Film-Tech Forum ARCHIVE: Non perf screen

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Posted by Richard May (Member # 2627) on 03-22-2012, 05:51 PM:

Is it at all possible to put a non perf screen in front of all 3 screen speakers. Will there be enough hf coming through. We have an Eiki projector that is reflecting the light off the perfs into weird patterns on the screen. I put white cardboard against the front of the screen and it disappears. I also put it behind the screen right up against it and it also disappeared. There is no room above the screen for speakers.

Posted by Scott Norwood (Member # 30) on 03-22-2012, 05:56 PM:

Assuming that the Eiki is a video projector, you have a moire issue. The pixel array is interacting with the screen perfs and causing weird circular patterns. It is the same sort of effect that one gets when folding a window screen in half. This is obviously very distracting to the audience.

I have yet to find a good way to predict if this will happen with a given screen and lens and video projector combination. If anyone knows how to do this, please tell! It does not happen with LCoS or CRT projectors, only DLP or LCD (because of the fixed pixel array).

The quick and dirty fix is to put up a white test pattern, zoom out the lens until the moire issue goes away, and then electronically scale down the picture to fit the screen. Another not-really-correct fix is to defocus the lens slightly. A better fix is to replace the screen with one with a different perf pattern and hope that the problem does not recur.

Posted by Paul H. Rayton (Member # 1817) on 03-22-2012, 06:38 PM:

We are assuming that the Eiki projector of which you speak is a video projector of some sort, right? Eiki also made 16mm film projectors of various sizes and types. If you're talking about one of their video projectors, just as an idea you might see if you could move this thread over (or close this one and re-start a new thread) in the "Digital Cinema Forum" section. I'd think you would likely find more people who may have encountered this issue over there, since it's so clearly associated with such types of projectors.

Posted by Marcel Birgelen (Member # 6801) on 03-22-2012, 08:08 PM:

quote: Scott Norwood
It does not happen with LCoS or CRT projectors, only DLP or LCD (because of the fixed pixel array).

Correct me if I'm wrong, but since when doesn't LCoS use a fixed pixel array?

The Moiré patterns or aliassing effects you're witnessing are probably happening, because your pixels are about the same size or smaller than your perforation. I guess, a screen with smaller perforation (holes) would probably be the best solution.

A screen without any perforation with speakers behind it seems like a very bad idea to me, because your screen will become one large membrane.

Posted by Carsten Kurz (Member # 5396) on 03-22-2012, 08:31 PM:

Defocusing ist still the best option if moiree is the issue.

- Carsten

Posted by Richard May (Member # 2627) on 03-22-2012, 09:41 PM:

Sorry guys. It is a video projector. You are right on Scott. Defocusing it slightly does improve it to a point. Going through the zoom range does not though. The only fix I see is getting a non perf screen.

Posted by Bobby Henderson (Member # 840) on 03-22-2012, 10:15 PM:

quote:
I have yet to find a good way to predict if this will happen with a given screen and lens and video projector combination. If anyone knows how to do this, please tell!

I'm sure there's some way to be able to predict if moiré will be a problem by doing some complex geometric math.

Having dealt with moiré in print, I think the easiest solution would be creating perforated screens that are more d-cinema friendly.

Movie screens typically have perforation patterns that are strong with vertical and horizontal angles. That sucks when the pixel grid pattern is horizontal and vertical. Basically, you can run into the same problem that often occurs when scanning a half-toned newspaper image (or anything half-toned) into Photoshop. The acquired image is prone to be riddled with moiré. This problem can be solved by rotating the image 15 or 30 degrees; it often disrupts the moiré pattern by taking it out of alignment with the pixel grid.

Movie screen companies could do the same thing: make the screen perforations in a pattern 15 or 30 degrees off horizontal-vertical axis.

I have seen ads for "video friendly" screens that merely make the perforations much smaller. But they still have the standard pattern with the horizontal and vertical alignments. Tiny screen perforations might be good for the crude 2K pixel grid. But as projectors improve to 4K or perhaps levels well beyond 4K the moiré issues could easily reappear. With the perforation pattern taken off the horizontal-vertical axis moiré shouldn't be an issue regardless of perforation size or projector resolution.

I wonder if this is something I could patent?
[Razz]

Posted by Randy Stankey (Member # 64) on 03-22-2012, 11:03 PM:

This might sound weird but try tilting the projector.
(i.e. Turning it on its roll axis.)

