Post Production: Basic Tips for Chromakeys

Post production is the final phase of production when the movie really comes together. This is the process of turning footage into a polished film. One of the things that happens in post production is a green screen shot being replaced with a background through the magic of chroma key. But even though it is a post production effect, a lot of its outcome relies on how it was handled in production. 

How Chromakey Works

Chromakey works by locking in on a specific color in your video and then making that color transparent so that a background under the video will be visible. The technology is decades old, but as digital technology has increased in recent years, the use of chromakey has grown with it.

Use a Green or Blue Screen

Let's say we are going to have our actors walking on an alien planet that we will later create digitally. Well for now we will have them act out the scene in front of a green or blue screen which we will then replace with the alien world later on with chromakey. Green is more commonly used than blue because less wardrobe and props are that color. Blue is also used because it is less reflective than the green, but more objects and wardrobe tend to be blue which can create problems.

Green Screens at Studios

Some studios have something that is called a cyc. A cyc is a section in a studio that is made out of plaster and does not have any distinct corners. This creates the illusion that the background is infinite. You normally see cycs used in commercials with an infinite white background, but you can also paint them green or blue for chromakeying.

Making Your Own Green Screen

If you don't have a studio then you can rent your own green screen. You can buy or rent one and then stretch it out to meet your needs. If you are doing this you need to make sure that the green screen is large enough and that it is not made of cheap material that is thin because this will create head aches when keying it out.

Light the Screen Evenly

Once you have your green screen set up you want to light it separately from your scene. It is extremely important that the green screen is evenly lit because this will make the screen one uniform color. An unevenly lit green screen is going to be composed of many shades of green and this can create a ton of head aches when trying to key it out.

Watch What You're Doing on a Larger Screen

When it comes time to key out the green screen, make sure that you're doing it on a large monitor. You want to make sure that the green is disappearing without any parts of your actor becoming transparent either. There's a certain finesse needed for chromakeying. An evenly lit shot on a thick green screen is relatively easy to replace while an unevenly lit shot on a crappy green screen requires some time and expertise.