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One Mirror, Two Projections

The front porch serves as a hollusion conduit using its two arches (there are actually two arches left) to host mosquito netting and projections.

In the first year I projected hollusions into the arch, I found that the five foot depth of the porch was insufficient to fill the arch space. In that year, I positioned the projector inside my office, which is the room inside the house from the porch, and projected out the window. I needed to open the glass and remove the window screen, but it worked and people were genuinely amazed by how I did it.

I blocked off access to that part of the porch visually and physically to prevent anyone from disturbing the effect, but I did notice a number of dads looking intently from different angles trying to figure out what I had done. When I noticed that, I told them.

The next year, I came up with the idea to use a mirror to reflect the projection back, allowing me to take advantage of using double the porch depth and keeping my office out of the decor. That worked really well.

In subsequent years, I kept projecting on the one side of the porch and occasionally added a white canvas painting tarp to the other side that I front projected onto. Eventually, the hollusion side became part of the computer coordinated show.

This year, my show utilizes both front-facing arches and will need two projectors and media players to fulfill. I'm working on the projector and media player solutions and have ordered new mosquito netting to test out for the space. My challenge is getting the image to the screen.

My first instinct was to use two mirrors for back reflecting. I also thought about using two short-throw projectors and no mirros. The latter was easily dismissed as being quite cost prohibitive. The former means taking on more awkward equipment. The mirror I had been using hangs on the wall in the guest room and the reflecting surface measures 27"x34". It's a big mirror and does a good job of reflecting the screen size from the projector in the first five feet of travel.

I would need another mirror of similar stature. Though, I could dual purpose it for another room in the house when it's not used the one night during Halloween.

However, it occurred to me that I could still use a single mirror to reflect two projectors pointing in different directions. The projectors would be setup in a V-configuration with the mirror. The selected projectors would just need 4-point key correction because of the additional lateral angle.

I do foresee two potential problems: 1) The 4-point correction is a software feature and image resolution could be seriously diminished with that much distortion; however, it is being projected onto mosquito netting and I'm not expecting high-definition output; 2) The light photons crossing each other could cause noticeable image distortion, or at the least, react more boldly with any fog that may creep into the proximity of the mirror.

Ok, the photon thing might be a bit too much, but the effect is additive and fogger smoke will be more illuminated in the area. Something to consider.

On another note, except for the basic, cheap projectors I use to project video loops onto props, no two projectors in my kit are the same. Since the hollusion arches are side-by-side, I believe strongly that I need two matching projectors in order for the effect to look authentic.

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