You want to learn Chroma Keying in after effects.But first of all have a look at what chroma keying is actually. You can get some basics of keying here.
What is Chroma Keying?
Chroma Keying is a visual effects technique in which a specific color is removed from an image or video and is composited on another element.
How to Chroma Key in after effects?
1) Bring Your Footage in Composition
Bring in your footage into a new composition and place it over a background which you want to use.
2) Apply the Keylight Effect to Your Footage
In the effects panel search for the effect Keylight , which is under Keying and apply it to your green screen footage.
3) Select the color to key out
Go to Effect Controls window , Select the eye dropper tool in keylight effect. Then click on the color in your footage which you want to key out.
4) Change the View Option
Change the view option which is set to final result as default.Open the dropdown menu and change it to combined matte.In this Transparent pixels will be shown as black and opaque would be white.It makes it easy to check the key and tweak it.
5) Adjust Screen Matte Settings
Expand the Screen Matte Dropdown below and adjust the undergiven options according to your footage requirements.Increase the clip black to completely remove the background.it will help to reduce the dark or noisy areas. Increase the clip white to make your subject opaque.These options would be enough to get you a quick decent key.
6) Change the view option to final result
Go to the view drop down again and change it to final result.
7) Refine your key
To further refine your key you can use refine matte,key cleaner and advanced spill supressor effects according to the requirements of your footage.
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