Visual Effects for Film

VFX is a tool for your story telling~

Why use a green/blue screen? The concept of “keying”.

For most people, when they think about shooting for visual effects, they think about green/blue screen. Why use a green/blue screen and how does it work? In this post, we are going to talk about the reasons for use green/blue screen and what visual effects artists have to do after receiving this green/blue screen footage. For this post we will give you just a basic concept of blue/green screen, and we’ll go further into detail in later posts.

To use green/blue screen, there is a simple rule:”The object you want should be in front of the green/blue screen” For example, the actors is what you want, but you want to replace the background. So you put actor in front of green/blue screen and make sure the green/blue screen completely covers this actor’s acting range. This means your green/blue screen needs to be big enough so that everything you want to keep is inside of this green/blue screen.

VFX, green screen

Your green screen need to be able to cover all the area that you want to keep.

green screen, matte painting
Now, let’s take more deep look of how green/blue screen works. In the compositing software(such as Adobe After Effects, The Foundery Nuke and many others), there is a tool call “key”. What this does is tells the computer all the pixel that is this kind of green or blue should be treated as transparent.(Note: Key tool works with other colors, but blue and green work the best because the digital images use Red, Green and Blue channels to make up the image. Red is not very good because actors skin tones often have a lot of red ) With this command being applied, the computer starts looking for all the pixel that are this kind green or blue color and then treats those pixels as transparent.
In this picture, you can see a lot of little square. Each square is one pixel. You can only see this when you zoom in a lot on a picture. If you carefully look at those pixels, you can tell that all the pixels are not exactly the same green. Some of them are lighter green and some of them are darker green. This is because every part of the green screen didn’t receive light in a exactly same way. The tiny wrinkles and bumpy surface of the green screen will cause some shadow or highlight and make each pixels show a different color. An unevenly lit green screen will cause this as well.


The key tool will find the same kind of green and treat them as transparent.

The keying tool will find all the pixels that is the same kind of green and then treat those pixels as transparent. The result will look like this picture shows. The artists will have to keep adding more similar colors to the key tool until the entire green screen turns transparent. But if you carefully look at the picture above that shows pixels, you can see there are some pixels that are close to the finger are the color between green and the color of finger. Those pixels are often ignored by the key tool. An experienced artists will have to apply other techniques to remove them.

green screen, keying

Hairs are so thin that they often can look green.

green screen, vfx

Half transparent objects will require more work in order to key out the green.

Above are two pictures showing difficult situations to key out green. As you can see, thin hair and half transparent object all contain a lot of green. If the artists tells the key tool to take out large range of green, those detail will be treated as transparent. If the artists tell the key tool to only take away certain green, the darker green will remain and look opaque, but what you want is for it to be half transparent. Those situations will take a lot more time and depend on the experience of the artist to apply different techniques to get rid of the green while keeping enough details. Avoid these situations will save you time and budget for your film making process.Now, you should have a very basic idea about why and how to use green/blue screen. You should also have a concept about what happens after shooting and how vfx artists can get rid of the green/blue screen. We hope through these concepts, you can avoid some difficulties and make your visual effects process more smooth and efficiency.

5 responses to “Why use a green/blue screen? The concept of “keying”.

  1. Nadim mir 2013/11/20 at 4:58 PM

    i do not understand visual effect

    • LightRay FX 2013/11/22 at 2:23 AM


      Thank you for visiting this blog. Sorry about your confusing.
      If you start from the first post “what is visual effect?”, you might get better idea.
      If you still can’t understand, please let us know how to improve.
      It’s our goal to help people understand and be able to apply VFX as a tool.


  2. harsha 2016/03/22 at 8:10 AM

    it was helpful. thank you πŸ™‚

  3. karuna 2016/12/09 at 12:16 PM

    It was very useful for beginners like me πŸ™‚

  4. Hari 2018/04/16 at 9:13 PM

    It was very useful for me to take an new step to into VFX in this blog Thanks Ya I really loveit ✌

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