# Combining Rear and Front Projection / any known issues?



## Celine (Feb 17, 2014)

We are working on a concept for a projection on a large conference stage, that would involve two rear projection screens for the slides and the live feed of the speakers and a front projection that is used for an wide screen opening show covering the whole 16-20 meters stage as well as a "background" animation that masks out the two rear projections during the talks. Does anyone have any experience with something like that?
We are wondering if if the light from the front projector projecting the background images during the talks could interfere with the power of the rear projected screens.


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## Footer (Feb 17, 2014)

Are you rear and front projecting on the same screens? Biggest issue that I can see is front projecting on a screen built for rear projection will not work all that well. Front projection screens are designed to reflect light... rear projection screens are designed to let light pass through. Two totally different things.


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## Celine (Feb 17, 2014)

Thank you Kyle for answering! 
The two smaller screens would be a part of the front projection only during the opening show, but we were looking into maybe getting Rosco's Twin White screen (that works for front as well as rear projection) or maybe just covering them during that time? During the talks there would be always rear projection on the two smaller screens but we hope to project a slightly animated environment around them from the front at all times. We can't avoid having these two screens, since the speakers need them, but would love to use the whole stage for a wide projection that is also mapped on some volumentric elements. it's would be sad to loose all that space and go with the other option: just using flat rear projections (maybe 3 larger screens), so we hope we can find a solution.
We love front projection, but another problem is it's a huge hall with a very low ceiling (ca. 5m) so far we've been working in theatre and never had that problem.
We would like to do something like these two nenatv / nenatv but we don't know how to best incorporate the two rear projections in there (the classic conference/talk set up with two screens and a speaker desk ).


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## Drew Schmidt (Feb 17, 2014)

The biggest issue I see is quite simply alignment. It's not a problem, just a potential problem. It's near impossible to physically align/keystone two projectors to layer on top of one another (or in this case on either side). Be sure you have a method of corner pinning. If all four corners of the screen are physically in the same location, then in theory ... on a flat surface ... every pixel inside that area will also fall on the same physical location. The question then is, how?

What software are you using to control you show? Anything like QLab or Isadora should be able to handle this. Or you could get more advanced and use syphon in conjunction with MadMapper.


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## Celine (Feb 17, 2014)

Hi Drew, that's kind of the problem we want to avoid with using front projection over the whole surface of the stage, the rear projectors would be turned off during the opening show and would be covered by the front projection. During the talks we would mask out their spaces. Masking so far has not been a problem for us on stage (concerning the background framing loops during the talks) , but we are worried that even if we mask out the spaces where during the talks the rear screens are, the light of the front projector could still wash out the rear projections, since these masks are digital not physical.


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## Drew Schmidt (Feb 18, 2014)

Sorry. I should have backed up.
First to answer your question, yes, technically, the front projector *will* shine light on the rear projection screens. Even if you digitally mask them. But in my opinion, any decent projector will have a negligible effect. Especially in comparison to any stage lighting going on. So the real answer to your question is ... not really, no, don't worry about that. Worry more about the stage lighting. 

What software are you using? Because as I said earlier, if you're projecting on the same surface twice, it would be wise to use that to your advantage and repeat the image. This will create a brighter image. But again, it requires something that can corner pin very well.


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