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This topic comprises 3 pages: 1 2 3
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Topic: Non perf screen
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 03-23-2012 05:27 PM
quote: Bobby Henderson
This same model could be applied to the perforations of a movie screen. Some care would have to be taken with regard to the size of the perforation dots and their distribution. In print work Stochastic screening will typically use more dots than a traditional half tone pattern. With a movie screen the goal is trying to maintain the white appearance of the screen while letting the sound pass through it. The screen needs enough dots to allow this, but not so many that the screen takes on a gray tonal appearance and reduces image brightness.
I guess you could create some mathematical models that create a dot pattern that evenly distributes the perforations across the surface, not all holes need to be of the same diameter for example. Also, this pattern doesn't have to be unique, you could probably repeat it. Like with all other perforation patterns, it's also quite easy to establish a minimum viewing distance. Beyond this distance, you should also not be able to see the repeat pattern in the seemingly random pattern.
I don't know how screens are perforated and the specific process will probably vary a bit between manufacturers, but I guess they use some kind of rollers with the perforation pattern on them and feed the screen through those rollers.
Of course, such a random perforation could have negative consequences too, like a weakened strength of the fabric. Maybe such a screen would start to deform after a while, if not produced from stronger materials.
quote: Scott Norwood
The reason why this does not happen with LCoS (at least not to my knowledge) is probably that there is no visible pixel "grid" and that the space between the pixels is either nonexistant or at least sufficiently invisible as to not interfere with the screen perforations. Also, LCoS has always looked a bit "soft" to me (I don't know why this is...maybe someone can explain that), which may be part of the reason why the pixel grid is not visible.
I've also not noticed the screen door effect on any Sony 4K LCoS projector until now. Seperate pixels look like small blurry dots, so they come with some built-in anti-aliasing ![[Smile]](smile.gif) . I don't know for sure if that's due to the technology or if they apply some depixelization using microlenses or if the image is purposely a bit out of focus. Theoretically, with LCoS, the space between the pixels is only limited by current semiconductor technology. With DLP, you will always need some space between the pixels to keep them moving freely, even if they expand a bit due to heat.
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Carsten Kurz
Film God
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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posted 03-23-2012 09:06 PM
Moiree does not need a visual pixel grid or low fill ratio to show, not in the sense of a certain pixel/grid coverage relation. It will always occur when regular patterns overlay each other with significant contrast.
It's just that with common display devices pixel fill ratios, the black grid spacing components create just another regular high contrast signal component that ads to the problem. Basically, they create a pattern of 'image pixels' plus surrounding always black parasitic subpixels. Still all regular and again raising moiree.
Why this seems to happen less on LCoS has a simple explanation - Sonys SXRD imagers already use tilted diagonal/parallelogram shape and raster orientation, which reduces the effect somewhat, together of course with the high resolution AND high fill ratio.
The effect varies across screens naturally, because the perf size and spacing of standard and microperf screens is more or less the same across all manufacturers, while the screen size varies by factors of 5-6 easily - and when referencing this to the 2k standard resolution across the screen width, that is a huge span of perf/pixel size relation. Some of them are more prone to moiree than others, and of course the lens focus setting and drift plays a dominant role as well.
Is moiree a real problem? With test patterns, yes. With real life footage - we would have heard about it from our patrons already during the last 5 years.
Maybe some day, we will see perfectly dithered perforated screens with no visible moiree.
I think also metallized 3D screens reduce the effect somewhat because they overlay another random microstructure - at least SOME benefit of silver screens.
- Carsten
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