# "Video Wall" project for musical



## bwhiteford (Feb 4, 2009)

Hey all,
I'm doing a musical coming up that's going to use 3 projectors that will rear project onto some sort of drop. I'm trying to figure out what the correct (and most cost friendly) fabric would be for a drop that looks nice with nothing projected on it, yet handles the projections well. Hopefully my question makes sense. Looking forward to your input.


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## renegadeblack (Feb 4, 2009)

Interactive Stage Curtains Demonstration


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## derekleffew (Feb 4, 2009)

For REAR projection, there's really no substitute for true projection screen material, such as Rosco US : Screens : Rosco Screens, Rosco Guide to Projections screens brochure.pdf. None are terribly attractive when nothing is projected, but the black tends just to disappear.

For FRONT projection, the options are broader.


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## ruinexplorer (Feb 4, 2009)

While I agree with Derek that there's no good substitute for a good quality RP screen, I have had success with strech fabric like this. It is flame retardant and works fairly well at either front or rear projection. I found the fabric to be fairly sturdy and inexpensive enough that I didn't care too much about it if it got damaged. One thing to note, as it is not made as a projection surface, the hot spot of the projector will be much more noticeable on RP. I highly reccomend not using a short throw lens on your projector since the image does not take kindly to being extremely off axis and you don't want to see the hot spot which you would see if you placed the projector properly.


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## museav (Feb 7, 2009)

I guess it depends on what you think looks good and what you are trying to do with the projection.

If you noted, some of the Rosco screen materials and most of the screen materials from projection screen companies like Stewart Filmscreen, Hurley Screens, Da-Lite, Draper, etc. will have values for viewing angles and gain. The two are often interrelated and also relate to both the viewing area and the projector brightness required. High gain screens typically have narrower viewing angles, they focus the light hitting the screen into a more defined area. Screen materials with a wide viewing angle are typically lower gain as they diffuse the light over a broader area.

What many people do not realize is how much this can affect the image brightness and thus the projector required. Using the Rosco screen surfaces as an example, the Black and Twin White can both be used for rear screen projection. Gain on the Milky White is a peak of about 0.35 while the peak gain for Black is about 2.3. That means that with the same projector the Black screen would produce an image over 6-1/2 times as bright as the Milky White screen would, but it would be viewable over a smaller area. That could mean the difference between needing a 2,000 lumen projector or a 12,000+ lumen projector for the same image brightness. On the other hand, when dealing with multiple images and.or wide audience areas, narrow viewing angles can be problematic and you can end up with a very small area that has good viewing for all of the images.

Another factor is ambient light. When ambient light hits the screen during projection a black or grey screen can make the image appear brighter by minimizing the effect on the black level and thus the better maintaining a higher contrast ratio. Darker screens also tend to not be as readily apparent when ambient light hits them while nothing is projected.

As ruinexplorer noted, you typically want to avoid wide angle lenses with high gain screens or very long throw lenses with very wide viewing angle screens. For example, with a wide angle lens the light at the edges of the screen is hitting the screen at a much shallower angle of incidence while the high gain screen is then trying to focus all the light back into a narrower 'beam' off the screen, the result is numerous image aberrations, drop off at the edges and so forth.

So it usually comes down to balancing all of these factors along with the 'look' you feel works when there is no image projected. If you go with a material not specifically made for projection then a number of these factors are typically going to be unknown, which is why it is so difficult to say what might work in those cases other than simply by trying it.


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## mrb (Feb 7, 2009)

another place to take a look at for screen material as well as other fabrics that may work is dazian fabrics


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## bwhiteford (Feb 9, 2009)

Sounds good guys, thanks for the replies. The other issue besides the correct fabric is how are we going to get the throw we need to get the image big enough on the screen. At the moment it looks like we're going to have to have the image split across the projectors. The center one would be straight on, and the last two would be crossing using built in horizontal keystone correction to fix the distortion. 

Does this sound terrible, or acceptable? Somewhere in between?


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## museav (Feb 9, 2009)

Crossing? Like below:

\ l /
\ l /
\l/
/l\
/ l \
/ l \
------ (It doesn't seem to want to keep the spacing, but you get the idea)

That could be a lot of keystoning, is there a reason you don't just have all three projectors straight on?

Also, if you simple have three images you might be fine but if you spread one image out across a large screen using three projectors you are going to need either projectors or external processing that can 'edge blend' all three images as one, you'll also end up overlapping the projected images a bit for this. And you would have to consider the resulting image format and resolution, even if you use 4:3 projectors the result for three projectors is 12:3 or 4:1 (a little less if you edge blend) which is a very wide format and you'll have to keep that in mind when selecting or producing content. One of the problems is that even if you working with a 1600x1200 resolution to create content, the area you'll have on the screens is only 400 pixels high, resulting in a fairly low vertical resolution image.