The idea is to stop the pixels from lining up with the holes or, at least, stopping them from lining up so often. Depending on the ratio of the size and spacing of the pixels to the size and spacing of the holes in the screen, you might only have to tilt the projector a couple of degrees.

If you want to know the math for figuring out Moire patterns, check Wikipedia: http://en.wikipedia.org/wiki/Moir%C3%A9_pattern

Posted by Marcel Birgelen (Member # 6801) on 03-23-2012, 05:23 AM:

quote: Bobby Henderson
I wonder if this is something I could patent?

Maybe you could invent screens with some kind of random perforation [Wink]

Posted by Dave Macaulay (Member # 813) on 03-23-2012, 09:06 AM:

The sound will be very bad with speakers behind a non-perf sheet vinyl screen.
Is this a "microperf" screen? Normal perfs are fine for normal cinema sized screens with decent seat-to-screen distances and "digital" video projection. The pixel size is large enough that each pixel covers several perfs and the brightness difference between 9 perfs and 10 is minor. You may see some subtle moire patterning with white field projection but it is not objectionable with normal movie images.
With a small normal perf screen the problem is severe. The brightness difference between a pixel covering no perfs and one covering one or two is definitely noticeable. A microperf screen has closer spaced smaller perf holes so the pixel size to perf pitch gets back to what a larger screen would have. These perfs themselves are less visible with seats close to the screen - common in small screen venues.
AMC has a lot of rooms with the speakers above the screen, and with the bass bins below the screen and the horn above. This sounds OK up to a point - most people don't notice it but if you're aware of the location you start to notice the sound coming from way up there. Even if you can't put the entire speaker above the screen, can you put the HF driver up there with the bass below or behind the screen? Screen attenuation is frequency dependent, LF goes right through and HF gets progressively attenuated as frequency rises. The screen material mass is the culprit. At low frequencies the sheet vibrates slowly so not much energy is used accelerating its mass. At higher frequencies the energy used to accelerate the sheet mass to vibrate it faster uses up more and more of the sound energy so less is left on the "outside" as actual sound.
You could also use a woven screen. These offer low acoustic attenuation and are available with anti-moire surfaces or a tilted cutting to misalign the weave and the pixel axes.

Posted by Bobby Henderson (Member # 840) on 03-23-2012, 01:43 PM:

quote: Marcel Birgelen
Maybe you could invent screens with some kind of random perforation.

That would indeed be another worthwhile solution. In printing, one can use Stochastic screening instead of an evenly distributed half tone pattern. Stochastic screening uses frequency modulation to create a somewhat randomly distributed dot pattern.

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.

Like others, I think solid screens are a bad idea. AMC tried it with their Torus screens, which required speakers to be positioned above and below the screen. The solid screen in combination with the cubic nature of stadium seated theaters created noticeable problems with echo, not to mention the other audio problems that are going to be there anyway.

Posted by Scott Norwood (Member # 30) on 03-23-2012, 01:52 PM:

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 am surprised that the screen manufacturers have not made an issue out of this (by offering screens that are designed eliminate or reduce the problem). It seems like the whole issue is the dirty little secret of the video projection industry that no one wants to talk about or even admit is a problem.

Posted by Bobby Henderson (Member # 840) on 03-23-2012, 02:27 PM:

Given the somewhat commercial nature of this web site (Film-Tech sells a number of cinema products) I wonder if Brad and his F-T colleagues ever considered marketing movie screens. A moiré-proof movie screen might sell pretty well.

Obvious questions: how do movie screen companies perforate their screens? I'm wondering about the process and how difficult or easy it would be to simply rotate the pattern 15 degrees.

Posted by Steve Guttag (Member # 268) on 03-23-2012, 02:38 PM:

Stewart does rotate their perf. grid to avoid Morie. Since their perfs are, more or less, human applied...they are also more random and thus don't show morie like others.

Random patterns have the issue that they are MUCH more visible than a constant pattern your eye can't fixate on. MDI's original mini-perf was an example of this.

Morie gets the most annoying when the pixel density is close to the perforation density. A popular misconception is that micro-perfs reduced Morie...they actually make it worse in most cases ad that puts the pixel density on the same order as the perfs.

Sometimes just zooming the lens a little bit one way or the other will get rid of the offending tiger-stripe.

Posted by David Graham Rose (Member # 1450) on 03-23-2012, 03:51 PM:

Greetings All

I would say no.

From Sawston goodnight

David

Posted by Richard May (Member # 2627) on 03-23-2012, 04:44 PM:

It's funny how common this problem seems to be after researching online. There are screens available that supossedly reduce or eliminate it.

