Green screen (or blue screen, which used to be more common) is a method of transforming the entire background behind an actor into something else, usually something so elaborate that it couldn’t be done using ordinary props on a set. Much of the modern Star Wars trilogy was filmed in this manner. It’s actually a fairly simple process, but requires some technology. Basically it involves actors acting in front of a big green sheet hanging from the wall. The main consideration is making sure the actor is not wearing anything green, because later you’ll get the computer to remove all traces of green from the picture so that it becomes essentially see-through. The computer is then able to fill the entire stage behind the actor with anything you wish; usually it’ll be some elaborate vista created with animated computer graphics.

For the purposes of the 2003/04 Film Club at Clounagh Junior High, we needed to show the school in the wake of a bomb blast. Forget about green screening for a moment. First things first: how do you show a demolished building without demolishing the building? Having some experience with Adobe Photoshop, I showed the kids how to use the Clone Stamp tool, and let them loose with a digital photo of the school. I chose the best result, then I imported the photo into Apple iMovie and applied the program’s smoke effect. Presto. One minute (or whatever length desired) of usable footage of a school in ruins.

We could have used the footage as it stood, and simply cut away to the actors’ faces, but I was determined to make the thing look as real as possible, and that meant showing actors in front of the wreckage. I cast my mind back to the days before green screening, to memories of those old movies where you’d be looking directly at the driver of a car. You’d see him moving the steering wheel left and right – rarely in proper time to the weaving of the road behind him, because, of course, the road isn’t real. The car is on a set, and there’s a projector behind it, out of sight, beaming an image of the road onto a screen. And that is exactly what we did at Film Club.

In the photo, between the two girls, just out of sight below waist level, is a multimedia projector connected to a laptop computer. Windows Media Player is playing the clip of the school we made earlier. The projector is projecting it onto a screen not more than four feet in front of the girls. My camcorder is about six feet behind them, and I’ve zoomed in just enough to avoid seeing the edges of the projector screen. Our actresses don’t have a lot of freedom to move. If one of them so much as raises a hand, it’ll go in front of the beam and cast a massive shadow onto the screen. But they can look at each other and talk, and it’s enough to complete the illusion. If your scene requires more movement, you can consider back projecting, but unfortunately our screen was too thick to allow that.

The final ingredient – and it’s easily overlooked – is the sound. I went outdoors and recorded a minute of outdoor ambience – wind, birds, traffic, etc. – and layered this onto the scene at the editing stage. If you listen carefully to the girls’ voices, you can tell by the reverberations that they’re not really outdoors, but on a budget as tight as ours (i.e. zero), you can only go so far (egg boxes all over the walls is something to consider, if you’re inclined to go the extra mile).