Green Screens and the Art of Filming Crazy Cool Effects

Green screens are part of a special effects process that's variously called "chroma key compositing," "chroma keying" and "color keying." But we'd say the most descriptive name is the one the British Broadcasting Corporation (or BBC) prefers: "color separation overlay."

In today's Hollywood, the method is pretty straightforward. First the studio sets up a flat, smooth surface — like a literal screen, a sheet of fabric or a painted wall. Whatever it is, the thing has to be uniform in color.

Next, the performers act out their scenes with the monochromatic surface right behind them. The effects artists use computer software specifically designed for this job to filter out everything in the shot that matches a pre-selected color.

So if, say, the backdrop is a vibrant green — and you've told the software to digitally remove that color — then the background surface will become transparent. It can then be replaced with a still image or a different video feed.

Just like that, the actors are magically transported from a green-lined set to some faraway planet. Or maybe a dinosaur paddock like the ones in "Jurassic Park." The possibilities are legion.

Usually, the screens and backdrops used in these visual effects shots come in one of two colors: Green or blue.

Why is that? Let's suppose you've got something in the foreground (like a costume piece or green screen suit) that's the same color as your backdrop. The software might fail to see the difference and then erase the object automatically.

Granted, this could work in your favor — but only if it's intentional.

So when selecting a backdrop color, anything that resembles human skin tones is probably a non-starter. Can't have the leading lady's hands and face disappear without warning. (Not unless she's playing Sue Storm anyway.)

Bright green and royal blue look nothing like human skin. And they're seldom used in costumes. For these reasons, they've become the two most popular backdrop color choices in scenes that depend on chroma keying tech.

Because modern cameras have an easier time singling green out, you're more likely to see green screens than blue ones on today's movie sets. Also, it's harder to get the lighting right with blue screens.

Don't get us wrong, though. There's plenty of room for both. In "Spider-Man" (2002), the title character swings around New York City in his trademark blue and red tights. Whereas his archnemesis the Green Goblin wears... well, he wears green. (Duh.)

For the flying scenes, the crew had to put Spider-Man in front of a green screen and the Goblin in front of a blue screen. That way, both characters stood out against their respective backdrops.

Besides accidentally wearing things that match the background color, there are a few other mistakes you'll want to avoid while playing around with the chroma keying process.

Try to light the actual "screen" (or wall or what-have-you) as evenly as possible. That'll make it much easier to edit out of the shot.

Also, since you're combining two separate images, definitely see if you can get the lighting to sync up. If the actor on screen casts a shadow in one direction, but the shadows in the background picture point the other way, your audience will spot the mismatch.

Green screens should be kept wrinkle-free, or you'll run into that uneven lighting issue again. Keep the screen taut while you film — and consider ironing or steam-cleaning it as needed.

Properly staging the actor(s) is important, too. Make sure none of the people or objects in the foreground are casting shadows directly onto the screen. For best results, put some space between them and the backdrop.

We live at a time when chroma keying isn't just for the major networks and studios anymore. Vloggers and YouTubers can now take full advantage of this technology. Zoom even has a virtual background feature which lets participants pose themselves in front of sandy beaches, city landmarks or whatever else tickles their fancy.

According to the company's website, this feature "works best with a physical green screen and uniform lighting to allow Zoom to detect the difference between you and your background."

Luckily, a household green screen may be purchased for under $50. Some models even come with their own editing software. So go forth and make cool visuals!