I've tried the zoom but it has to be zoomed almost all the way to get rid of it.

Posted by Marcel Birgelen (Member # 6801) on 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]

. 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.

Posted by Mike Olpin (Member # 1115) on 03-23-2012, 06:36 PM:

How large is the screen? If it is on the smallish side, you may consider using an acoustically transparent fabric screen such as this one:

http://www.accuscreens.com/Screens.asp?detail=364&screen=AccuScreen_Fixed

Posted by Richard May (Member # 2627) on 03-23-2012, 07:12 PM:

22 feet wide.

Posted by Carsten Kurz (Member # 5396) on 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

Posted by Louis Bornwasser (Member # 3063) on 03-23-2012, 10:02 PM:

I saw this problem in 2D with How to Train Your Dragon on 35mm film. The 2k intermediate generated enough interference so that the holes on the screen generated swirling (clockwise) patterns. Yes the picture was very sharp and bright. 65 ft/lm. Louis

Posted by Richard May (Member # 2627) on 03-24-2012, 03:29 PM:

Patrons tolerate a lot on the screen. Don't think just because they don't complain it doesn't happen. This has been a problem for years at this theater. Only film makers have complained about it.

Posted by Brad Miller (Member # 2) on 03-24-2012, 04:18 PM:

It just depends on the size of the screen vs. the perforation design as to whether they "line up" and introduce moire.

What SHOULD be happening is the screen manufacturers should have a 2K and a 4K projector and run some tests to find out with X width in 2K, moire will be introduced between Y and Z feet, so they know to use a different hole pattern for those size screens, etc.

Its easier for people to turn their brain off and not do the extra work in making a product better suited for the application. If Film-Tech manufactured screens (which we don't and I don't see us ever getting into that), we would absolutely be doing this.

For the time being, if moire is noticeable I try scaling the image a little bit. Most often, scaling introduces less artifacts than moire...but it must be evaluated on a case by case basis.

Posted by Richard May (Member # 2627) on 03-24-2012, 08:08 PM:

I'm assuming you mean upscaling to a higher resolution?

Posted by Monte L Fullmer (Member # 2797) on 03-24-2012, 10:21 PM:

Just wondering on something here..if it's a valid cause or not...

Is your port glass the common, leaded variety, or is it a lead free glass?

Leaded is basically a unpure class that loves to eat light and create some slight distortions.

Posted by Steve Guttag (Member # 268) on 03-24-2012, 11:33 PM:

Stewart does indeed get the projector information and use that when making the screen.

Posted by Brad Miller (Member # 2) on 03-25-2012, 02:05 AM:

No Richard, I mean zooming the lens to make the image too big for the screen (just enough to make the moire go away) and then scaling the image DOWN on the chip. It costs a little bit of light and if you aren't careful can create noticeable artifacting, so there is a bit of trial and error involved.

Posted by Richard May (Member # 2627) on 03-25-2012, 06:09 PM:

Yeah I tried that already. I had to zoom so far before it went away. The light loss was a lot.

Posted by Brad Miller (Member # 2) on 03-26-2012, 12:53 AM:

In that case a microperf screen is probably in your future.

Posted by Richard Fowler (Member # 893) on 03-26-2012, 01:41 PM:

I assume you have a 1080 HD LCD projector hitting your 20 - 22 foot screen...expect to see some moire artifacts if you are projecting on a Technikote or the "floral hole pattern" on a couple of older MDI screens. Micro perf will be less objectionable. Get some screen sample swatches to do a projection test.

Posted by Richard May (Member # 2627) on 03-29-2012, 04:44 PM:

If I turn the projector on it's side, it completly goes away. Maybe I'm thining about this wrong, but what if I order a screen and reverse the height and width and then rotate it 90 degrees......

Posted by Randy Stankey (Member # 64) on 03-29-2012, 11:22 PM:

What is the minimum angle do you have to rotate the projector with respect to the screen in order to make the problem go away or, at least, be less noticeable?

Sometimes it's only a few degrees one way or another.

Posted by Frank Angel (Member # 248) on 03-30-2012, 12:36 PM:

The so-called "acoustically transparant" screens have been around for quite awhile -- they are basically made using a very fine layered woven plastic material -- picture a wicker basket. One layer of the weave is on top of the other so no holes show, yet sound isn't blocked (much). Problem is, so far, I am not aware that they make this stuff in more than 15 ft widths. Seems it would be a great market if they could manufacture it for commercial cinema use. Only trouble...so far, I have never see this material in a silverized version, but that doesn't mean it couldn't be made (Brad?).

