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Author Topic: DLP, screen perfs, and moire patterns Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 08-13-2011 08:51 AM                        Is there a way to calculate or otherwise predict whether a particular combination of DLP (or LCD) projector, lens, and screen perf pattern will produce a moire pattern at a given image size? I have run into this a few times, with both standard and D-cinema video projectors, where the right (wrong) combination of these factors will produce an objectionable moire pattern on the screen. For those who have not seen it, this is the sort of pattern that can be observed by folding a standard window screen in half. It makes the picture basically unwatchable if there are any bright, solid-color areas in the image. I am involved with a film and video installation at a performing-arts center and want to make sure that this will not be a problem for them. Is there any way to do this short of installing the video projector and testing with various samples of screen material prior to ordering and installing the actual screen? (And is there any risk that there is enough variance in screen perfs that a small sample of material may produce different results than the actual screen?) In the past, the workaround that I have used for temporary setups is to zoom out the video projector and scale down the image. While this loses resolution, it is less objectionable than the moire pattern. Unfortunately, the venue in question is right at the short end of the lens that they will need, so this will basically not be possible. Even if it were, I would not consider it to be an acceptable option in a permanent installation. This is sort of a low-budget deal, so it needs to be done right on the first try. Does a micro-perf surface provide any advantages here? Is there any reason to consider that for a full-size, 700-seat theatre? In case it matters, the screen will be about 13x31' (common-height masking); the video image will be about 13x23' (for 1.78:1). Video projector will be a Christie HD12K (16x9 chip, 1920x1080) with the 4.1-6.9 lens. Throw is 113'. The lens won't quite zoom out enough for scope, but they can live with that. The screen type has not been determined yet, but will likely be either a matte or 1.4 gain Harkness. For now, the loudspeakers will be flown above the screen (shared with the stage system), but they will be getting a perf screen in hopes of upgrading to cinema speakers later. Film projectors will be Centurys with 2.5kw xenons in Super Lume-X lamphouses. The screen frame was already in place and is in good shape, so changing the screen frame is not an option.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 08-13-2011 01:09 PM                     Scott...it all comes down to pixel density...as the pixel spacing and size approaches the perforation hole spacing and size...the morie issue becomes more acute (it is always there...just not as objectionable). Believe it or not, microperf can actually accentuate the morie problem as it often makes the peroration size/spacing similar to pixel. Some screen manufacturers, like Stewart, rotate their perforation pattern so they don't line up with a projector's pixel grid. Also, since Stewart applies their perfs after the fact, they are not terribly uniform to begin with...as such, they rarely have morie problems. -Steve  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 08-13-2011 07:24 PM                     When it comes out bad, there is always the option to defocus slightly to reduce moire patterns. I doubt that microperf will help here. - Carsten  |  IP: Logged Brian Guckian Jedi Master Film Handler Posts: 594 From: Dublin, Ireland Registered: Apr 2003 posted 08-14-2011 12:46 PM                     The mathematics behind this are certainly very interesting; it appears that to avoid generating a moire effect, the difference in spatial frequency between the screen perforation spacing and the spacing of the projected pixels should be significant, as Steve implies. For a 23 foot video image, a HD projector should project pixels roughly 3.65mm x 3.65mm in size (7010mm / 1920), albeit not accounting for e.g. the gaps between the micromirrors with DLP projectors or between pixels on LCD projectors. If one can establish the (horizontal and vertical) perforation spacing for the desired standard perf or micro perf screen surfaces, this can be compared to the projected pixel size / spacing from the projector imager. I'm not sure if micro perforation surfaces are ideal in large auditoria due to the HF attenuation, but perhaps they have been used successfully in such circumstances.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 08-14-2011 01:02 PM                     In reality, it is even more complicated than this, because DLP pixels have a shape (square) and gaps, and both create their own spatial frequencies than can interact with the perforation. - Carsten  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 08-14-2011 03:36 PM                        Saw same problem on 35mm film of Train Your Dragon. At first looked like video noise on same color backgrounds. (The problem was video pixels vs. film grain.) Louis  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 08-14-2011 03:38 PM                     Or instead of spending all that money on a Stewart screen just step up to 4k DLP with the extra money you would have been spending on the screen. While there is no surface as nice as a Stewart going to 4k eliminates moire completely and affords many other benefits at the same time. Just sayin... Mark  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 08-14-2011 