What is chroma key? The use of this technique has become integral to modern filmmaking. Whether alongside the use of green screens or blue screens, chroma key effects have become the most commonly used VFX technique in the entire film industry. In this post, we’ll provide a chroma key definition, explain the importance of this technology, and the reasoning behind using different colors.

Importance of chroma key

Explaining the chroma key effect

Chroma key is a term most associated with the desired effect achieved from the use of a green or blue screen, or by using green or blue paint. Let's explain what chroma key technology is and how to use it.

CHROMA KEY DEFINITION

What is chroma key technology?

The Chroma key technique is the process by which a specific color is removed from an image, allowing that portion of the image to be replaced. This color can be any solid color, most commonly blue or green. Chroma keying can also be used as a verb for the act of removing a solid color from an image. This means that the sentences, “I used a chroma key on the video,” and “I chroma keyed this video” are both correct. The technique can also be referred to simply as “keying” or “keying out.”

Chroma key app

How does a chroma key work?

Proper chroma technique may seem daunting from a distance but it is actually one of the easiest VFX to pull off. The simplicity of the key effect doesn't mean that executing it well doesn’t involve a good amount of technique and craft.

One of the first techniques was called the Sodium Vapor Process, which was originally developed for Disney's Mary Poppins. As the following video explains, it was an ingenious method to work around the glowing edges that plagued early blue screen processes. 

Science and Innovation in Early Chroma Keying Tech  •  Mary Poppins

A quick and dirty effect can be pulled off with very little effort, but a clean and professional-grade version takes a little bit more time and care put into the execution. In general, the assumption might be that the specific color being using is the most important consideration. In actuality, it is the green screen lighting that determines the quality of the effect.

Using a green screen like a professional is within your grasp. You can execute a chroma key in After Effects, using the plugin in Premiere, or through the use of chroma key apps. The below video offers an in-depth guide to filming with a green screen.

The importance of chroma key & green screen basics

You don’t need studio backing or a huge budget to use chroma key software. This is a relatively simple effect that you can pull off single-handedly for free or extremely cheaply, even using an app which will automate the process, but you will find better results when using a full editing program.

You can even make your own green screen. The below tutorial shows how to execute a simple key in Premiere.

Chroma Key Premiere Tutorial

Or, for a more advanced tutorial, check out the following example using additional elements to execute this effect in After Effects.

Chroma Key After Effects Tutorial

Executing this effect in Photoshop is a somewhat different process. Unfortunately, the workflow is different in every software program, so it may take time to adjust. To learn how to remove keyed backgrounds in Photoshop, check out the below video.

Keying out backgrounds in Photoshop

Learning to incorporate green screens and chroma key green paint into your filmmaking toolbox can open up a whole new range of possibilities within your creative projects.

Chroma color

Chroma Key with different colors

There is plenty of room for experimentation in video. Achieving the cleanest key possible means avoiding the common problems. This typically comes down to evenly lighting your screen but there are also some tips and tricks to clean up problems if your video has already been shot. Filmmaker IQ shares some tips for using green screens in the video below.

Filmmaker IQ gives tips for avoiding keying problems

Green and blue may be the most common chroma colors but they are not the only options. Technically speaking, any solid color can be used as a background. Red, purple, orange, brown, any solid color at all. The reason why green and blue are used most often is that neither green nor blue is present in human skin tones, and people are the most common subject that are keyed.

Green and blue also have a higher luminosity than other colors, meaning they register more brightly on digital cameras naturally. There are a number of small differences between green and blue screens but for the most part, they can be used interchangeably.

The most important thing to keep in mind when choosing which color to use as your key is to avoid overlap with the subjects in your frame that you do not want to remove.

If your subject is wearing a green shirt, use a blue paint instead of a green unless you want their shirt disappearing along with the background.

UP NEXT

How to Use a Green Screen Like a Pro

Now that you have an understanding of what keying is, you may want to try doing it yourself. Before you jump right into it, be sure to take a look at our step-by-step guide for using a green screen like a professional.

Up Next: How to Use a Green Screen →
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  • Sam Kench is an internationally-awarded screenwriter, independent filmmaker, and film critic. Lover of foreign films; hater of American remakes.

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