As for the issue at hand, Richard, a down and dirty, albeit a work intensive fix is based on the projector tilt concept. If you can live with a slightly smaller screen image, you could "tilt" the screen. Basically restring it so that it is not sitting parallel to the frame. In other words, pull one corner closer to the frame than the other causing the screen itself to be sitting crooked in the frame, assuming your frame is somewhat larger that the screen fabric as most are (usually about 6 inches to a foot). Then you adjust the mask so it frames out the off-centered screen. This in effect will skew the perfs and HOPEFULLY eliminate or at least lessen the moire problem. Only downside of this will be that you will have to reduce the image size a bit to compensate for the off center tilt.

Another solution, since you say you are actually able to eliminate the moire just by placing a white material behind the screen right up against the perfs, you might want to try a piece of white china silk. That is pretty close to acoustically transparant and it might do the trick in term of both letting the sound thru and still killing the moire. The problem might be that it is not a rigid material and it may be difficult to keep it in close contact with the screen. It would be the same as pressing a piece of foamcore against the screen.

As for a non-perf screen....no way will you be happy with that. In fact, I would say it's out of the question.

As for figuring out perf size vs projector pixel size, evidently that CAN be predetermined because there is one home theatre screen manufacture who asks for the projector make and model so they can perf their screen material at a specific pattern to eliminate moire, optimized for that particular projector (or so they say).

Posted by Martin McCaffery (Member # 37) on 03-30-2012, 01:02 PM:

As we're talking digital, could you just tilt the projector a few degrees and then use keystone correction?

Posted by Frank Angel (Member # 248) on 03-30-2012, 01:18 PM:

Wouldn't keystone correction just rearrange the pixel pattern so it again matched the perf pattern? I guess, it depends how much tilt would be needed to eliminate the moire. If just very slight, that would be much easier than tilting the screen materal. On the other hand, it would have to REALLY be a slight tilt; seeing a skewed image on the screen is pretty awful if it is noticable, and I'll say this, if I see credits not perfectly parallel to the screen edge, I have to up my meds.

Posted by Richard May (Member # 2627) on 03-30-2012, 05:09 PM:

Randy and Martin......It took much more than a few degrees before it went away. The only way I see, other than "trying" a different type of screen, is to put something up against the back like Frank says. Or like I said, order a regular screen and turn it 90 degrees.....I can't see why that won't work if by turning the projector 90 degrees, it goes away.....

Posted by Randy Stankey (Member # 64) on 03-30-2012, 06:41 PM:

How far of a turn? Like 45 degrees?

I don't know if this will amount to anything but I'm thinking about this problem to see if we can puzzle something out.

Basically, a Moire pattern is a visual beat frequency:

Play two audio tones at the same time and you hear a pulsation that varies in proportion to the frequency difference between the two.

Superimpose two visual patterns and you'll see a "pulsating" pattern of waves corresponding to the difference between the size/distance/period of the two patterns.

If we know the beat frequency between two sounds and we know the frequency of one of the sounds, it is possible to deduce the frequency of the unknown sound.

I don't know if this is possible but let's ask the question: If we know the dot size/resolution of the projected image and the amount needed to turn in order to make the patterns go away, is it possible to deduce something about the screen, the picture or the relationship between the two which can help us change some variable that will make the problem go away?

Posted by Richard May (Member # 2627) on 03-30-2012, 06:44 PM:

I will have to do it again but I think it was at least 45 degrees.....probably more. If everyone lays on the floor, I could keep the projector 90 degrees..... [Wink]

Posted by Richard Fowler (Member # 893) on 03-31-2012, 06:15 AM:

Your perf pattern sound like it is off set 50% row to row in an alternating layout....a moire producer at the right resolution.

Posted by Scott Norwood (Member # 30) on 03-31-2012, 08:38 AM:

quote: Richard Fowler
Your perf pattern sound like it is off set 50% row to row in an alternating layout....a moire producer at the right resolution.

Ah...now we're getting somewhere. Is that how you tell if a particular screen will be a problem?

And do they still make new screens this way, or have the screen manufacturers changed perforation patterns since video has become common in theatres?

Posted by Richard May (Member # 2627) on 04-18-2012, 12:37 PM:

The microperf screens came in. I installed one this morning. The pattern is totally gone now. That was the way to go.