# Rear projectors and forced perspective



## TechWench (Dec 26, 2004)

Im doing a show in february and it calls for a bunch of projections that are really key to the show. The TD wants to use a rear projector. the screen he wants to use is fabric [not sure what kind or anything yet]. 
there are 3 screens right next to eachother. the middle one is going to be quite big [its where most of the action happens.]. now this set is going to be forced perspective. and a thrust stage. ive never done this before. 
im the SM for this show and i wanted to do a little research on the topic of the rear projector and forced perspective to help him out. what kind of rear projectors are out there and what are the best kinds. 
i did a google search and nothing too helpfull came up! grr..
also, what are some tips for this forced perspective thang. 
[its just odd to me that its in a full thrust stage.]

any help is wonderfull...thanks!


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## Sombra2 (Dec 26, 2004)

are you talking about rear projector like the projector will be back stage and shooting toward the audience or rear projector like the projectors will be in the back of the audience shooting toward the stage?


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## TechWench (Dec 26, 2004)

*oops*

sorry, i should have clarified.
it will be behind the stage shooting towards the audience.


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## Sombra2 (Dec 26, 2004)

ok, i haven't done this before but i know projectors and you be able to use a regular projector just have it a mirror image or maybe just use it regularlly. Mirror or not it should work just the same as a regular front projector.


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## SuperCow (Dec 26, 2004)

We did just that for our recent production. The projector was actually directly beneath the screen, pointing away from the audience and at the back stage wall, on which we hung a mirror. From the mirror, it reflected onto the screen. It required a bit of keystone correction, but it worked really well, all things considered.


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## Peter (Dec 27, 2004)

We have a mondo big projector in our booth that we have used for this stuff (although obviously it wasnt rear projection) but one day i was flipping through it's options, and it had one setting to change and it flipped into rear projection mode, where it merrord the image for you. If you do use a merror, be warned that you will loose abit of your brightness when you bounce off of it. 

Whatever you do, try and get pics, i want to see what this looks like when you pull it off!


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## ccfan213 (Dec 27, 2004)

also using a mirror could be risky as far as aim, i am assuming it wont be on the whole time, and in the rush to do a set change backstage someone could not aim the projector properly at the mirror or the mirror properly at the screen, also as far as loosing brightness goes, im pretty sure you just add up the distance from the projector to the mirror and the mirror to the screen to determine the difference. my eye doctor did something like this once to test my vision from 20ft away in a 10ft (or was it 5ft) room.


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## Sombra2 (Dec 27, 2004)

you have two options to keep it clear without using a mirror or if your projector doesn't have a mirror mode. you can hook your projector to a computer and on some computers there an option to mirror the output, or a cheap projector which is get a overhead projector which many teachers have lieing around in their classrooms and get a transparent color lcd screen, many math teachers in my school have black and white ones that link to their calculators, anyway get a transparent colored lcd screen and just turn it upside down to mirror the image.


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## bdesmond (Dec 27, 2004)

Mirror/flip/roate settings are pretty much standard issue on lcd projectors. Usually if you ceiling mount a projector, you hang it form the bottom, and thus the image has to be rotated. 

As far as rear projection goes, the true screens for doing this use a special type of material. I saw this done for a play with a mirror, a standard flat, and an el cheapo projector. I never saw it in action, but from what I heard it worked just fine. The projector was hung under a platform on top of which the flat was mounted. How exactly the mirror was hung, I don't know, but, they got it to work. 

Another thing you'll need to consider more or less depending on your set is getting the power and signal to the unit. This isn't a big deal in general, two cords assuming you've got a composite signal, but, if the projector is out in the set, you'll need to dress & hide the cables most likely. 

Finally, you've got to have a source. When I saw this done, a PowerPoint presentation was used with Photoshop stills for slides. You can embed stills and video content into a PowerPoint slideshow, works well for this. If you're going to use a computer to run this, you'll need a scan converter in order to run a composite signal to the projector. If your projector has a computer input, and you're intent on or have to use it, you're either going to need to purchase a lengthy male/male VGA cable to get from the projector to the computer, or you'll have to park under or next to the projector. 15 conductor cable, whch is what VGA runs on is rather bulky and difficult to run, especially compared to some nice flexible coax. 

I'd recommend that you go the scan converter route, and also get a simple switcher to go between the scan converter and the projector. This way, you can cut to black when you're not running slides, video, whatever. Otherwise, you need to have someone to go put a piece of cardboard in front of the projector, or hit a video mute button on it if it has one in order to switch sources. Otherwise you're going to end up with a blue screen, color bars, partially sync'ed image, or some other unwanted picture. 

Yet another option, if you either have the equipment already, or have a chunk of money for rentals is to rent a long throw and front project the image from a balcony, catwalk, etc. A long throw projector is probably going to run you in the $200 - $300 a day range to rent it.


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## len (Dec 27, 2004)

What kind of image is being projected? Is it a simple metal gobo image? Full color litho gobo? Video? How big? Can you split the gobo and do multiple source? The type of projector you'll need will impact the advice. Also, fabric may not work, depending on what it is. A lot of them are just too opaque to do any good. Rear projection screen material will decrease the intensity anywhere from 3 - 10% depending on quality.


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## SuperCow (Dec 27, 2004)

In our case, there were no scene changes in that part of the stage, so the projector, mirror, and screen never were moved.


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