Matte refinement overview
Keying is rarely a one-step process. Although adding a keying filter is the first step, there is usually additional work that must be done to create a good key that retains detail around the edge of your subject. You can use the mask tools and alpha channel filters to turn a decent key into a great one:
Create a garbage mask to crop unwanted background objects in the shot that can’t be keyed, such as the edge of a blue screen stage, lighting rigs, or tape that appears in the background.
Create a holdout mask to restore part of a foreground image that has been removed by the key.
Add a Matte Magic filter to refine matte edges.
Add a Spill Suppression filter to manually neutralize a key color.
Use multiple keys to isolate difficult-to-key regions in your foreground subject.
Important: When using masks with keying, apply any masks after applying the keyer. Additionally, be sure to apply the masks to the same image layer as the keying filter itself.