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## bwhiteford (Feb 9, 2009)

Your drawing is correct. I wish we could have them all straight on (i'd sleep better at night) but the distance between the screen and the back wall is only about 8 feet. There is a 12 foot wide elephant door in the middle of that wall that leads into our scene shop. The shop is plenty big, so my idea is to have the center projector straight on and cross the other two like my previous post so we can shoot them all through the elephant door. If only the entire back wall was a door, but no facility is perfect 

I think I can correct the keystoning as these projectors offer both horizontal and vertical keystone correction.

As far as the image goes, we're just showing stock movies and graphics utilizing a Matrox TripleHead2Go Box, so it's alright if it appears a bit stretched. To blend the edges, we're going to produce videos that have that type of blending in the actual video so we don't need to purchase expensive blending processing equipment ahead of time.

This is a great conversation, let's keep it going!


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## derekleffew (Feb 9, 2009)

bwhiteford said:


> ...I think I can correct the keystoning as these projectors offer both horizontal and vertical keystone correction. ...


That seems a significant amount of horiz. keystone correction. Are you sure the projectors offer that much?


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## bwhiteford (Feb 9, 2009)

We did some testing to see how much we could get away with, and we were quite impressed. Without cutting holes in concrete, I'm not sure if theres any other way we're going to get rear projection in this space, so hopefully it works. I suppose the best way to get the keystone correction right is to set vertical keystone when the project is focused straight on, then set horizontal keystone when the projection is at an angle.


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## museav (Feb 10, 2009)

bwhiteford said:


> As far as the image goes, we're just showing stock movies and graphics utilizing a Matrox TripleHead2Go Box, so it's alright if it appears a bit stretched.


A bit stretched? You are going to have probably a 3072x768 or similar virtual display, maybe more like 2662x768 due to overlap for edge blending. That means either a very stretched standard format image or just a narrow vertical band of a more common format image. This may work fine, here are some examples with three edge blended projectors (DPI2001d and DPI2001a), but just be aware of the format you have to work with.


bwhiteford said:


> To blend the edges, we're going to produce videos that have that type of blending in the actual video so we don't need to purchase expensive blending processing equipment ahead of time.


Edge blending is more than just creating a wide image or stitching multiple images together, it accounts for the projector edge drop off and overlap required to get a seamless image. There certainly is software that will do this but there is definitely a difference between stitching and edge blending.


Keystone interacts and you have to consider the total correction required, not just in one direction. You have not identified what projectors and lenses you are using, but I would always try to use the best possible physical location first and then lens shift before using keystone correction. And a projector with 3-D or point-by-point correction might be better than simple horizontal and vertical correction. But the only way to know for sure if or how well it will work is to try it.


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## bwhiteford (Feb 10, 2009)

They are NEC NP3150 projectors with the standard lens (whatever it comes with). Does that help anyone?


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## museav (Feb 10, 2009)

bwhiteford said:


> They are NEC NP3150 projectors with the standard lens (whatever it comes with). Does that help anyone?


Yep, and mostly good new. Those have horizontal and vertical lens shift and with the standard lens you should be able to take advantage of all of it, although the horizontal shift is still limited (which is typical). They also use Silicon Optix processing and 3D Reform, so they essentially a very flexible geometric correction capability. Overall, probably one of the better products you could have to address the physical situation but you should still verify it before going too far.


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## ruinexplorer (Feb 11, 2009)

Don't forget that the projector that is straight on will end up having more output than those being cross-shot and keystone corrected. I am not familiar with that particular model, but some projectors have the ability to dim the image which may be necessary for you to have an even projection (if necessary due to content).


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## bwhiteford (Feb 24, 2009)

We're figuring out the projector situation still, but we would like to get the screen ordered. Does anyone know if that's something that can be rented, and if so where from?

I found this:

Projection screens - I. Weiss

but i'm not sure they rent them.

Thanks as always,
Brooks


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## museav (Feb 24, 2009)

Most rental and staging or events companies rent screens and/or projectors. The difficulty may be in finding the size and type of screens that you want.


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## derekleffew (Feb 24, 2009)

See this glossary entry: http://www.controlbooth.com/forums/glossary/9221-ratio-projection-image.html, and look in your local yellow pages under "Audio Visual". 

The show I'm working on right now has a 22.5'x40' main screen and two 15'x20' side screens.


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