I have run into a problem. I am working with some video. It depicts a boy watching a movie that is being projected onto the side of a grain elevator. The projection slowly moves across the side of the elevator and on to the side of it so that it is on 2 walls.
I have created my stand in geometry of the elevator. I started by attaching the movie to a spotlight's color channel. I have then projected it onto the building.
Now how do I get the projection square? By barn doors?
Secondly, how do I get the beam to show up, choose fog and then do I set the movie to the fog color channel?
Third, and MOST IMPORTANT, how do I get just the projected image to show up and NOT the geometry as well. I tried the primary visibility thing but it makes the projected movie disappear as well.
I'll address your question about making the movie square - I assume you mean you don't want it to distort perspective-wise. I'd say just to cheat it in the scene. In other words, don't position the light source where the projector is in the scene. You'll probably have to find an acceptable middle ground where it still looks like it's being projected from the projector, yet isn't skewing all over the place.
As far as just rendering the projected footage only: make a render layer with everything in the scene except any other lights but your spot. reassign all of the materials on all geometry to probably a grey lambert. Then render that render layer seperately for compositing.