09:46 PM                     I was in no way implying that one should use Stewart screens in mainstream theatres...the cost would drive exhibitors even more nuts. In screening rooms and places where tickets are not even sold, that is another story. Going 4K may or may not help...it all depends on the pixel density versus perforation. -Steve  |  IP: Logged Brian Guckian Jedi Master Film Handler Posts: 594 From: Dublin, Ireland Registered: Apr 2003 posted 08-15-2011 07:11 AM                     quote: Carsten Kurz In reality, it is even more complicated than this You're right, because an additional factor must also be the diameter of the perforation versus the size of the pixel as it appears on the screen. Consider that one manufacturer's standard perforation diameter is 1.2mm and their micro perf diameter, 0.5mm. Taking the first value, this may appear sufficiently different to the 3.65mm x 3.65mm projected pixel size mentioned above, but if one allows for the gaps between DMD micromirrors, or between pixels in an LCD panel, then that size effectively reduces, and may sufficiently approach the 1.2mm perforation diameter to cause a moire effect, even if the pixel v. perforation spacing is satisfactory. It must be possible to devise a mathematical formula that predicts when a moire pattern will occur, based on a set of known parameters.  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 08-15-2011 08:25 AM                        As we move toward full digital, the industry may just need to move away from standard perf screens that were acceptable for film presentation and look to other alternatives. For example, SMX Cinema Solutions makes a woven plastic screen that is even more acoustically transparant that the traditional perf screen -- evidently like everything else in physics, there are degrees of acoustic transparancy. This material does have miniscule spacing within the weave (think wicker basket), but it is not uniform and that makes all the difference when it comes to moire patterns forming. And even there, the company cuts the material so that the weave is not horizonatal, but at a slight angle. I don't believe this company at the moment supplies sheet large enough for large theatre size screens, but if this kind of material is a real solution to the moire pattern problem, perhaps the industry will have to think out of the box and abandon perfs as we know it and move to a screen that is a better fit for digital exhibtion. BTW, I would guess angling the screen pattern as is done with the weave screen, could be applied to perf screens as well. If you had a sheet that was cut so the perfs were slightly angled off the horizontal, it might help reduce moire, given that the pixels in the chip are on the horizontal. As I said, just a guess. Then again, perhaps some screen manufacturer needs to come up with a perf pattern that is totally randon. No reason why they all need to be lined up in the traditional patterns. Seems like it wouldn't be terribly difficult to punch holes ransomly. Just give me an ice pick and a patent application.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 08-15-2011 08:29 AM                     I have not seen any moire on any of the 4k installations I 've done on screens up to 62' wide. I've purposly tried to get it to appear by projecting various test patterns and such but to no avail. Of course if you blew the image up large enough it would come back... you'd have to be somewhere near the square of the 2k pixel size before that might happen though. The image would be huge... Probably larger than you could adaquately light up. Mark  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 08-15-2011 10:06 AM                     Maybe manufacturers will introduce laser-perforated screens with random perf spacing and size sooner or later. Would invert the previous random grain on film to regular perforation on screen situation. - Carsten  |  IP: Logged Jim Cassedy Phenomenal Film Handler Posts: 1661 From: San Francisco, CA Registered: Dec 2006 posted 08-15-2011 10:58 AM                     I saw a custom installation in a private screeing room a couple of years ago where the moire problem had been 'solved' on a 9x12ft screen by attaching about 100 small surplus pager vibrator motors around the cirumference of the back of the screen, causing the entire surface to vibrate. It seemed to work, except that if you sat in the first row, there was a low frequency hum from all the pager motors audible during quiet parts of the sound track. If you sat further back, it blended in with the room's ambient noise level.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 08-15-2011 12:36 PM                     Just pump up the volume and your moire is gone... - Carsten  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 08-16-2011 10:27 AM                        Thanks for the replies. Unfortunately, there isn't much in the way of constructive ideas here. The venue considered a D-cinema projector, but decided against it for various reasons, so 4k is out. A non-perf screen is an option, but I really want them to preserve the option of installing proper cinema speakers at some point when they find out that sharing a sound system for live performances and films never works well. There is a Hurley perf screen on the frame now. It is probably at least 20 years old and discolored and in need of replacement. If we test the setup on this screen and it works, can we trust that a new Hurley screen would also work without issues? I would prefer Harkness, but am more concerned about moire issues than getting the perfect screen surface.  |  IP: Logged

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