Keying is defined as the process of replacing part of a piece of footage with footage or an image from another source.
Alpha channels are important as these provide the transparency or opacity of an image these are represented in black(blocks out info) and white(shows info).
When filming on a green screen there can be variations of green on the curtain, even wrinkles causing even darker variations. By separating it into separate keys this allows you to get a more precise key on areas around the head, arms, torso and legs. Without this, the green can occasionally spill onto the character.
This is a single image wrapped round into a sphere, this helps to simulate environmental light by emitting light from the image itself, this can be done by using 360-degree footage as the wrap around.
Skarlet 