Projectors with alpha channel?

[Steve Roberts]

Umm … alpha channels don’t apply when it comes to projecting light (film or slides) on something. Think about it, when you’re projecting black on a set, it doesn’t add anything to the existing light on the set. When you’re projecting white, you’re lighting up the set with the projector bulb.

[David Bogie]

Head to the school library and see what they’ve got for past issues of theatrical lighting magazines or hit the Web to see what’s happening in this interesting application of video projectors. It’s been going on for several years.

There’s nothing different about using a video projector than using a slide projector, movies, or a moving disk image in a leko or Trouper. As noted, black is the absence of light form the projection.

The main issue you must deal with is the scene’s ambient light overpowering the projector’s relatively weak beam. If you’re hitting side curtains, you’ve got to keep them masked off so your lighting designer will need lots of time to integrate video projectors in to her light plot.

bogiesan

This is my standard sigfile so do not take it personally: “For crying out loud, read the freakin’ manual.”

[Graham Quince]

I did this exact set up for our school production last month. You want to use black as your alpha channel equivalent. There’s no such thing as black light (not getting into UV here), black is the absence of light, so what your projector throw out is the faintest of faint white. If you project a black screen onto a black curtain, you get a very faint grey square. If you project white you get a strong white square.

We mounted the projectors as close to the screens as possible and keystoned the lens correction. We did this so our actors could get really close to the backdrop without getting in the way of the projection. At times it looked like we’d rear-projected they were so close.

If you keep your stage lights off the backdrop, there is no problem with the projector being washed out.

The only other advice I can offer is to keep the cables out of the way. One of our kids managed to kick the VGA cable out of the PC on opening night and we had to wait for the interval to reset.

Graham

https://www.shiveringcactus.bravehost.com – Free FX for amateur films

[Dick Smith]

i say the answer is interpretive dance

_dick

[Thomas Leong]

Here my two cents –

There are a few cheap ways to mask off the hard edge of the projected screen area to get rid of that rectangular look even when video black (which is actually greyish) is projected –

1. Tape up the edges of the projector’s lens with gaffer tape, or block the light off at these edges with a book/videotape covers and the like. The closer these ‘mechanical gobos’ are to the lens, the softer the light fall off will be.

2. Wash out the edges with stage lights. Problem is the spill from these lights onto the rest of the screen area.

3. Over project the image, i.e. projected screen area is larger than the physical screen, and wash out the spillover with stage lights. To avoid the flames getting too large to be realistic, you could pre-comp the flames into a larger comp with a black background.

Thomas Leong