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Puppet Kaos - where Kelvin Kao plays with puppets and tell random stories

How to do a green screen effect: the theory

In my post Editing Trick: How to have two puppets on screen at the same time with only one puppeteer, I said that I’ll talk about the green screen effect. The green screen effect is something that’s very widely used in film and TV productions. Huge portions of movies like Star Wars and 300 are filmed in front of a green screen so they can use a lot of computer generated images in the background. It’s a very useful effect. So, let’s talk about how green screens work.

When you use a green screen, you first film the foreground object (usually some kind of character) in front of a screen that’s green (first picture). And then, you insert your background (second picture) in the back (where backgrounds belong). And you’ll have a composite like the third picture:

What the software will do for you, is look through every pixel in the first picture for green. When it sees a green pixel, the pixel becomes transparent so the background layer can show through. So whatever was green in the foreground picture is replaced by what’s in the background in the final result.

When you use this method, you should be very careful because if the character is wearing anything that has green on it (like the question mark on the shirt in the following picture), it will disappear too! And then you get a shirt that’s partially transparent and you can see through the guy’s body and see the background. So be very careful.


What if you want to have a foreground character that wears green? In that case, you might want to use a blue screen effect instead. That is, film the character in front of a screen that’s blue, and in editing, let the computer pick out all the blue pixels instead of the green ones. In many movies, both are used depending on what’s in the foreground.

Why don’t we use red screens? It’s because that our human skin has red in it and you might end up with strange effects that you don’t want. Of course, this might or might not be an issue for puppets. I haven’t tried it out yet.

In the next post I’ll talk about how to do it with actual footages.

Related posts:
Editing Trick: How to have two puppets on screen at the same time with only one puppeteer


  1. April 14th, 2009 | 1:11 am

    […] already written a tutorial on how to do a green screen effect before, but I thought the example wasn’t the best because it wasn’t the best example […]

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