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Discussion Starter #1
I was looking at an InFocus DLP projector at a store, when I notices the image was not as good as I expected. It had an unusual appearance, almost like a screen door effect, but not quite as bad. I have seen this projector in action before, and this was not an issue. I later found out it was being displayed on a perforated screen, with the speakers behind it.

Is this a common problem with DLP projectors and perforated screens?
 

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Likely what you saw is Moire patterns. This is basically where the pixel structure of the projector matches up just so that it conflicts with the perf pattern on the screen. It can cause image problems which are objectionable. If a perfed screen is something you have to get for your setup, then it is best to try and keep the screen larger than 110" diag. Cygnus makes a de-pixelizing lens that usually can correct for this as well, so that is another option.


Thanks!
 

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Quote:
Is this a common problem with DLP projectors and perforated screens?
No, It is a problem for all Fixed Panel Display Systems. LCOS, DLP and LCD all can experience some aspects of the Moire effect.
 

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Oswald,

Perhaps you were observing the perforations themselves. Moire manifests itself as vertical wavy lines. The holes on Stewart microperforated screens disappear at around 8-10 feet (depends on your vision). Da-Lite uses slightly larger perforations, and therefore, you need to be a little further back. BTW, as the resolution of a projector increases, so to does the size of screen necessary to avoid moire. Hopefully the future upgrade to 1920x1080 native resolution projectors won't come back to bite us perforated screen users in the bum.
 

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Discussion Starter #5
Jason;

How does having a larger screen help, since the larger the image the more visble pixel structure will appear? I have read review on this cygnus lense, and my understanding is that it softens the image when used. The size screen I am considering is 110" diagonaly(16x9).


Scott;

They were not the perforations itself, I got a good close look at those and they did go away at about 9' for me. However when the the PJ was turned on it was these vertical shadow like lines that became very visible.
 

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Oswald,

You were definitely seeing moire artifacts. Moire occurs from the lining up of the grid pattern of the projector with the holes of the perforated screen. As you increase the projected pixel density you will hit a point where moire becomes visible. Since the projected pixel density decreases as the image size increases, one way to avoid moire is to use a screen size big enough to avoid the problematic pixel density. This is also why I think that we may get into problems as we move to 1920x1080 resolution projectors. What works with todays 1280x720 projectors may not work with tomorrows 1920x1080 projectors. Of course, the grid pattern will be that much finer so the moire may be much more subtle. As Jason has pointed out, the lens from Cygnus will solve the moire problem. If this is an avenue that you do not wish to pursue, you may want to consider Screen Research screens which are acoustically transparent fabrics rather than perforated vinyl and do not exhibit moire.
 

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Discussion Starter #7
Thanks Scott I understand now. Its just that I have seen other perforated screens with DLP's, and never noticed this effect. The screens were 100" diagonally. Is it because some projectors are more prone to this, or was it just dumb luck the other PJ's just happen to misalign with the screen perforations?
 

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Maybe the other DLP's were lower resolution, or perhaps the screens you viewed had a lower density of perforations that the Stewart screens.
 

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my take on it:

- tiny bit of defocus helps a lot (the higher the source resolution and pjs resolution, the "more" (...) lattitude you have with defocusing)

- use of an anamorphic lens help tremendously (in all formats or for scope-like formats with 16/9 native pjs)

- moiré is also very much dependant on the fill factor, the higher, the less risk of moiré (hence why CRT and DILA are virtually not prone to this side effect)

- make sure the back of the microperforated screen is absorptive black fabric

- the larger the screen, of course, the further away in proportion you will sit away from it and thus will not be able to see the microperforations.


The good alternative is scope screen, non perforated, center speaker just underneath and tilted upward to reach spectators heads.

Back to perforated screens: if you use a small one nonetheless, place the left and right speakers on its sides and not behind unless it's at least +/- 9ft wide , to preserve a good stereo imaging.

The Screenresearch is interesting but I heard the fabric structure itself "could" be visible in bright scenes. Haven't seen one yet though.
 

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David600,


The fill factor does not affect whether or not moire is present, but it does affect how severe the moire is. When I tested various projectors on my Stewart microperforated screen, DILA exhibited moire at smaller screen sizes than the other projectors due to its higher resolution, and therefore, projected pixel density. The moire, however, was never objectionable unlike that exhibited by DLP projectors and especially LCD projectors.


