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This topic comprises 2 pages: 1 2
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Author
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Topic: The junk called IMAX with Lasers
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 05-15-2017 09:07 AM
quote: Geoff Jones
Is it really okay if there's just a little speckle? That sounds sorta like saying, "Oh, the film was barely scratched."
The Dolby Cinema presentations I've seen, there was, in practically all scenes, no speckle visible at all. Only in extremely bright scenes and if you go looking for it, you will see a tiny bit of speckle is still in there. Most people will probably mistake it for noise if they notice it at all. Only when the camera pans you can identify it as speckle.
The IMAX presentation I've seen looked like a unicorn barfed all over the screen. Pearlescent glitter all around in any bright scene.
None of the current projection technologies is entirely artifact fee, but there's a difference between something you need to go looking for and something that screams in your face almost all the time.
quote: Geoff Jones
Your new technology looks like shit unless you physically shake or vibrate the screen? Does it get shaken evenly across the entire surface? What effect does shaking the screen have in the long run?
Good questions actually. I guess the idea would be to uniformly shake/vibrate the entire screen. What it does to a screen in the long run might also be interesting, especially if you're considering silver screens that do have a fragile, special coating on the surface.
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Dave Macaulay
Film God
Posts: 2321
From: Toronto, Canada
Registered: Apr 2001
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posted 05-15-2017 11:13 AM
Laser speckle is created in your eye, it's interference patterns from the coherent light. Oddly, if you move your head side-to-side the speckle might move... one way (can't remember which) means you're nearsighted and the other way means farsighted, not moving means neither. I'm not sure if this is true, but it worked as described for me (nearsighted).
The shakers move the screen in and out a couple of mm in a random pattern: that changes the wavefront reaching your eye and, although the speckle is still there, it's also randomized and your brain filters it out.
Speckle is most visible with green laser light, less with red, hardly at all with blue.
Phosphor lasers use the blue laser light directly for blue - the speckle is pretty much unnoticeable - and hit a phosphor target with another blue laser which emits yellow light, that's split into red and green. The phosphor's yellow fluorescent light is not coherent, it's just like light bulb light, and it doesn't create any speckle. The phosphor is usually on a spinning disc to allow cooling - and the phosphor does wear out eventually.
"True" laser illuminated projectors all need to do speckle reduction somehow. I think there are some systems that use a complicated spinning disc "lens" between the laser and light engine but I haven't seen that.
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 05-15-2017 07:43 PM
quote: Joe Redifer
Are they using nearest-neighbor scaling, thus resulting in non-uniform pixel sizes across the screen? Like, did they just buy a cheap HDMI scaler made in China off of Amazon? You'd think they'd have something that could at least do pixel averaging.
The non-square pixels are a result of their weird setup.
Their projectors use industry-standard TI DLP chips. I'm not sure if they use 4K DCI or 4K UHD chips, but either way, they don't come in the aspect ratio of 1.43:1.
So, in order not to waste too much light, they're using anamorphic lenses to stretch the image vertically, instead of horizontally.
In order to make a normal DCP with "square pixels" fit on screen, you obviously need to rescale it. So, the weird vertically stretched pixels are a result of the anamorphic setup.
The scaling wasn't nearest neighbor scaling, it was obviously a bit more sophisticated bilinear or bicubic affair, but it still resulted in pretty ugly artifacts. Maybe their scaling algorithm didn't properly account for the anamorphic nature of the scaled image.
I'm not sure if Alien: Covenant is a 4K release. Many scenes seem to be shot on the Alexa XT, which isn't a 4K or 4K+ camera. Also, we were watching a pre-release show.
Maybe, we were watching the "ordinary" DCP and not the DCP with "IMAX magic" enabled. Still, that would make no real sense, because I clearly heard the IMAX 12-channel sound system in action. Also, the IMAX feature trailers in front of the actual feature had identical issues.
quote: Geoff Jones
Thanks for the explanation Dave Macaulay. That's helpful. It reminds me of the "rainbow artifacts" that you get from DLP projectors. Some people are more sensitive to those than others. I see them, but when I was shopping for my first digital projector, I remember thinking that even if I didn't see them, I wouldn't want a projector that might look bad to some of my guests.
I think all people are almost equally sensitive to laser speckle, although some might not be sufficiently irritated. My girlfriend called it rainbow dots and she also was pretty irritated by their presence.
The rainbow effect is only present in single-chip DLP projectors with color wheel. It's usually much more pronounced in older models than in modern models and some people apparently cannot see it and others do see it, but don't mind the defect. I personally cannot stand it.
It's the same thing you get when you look at a bright laser pointer on a bright surface. You'll see something that looks like a lot of small dots. Now, imagine this in three colors and then you have an idea how speckle looks like in a 3DLP laser setup.
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Carsten Kurz
Film God
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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posted 06-19-2017 06:17 PM
IMAX screens converted to lasers previously used silver screens for 3D. At first, it seems natural to leave them in, even if the do rebuilding of the front speakers as well, and even if they do not need them for 3D any more (IMAX laser 3D is color separation, not polarization). BUT when you look at the speckle these screens show, it appears as a very bad decision. I can only assume even their dual laser projector setup is not bright enough to use a lower gain screen, so they stick with it. I know one IMAX laser installation that started operation with the pre-existing silver screen, and after some weeks, received a NEW silver screen...nuts...
- Carsten
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 06-20-2017 06:44 AM
Installing a new silver screen, to keep the existing 3D setup operational might be a valid reason to keep using a silver screen.
Then again, the Brussels theater never had a 70mm dual-projector setup. The image, despite being a dual laser setup, wasn't overly bright, nothing compared to the Barco 6P and Dolby Cinema setups I've seen until now. So, maybe the silver screen is needed to achieve the necessary brightness.
I can't help it, but to me "IMAX" for motion picture presentations still feels like a hack. The native screen aspect ratio doesn't fit any common movie format very well, the oversized screens and difficult room geometry most likely also introduce all kinds of challenges to get the sound right.
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