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Topic: Real D & Dolby 3-D - What is the difference?
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Julio Roberto
Jedi Master Film Handler
Posts: 938
From: Madrid, Madrid, Spain
Registered: Oct 2008
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posted 12-18-2008 10:09 AM
Well, indeed the brain is very forgiving to stereo color disparities (differences in color per eye) and it's hardly is a problem as long as the luminance levels are kept the same (which hardly ever happens in anaglyph system, hence the headaches and difficulty of fusing the images).
Look at this graph.
http://img58.imageshack.us/img58/7522/infiteccolor1cg5.jpg
Dolby 3D has a reduced color space, but reasonable.
Also, each eye has a different colorspace.
But everything inside the red triangle in the map i.e. can be displayed fine on both eyes while each subimage being properly blocked by the proper eye.
But of course you need to "move" (color correct) each image independently to make it fit in there as best as possible.
I don't know how much people would object to not color corrected images. For once, same colors (those outside the gamut) would look washed out (they would basically be sharply cut off by the filters). Then, the remaining ones would seem to "flicker" a bit (as each eye would be seeing them differently).
Uncorrected white would not look white.
Certainly not the kind of thing you want in a festival neccesarily, but perhaps doable.
Two "identical" (stereo pair) black and white pictures .... hummm ... I wonder how that would look. I have never actually seen a black a white image through an infitec system.
Where do you get that much stereo black and white material? Most movies in the 50's were color. Sure a few black and white too, but not that many.
And we won't mention that about +80% of the light is lost.
But at least there won't be buttery popcorn to leave fingerprints on the $28 glasses ![[Smile]](smile.gif)
Dolby has the advantage of the best extintion ratio, though (least ghosting). It's "roughly":
-Better than 250:1 for linear polarized (as long as you keep your head perfectly aligned with the screen). You lose >66% of the light. Of course having to use a silver screen, with gains of over 2.0, it also means that you get most of that light back. Kind of. Most sensitive to orientation of all systems (i.e. those not sitting centered from the screen may start seeing more ghosting). It uses the cheapest glasses and filters.
-Better than 100:1 for circular polarized (like Real-3D, you can rotate your head around). Worse extinction of all systems (largest ghosting or "double images"). You lose >70% of the light. Not so sensitive on orientation. Glasses and filters are a bit more expensive than linear.
-Better than 1000:1 for Dolby 3D. Move your head around all you want. Least ghosting by far. You lose >80% of the light. The color space is somewhat reduced and changed, but very generous and unnoticeable when color corrected. The glasses and filters cost an arm and a leg. Not sensitive to orientation or any such issues. Unless you use a gain screen to get some increase in brightness. But even then, the angle doesn't affect the extinction, only your usual potential problem with hotspotting and the like that happens with high gain screens.
Infitec had been selling their filters for home projectors for about $100 pair for a while before they made the deal with Dolby 3D for digital cinema. Several vendors sold an external color-correction box. The Infitec glasses have always been over $40 a pair, even $60 at some point. (Dolby just lowered theirs from $38 to $27).
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Julio Roberto
Jedi Master Film Handler
Posts: 938
From: Madrid, Madrid, Spain
Registered: Oct 2008
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posted 12-19-2008 12:03 PM
I meant you lose over 80% of the light coming out of the projector, or rather, that the projector would be capable of delivering to the eye without the 3D system (filter/glasses combination).
Think that, for the very nature of the system, the need to split the light between 2 eyes so that only 1 sees it while the other blocks it 100%, necessarily means that such a system can only have a brightness of 50% of the level it would normally output at best (considering a "natural" unpolarized light source of wide "natural" spectrum).
Because no system is "perfect" and each layer of filters adds an additional 2% or 3% percent loss at best, even the best theoretical system would imply a light loss of 54%. Such system doesn't exist, and the very best you can get good results with is about 63% loss (37% efficiency).
Infitec filters, at least when they first started to manufacture them, had 12 filters plus other surfaces. Something like 16 layers total, IIRC.
And you have the ones in the projector and then the ones on the glasses to go through. Yeah, it's no fun for the lamp to keep up with that. Hence a gain screen is, from my point of view, almost a requirement for most Dolby 3D systems.
I don't know what currently Dolby specs, but usually Infitec systems only allow about 15% of the light to cross them. Coupled with the glasses, it wasn't unusual to see only 13% of the total light you started with. I think Dolby's is only a couple of % points above that, something like 18% efficiency (blocks 82% of the available light). Maybe a bit more, though, but definetly under 20%.
When you use a gain screen, you of course see an increase in brightness for some people in the audience. I'm talking comparing with the result in a 1.0 gain screen you would get with a "plain" projector and one with the 3D system fitted when looking through the glasses to a 1.0 gain screen (if such thing existed for polarized systems, i.e.).
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Julio Roberto
Jedi Master Film Handler
Posts: 938
From: Madrid, Madrid, Spain
Registered: Oct 2008
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posted 12-19-2008 03:24 PM
quote: Jack Ondracek
OK... how bout this?
Is there any particular reason, aside from screen gain and the cost of glasses, why Dolby 3D wouldn't work at a drive-in?
I think it should work fine. But it would be hard to get reasonable light levels. Perhaps stacking 4 projectors. But I don't know if any servers support quadruple projection. A very high gain screen in a drive-in may not be very effective, as it probably needs to cover a wide angle area.
Here is part of a flyer by Barco when they were selling the infitec system. The active version (basically like what Dolby 3D has now) only allowed 7% of the light to reach the eyes. Or, rather, 7% of the perceived brightness that the same projector/screen would've been capable of if it didn't use the 3D system.
http://img511.imageshack.us/img511/1339/infitecbarcohr5.jpg
Efficiency in dual systems seems to be higher now, possibly due to the blanking interval in active systems:
http://www.barco.com/entertainment/en/stereoscopic/lumens.asp
Real-3D may not work fine due to a number of issues. Real D themselves admit that their system is not recommended (doesn't work well enough) for images over 60 feet wide (due to the limited angle to which the zscreen works). Not sure with their newer XL model.
On very large screens, I think a larger zscreen would be needed and placed a bit farther from the lens, but not much. Anything above about 15º and the zscreen start to not effectively rotate the polarization in the corners of the image.
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