One of the most famous examples of compositing is the green and blue screen. In a still image it’s easy to cut out an actor and place them atop a background. Once they start moving however, the process of cutting them out becomes laborious. That’s why methods have been devised to automate the cutting out based on calculating what is actor and what is background automatically. Though there are many ways to do this, green and blue screen are the most memorable.
The process of removing the background based on color information is often called a “chroma key”. There are several important tricks for “getting a good key”.
If possible, get a shot of the green part that’s supposed to become invisible. A good keyer will often let you feed this into it to help with the key.
Get the actor away from the green screen. Shoot as far forward as possible. The green light “spill” that reflects off the screen will destroy your edges.
Get the green screen evenly lit (as much as possible) and make sure it’s only around 1 stop brighter than the talent is lit.
I generally shoot at a faster shutter speed and deeper aperture to maintain as much detail as possible. Get a clean key and then add destructive elements like depth of field blur and motion blur in post.
Pull key in parts: A “core” key, a hair (and/or other extraneous detail key) and whatever other unique edge requirements present themselves. Just don’t try getting the ideal settings for the entirety of the figure’s edge with a single key.