fps, reducing the
strobing effect by 50%. This, off course, requires special hardware which is
really expensive so only few systems run at such high speed (like Showscan).
A third solution to avoid strobing is to avoid large screen projections.
35mm, 5p. 70mm and 8p. 70mm projections show less strobing simply because
the smaller screen size and therefor the global difference between a point
on an image and the same point in the following image of the sequence is
smaller as in a 15/70mm projection.

Until now I haven't addressed 3-D Imax yet. If regular Imax projections
already show bad artifacts like strobing, 3-D Imax will be worse. Instead of
ONE strobing image being projected, now there are TWO stobing images being
projected. Altough, in theory they strobe in sync, our brain - already
overloaded by the extra addition of the varoius aspects of depth
perception - has trouble correcting even the slightest out-of-sync
projections (3-D projection is still a mechanical process and it might show
wear or parts might get too much play), so our brain fails to correct and we
perceive heavy strobing with a terrible head-ache as a result.


The bottomline is, for most pleasant viewing of 3-D Imax films, we must
avoid strobing, therefor avoid pan's, our objects should move the right
speed and never violate the Imax window.
Follow these rules and you get the must dull experience in 3-D Imax... so
let's "bend" these rules once and a while!


Peter

--
Peter Muyzers
3D Animator / Digital Effects Artist
peter.muyzers@xxxxxxxxx

Engstegenseweg 88/2
3520 Zonhoven
Belgium
+32 (11) 812 796

* I do not seek, I find *
        - Pablo Picasso


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