# projecting a background



## Ljdonnelly (Dec 21, 2011)

we are doing a production of all shook up and we have decided to try to project the background for a few scenes. the problem is we can't figure out how to do it. we have a 30 foot backdrop and are using two projectors mounted in the ceiling of the stage to cover the back drop. 
one option we were discussing is to create a composition in photoshop that would be wide enough then split the signal in two and run it to each projector. but we cant figure out how to split the signal. 
the second option would be to run the same image twice which we know how to do but we prefer option one. 
any adive on how to make option one work is greatly appreciated. we can get software and hardware but budget for this is not that high. not cheap but not rediculous. thanks


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## museav (Dec 21, 2011)

Are you trying to project over the entire backdrop? Is it 30' high, 30' wide, 30' diagonal or what? Are you going to be able to keep all stage light off of the entire are where you will be projecting or will some portion often have light on it?

What is meant by the projectors being "mounted in the ceiling of the stage"? Where would they be located relative to the 'screen'? What do you have for the projectors and the computer source? Where would the computer be relative to the projector locations and do you have the requisite cabling either available or installed?


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## Ljdonnelly (Dec 21, 2011)

we are projecting over the visble part of the backdrop.
it is about 25 - 30 feet high and about 15 feet tall 
yes we will be able to isolate it
we mounted the projectors hanging from the ceiling about 30 feet from the backdrop
using either a mac or a pc laptop which ever works
the projector is a nec vt series (i will post exact model later) 
the computer will be about 30 feet away and we are going to rent the cabling


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## MarshallPope (Dec 21, 2011)

Assuming you already have the positions of the projectors taken care of and whatnot, your simplest route will be a Matrox Dual Head to Go - HERE. It will stretch/split a video signal into two signals. I feel like your biggest issue, though, will be getting the coverage you want.


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## Rdonham (Dec 21, 2011)

We projected some backgrounds with a Matrox Triple Head to go and 3 projectors, Mac, and Pro presenter with edge blending. Worked very well.


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## Jawbone (Dec 24, 2011)

I agree, a matrox dual head or just double stack your projectors and keystone them to one image and use something like vpt (free) or resolume for fine keystoning.


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## museav (Dec 28, 2011)

I believe that the DualHead2Go simply makes two display devices, of the same resolution, appear as one larger display. It does not handle edge blending, format conversion and so on, that would have to be addressed in the video production or graphics hardware/software. However, processing the image to display over two projectors is one thing, getting the desired image size, format and brightness from the location available may be another.

For example, say the desired overall image is 15' high and 25' to 30' wide. Assuming a 10% to 20% overlap for edge blending that means each image would need to be 15' high and 13.16' to 16.67' wide. That is a 0.88:1 to 1.11:1 ratio format, basically square rather than the typical 5:4 (1.25:1), 4:3 (1.33:1), 16:10 (1.6:1) or 16:9 (1.78:1) rectangular image format of projectors. In fact the 5:3 (1.67:1) to 2:1 ratio for a single projector creating a 15' high by 25' to 30' wide image is much closer to the format of any typical projector. This suggests that for a 15' high by 25' to 30' wide image stacking or overlaying the projectors may be more effective than splitting the image horizontally.

The other factor in this is what image size the projectors can create given the apparently fairly fixed location. I am not sure that any of the VT projectors may provide a 15' high, 16:10 or 16:9 image with a 30' throw distance, they would likely need to be further away or would be limited to a smaller image. A 15' high by 25' to 30' wide image is also a very large image for the brightness of the VT projectors, so you may have to consider how bright the backgrounds need to appear, the projection surface will also be a factor in that.

You may also need to consider where the projectors are vertically and side-to-side relative to the projected image. You may be able to correct for some vertical keystone but that is not without tradeoffs while I think the VT projectors, or at least many of them, do not support lens shift or horizontal keystone correction.


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