The fabric structure of the Screen Research ClearPix2 material is not noticeable at any reasonable viewing distance.
 

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Oswald-


Another option is the Celtic Cloth from Dazian. I just received a few yards of it yesterday and made it into a 110" diagonal screen for my theater. Since there are no perforations, there is no moire problem. Also there are no concerns with seeing perforations either. I am currently trying to decide on a screen choice myself for my Sharp 12000. The Dazian is certainly a frontrunner at this point. Oh yeah, you can't beat the price either. :)
 

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The Stewart Cinemaperf does not exhibit this problem that I call reverese pixelisation, not moire.


You are actually seeing the pixel structure projected behind the screen. My definition.


With lesser bigger holes you actually get depixelisation and contrast enhancement.
 

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I am not sure what you are referring to. Light that is passing through my microperforated GrayHawk is absorbed by black material behind so there is no way that I am seeing the pixel structure projected behind the screen. I am not saying that what you are describing is not possible, but it is not moire.
 

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Discussion Starter #14
Stewart says that the Cinemaperf will also have this problem with DLP's using screen sizes of less than 100"! They did not reccomend it for use on small screens, just as Scott was telling us.
 

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Quote:
Originally posted by Oswald Pascual
Jason;

How does having a larger screen help, since the larger the image the more visble pixel structure will appear?.
Oswald,


First you have to understand how Moire is created. Take two pieces of identical

screen and look through both at the same time. Hold them a short distance apart.


You will see interference fringes - black bands. This type of interference happens

when you have patterns that ALMOST match in their "pitch" - the spacings of the

repetition.


Moire with projectors happens when you have interference between the spacing

of the holes in the screen, and the spacing of the pixels.


Now if you make the image bigger as Jason states - there are a fixed number of

pixels in the projector - and if you make the image bigger - the spacing has to increase.

[ The distance from the center of one pixel to the center of the next pixel increases because

the pixels are bigger].


However the spacing of the holes in the screen doesn't change with the size of the

image or screen. The screen manufacturer puts the holes the same distance apart

regardless of how big the screen is. A larger screen has more holes - so the the

number of holes per inch is the same.


By making the image bigger - the spacing of the pixels is no longer ALMOST the

spacing of the holes - which is what causes the interference.


That's why Jason advice works. The best thing to do is to call the screen manufacturer

and tell them what projector you have, and how big an image you plan to have - and

they can tell you if Moire will be a problem.
 

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Discussion Starter #17
Thanks Dr. Gregory Greenman

I now fully understand this problem and why manifest itself. The larger the screen increases the space between the pixels while the screen holes remain fix therfore no more moire effect.


Unfortunately the increase in pixel spacing reduces image quality, and one already has a loss of light due to the perforations, so this would cause other problems as the bulb gets older. Also if one was ever to upgrade to a higher resolution projector the moire effect would rear it's ugly head again on the larger screen!


Maybe I am better of without the micro perf.

The problem is I have 8' cielings and my center channel speaker is design to be vertically tall instead of wide, so I only have a small space below the screen to mount my speaker. I cant simply turn my speaker on it's side becuase of the tweeter arrangement.
 

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Quote:
Originally posted by Oswald Pascual


Unfortunately the increase in pixel spacing reduces image quality, and one already has a loss of light due to the perforations, so this would cause other problems as the bulb gets older. Also if one was ever to upgrade to a higher resolution projector the moire effect would rear it's ugly head again on the larger screen!
Oswald,


Again I would encourage you to call the screen manufacturer and fully explore your

options.


Some projectors - like D-ILAs and newer DLPs - with high fill factors are resistant to

Moire.


Check with the screen manufacturer as to how big you have to make the screen in order

to avoid Moire.


If the selected projector is powerful enough - in terms of its lumens output - you can

afford to throw away light through the perforations, and still have the proper illumination

of the screen. That is, get a projector that would actually be too powerful for the screen

if it were not perforated - and the perforations would bring the intensity down into the

optimal range.


Again - please explore all your options - it does make the theater experience more

"real" to have a perforated screen - just like in a real theater.
 
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