Keyer filter overview
The Keyer filter is intended for subjects shot in front of a blue screen or green screen. However, you can also use this filter to “pull” chroma keys from any range of color within a shot. The keyed portions of the image are rendered transparent, allowing background images to show through.
At its default settings, the Keyer filter attempts to work as automatically as possible. When you apply the Keyer filter to an object, Motion analyzes the visible frame in the Canvas to detect a dominant color such as a green or blue background. The dominant color becomes the initial sample that generates the tolerance, or core transparency, of the underlying key; this key is rendered as transparent in your composite. You can override the automatic initial color sampling to key any color, and you can refine your settings using the parameter controls in the Filters Inspector.