What Is a Green Screen (Chroma Key) & How Does It Work?

A green screen converted to an ocean background
A green screen lets you change the background of an image or a video.

In videography, a green screen or chroma key is a technique used to change the background of a video to something more intriguing.

This guide teaches you what is a green screen and how it works.

Green Screens in Videography

In videography, having the right type of background can enhance the viewer’s experience. For example, an interview that takes place in a boring environment is not that appealing to watch.

To make the video more interesting to the viewers, the background has to be chosen carefully. This can mean the whole film crew has to travel far to achieve a nice background for the shot.

But it’s not always affordable, feasible, or even possible to shoot a video in a beautiful environment with an outstanding background. Also, in some situations, the desired backgrounds might not even exist in the real world.

This is where green screens help. With a green screen, all you need is a pure green wall to shoot the video anywhere in the world or virtual world. The green screen experiences are realistic, and the viewers cannot tell whether the video is actually shot onset in the background or not.

But how does a simple green screen turn into a nice view so naturally?

What Is a Green Screen?

Image Credit: thinkenvy.com

Green Screen or Chroma Key is a backdrop of bright pure green canvas. The green screen makes it possible to change the background to any image, video, or live feed by the editors. In a sense, the green screen is a placeholder for the background.

With modern-day technology, the transition to the edited backdrop is natural and looks indistinguishable from actually shooting the film in that place.

The professional-level green screens are made of stretchable nylon spandex. But honestly, any bright green fabric can get the job done—at least to some extent. Some people might even paint the studio walls with green color to mimic green screens.

Why Green?

Now you might wonder why green screens are green, not red or blue, for example.

There is a simple explanation for this.

The green color is rarely used in everyday fashion or decor. Thus, the background color of green is not confused with the clothing, hair color, or such.

Other colors, like yellow, orange, brown, or red could also be used instead of green. But these colors are found in different shades all around us. This makes changing the background much harder.

Consider using a brown screen instead of a green screen. If a person with brown hair stepped into the view, the chroma key technology would think the hair is part of the background, not the person.

The green plant would make a green screen editor scratch their head.

How about a Blue Screen?

Blue is one of the rarest colors in nature. Very few animals, plants, or artificial objects are blue.

So why no blue screens instead of green screens?

The green color is not only a rare color, but cameras are also sensitive to it. As a matter of fact, digital cameras capture green color twice as much as any other color.

In other words, replacing a green color with the desired background is the easiest for the post-production teams.

Another benefit of the green color is you need less light for it to be bright enough. This saves money as the electricity consumption doesn’t need to be that high.

Even though it appears that the green screen is the way to go, there are some exceptions. As a matter of fact, blue screens are indeed sometimes used.

Why Blue Screens Are Used Sometimes?

Even though the brightness of the green screen is a great feature, it’s also why sometimes a blue screen is used.

Thanks to the brightness of the green screen, it can spill green color to some other relevant parts on the set. If an object reflects green light, it disappears into the green background.

This is typical when dealing with shiny objects that easily reflect light.

When filming a darker scene, a blue screen is much more post-production friendly. This is because the blue screen emits less light and is unlikely to melt objects into the background.

Using a blue screen is still more expensive because it requires much more light to work.

Next, let’s take a closer look at how the green screens work.

How Does a Green Screen Work?

To make a green screen work, the background surface has to be uniform. To accomplish this, the canvas must be straight without wrinkles that introduce unnecessary contrast.

One of the main techniques that make the green screen work is called keying. Let’s take a look at the keying process.

Keying

Keying means removing the green screen in the post-production phase with a video editing tool.

Make sure to check the best green screen editing tools!

When the green background is keyed, you are left with a transparent background. To this background, you can insert anything from images to videos. For example, you can insert an ocean or city view into the background.

There are two main ways to make the green screen transparent.

  • Chroma keying
  • Luma keying

In the next two sections, you will learn what these techniques mean.

Chroma Keying

Choma keying is the most common keying technique used in the post-processing of green and blue screens.

The idea is simple.

Each color has a chroma range called the chrominance value.

The chroma keying splits the video into separate layers based on the unique chrominance value of the video.

In other words, you can transform all the parts of the video (or image) of the specific chrominance value to the desired background.

Luma Keying

Another popular technique to make a green screen work is called luma keying.

Instead of controlling the transparency of the background based on the color, brightness is used instead.

In luma keying, the layer transparency is set based on the brightness (luminance) level.

Because the green screen is the brightest object on the set, the green layer becomes more transparent than the rest of the scene. This makes it possible to replace the background reliably with another scene or image.

The luma keying is a strategy used when editing still images.

Summary

A green screen is a popular technique to change the background of an image or a video.

Instead of traveling far to an appealing set, all you need is a studio with a green screen or green walls.

The green screen works so that:

  1. The post-production team removes the green background from the view.
  2. The background becomes transparent.
  3. A new background layer is applied below the image or video.

To change the background, two popular techniques are used:

  • Chroma keying to remove the green background based on green’s unique chrominance value (color).
  • Luma keying to remove the green background based on the brightness levels.

The reason why green is used is that it’s a rare color that people seldomly wear. It’s also bright and requires less lighting which cuts costs. Other than that, there is nothing too specific about green. Sometimes blue screens are used too!

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About the Author

Artturi Jalli
I'm an entrepreneur and a blogger from Finland. My goal is to make coding and tech easier for you with comprehensive guides and reviews.