The Green Screen Effect, also known as Chroma key, is the process of shooting against a blue or green screen, removing that screen colour with special software, and adding in a more exotic background in post processing.
Chroma key is a widely-used technique that allows foreground objects to appear in a video scene even though they were not present – and often could not have been present – when the scene was shot. When an actor jumps into a volcano, or battles a giant insect, it is often the case that chroma key was used in the scene.
Chroma key effects are often called “blue-screen” or “green-screen”. This is because the foreground action is shot in front of a blue or green background. The background is then electronically removed, leaving only the foreground action to be superimposed on the actual background of the final scene, which has been separately filmed.
Blue and green are the generally-preferred colours for chroma key because their removal from an image will not affect human skin tones. Any colour however can be used to do chroma key special effects.
Chroma key tips
Successful use of chroma key depends on carefully setting up your shot, and will need trial and error to get the details just correct.
Light the backdrop as evenly as possible: Use multiple lights on the backdrop to ensure that it is well-lit across its whole area and without hotspots. Diffuse sunlight, as produced by a light overcast sky, can work well when shooting out of doors is an option.
Don’t let the subject shadow the screen: The subject should be at least one meter in front of the backdrop. This will help to avoid shadows being cast on the screen.
Choose foreground colours carefully: Don’t wear green if you are shooting on a green screen, or blue for a blue screen; those areas will become invisible later in post processing.
Make a smooth profile: Chroma keyers do better with a smooth edge than a jagged or complex one, so try to present a smooth profile to the camera. Hair is particularly tricky, and should be slicked down if possible.
Use tight framing: The wider your frame, the larger your background needs to be, and the more difficult it is to film. Shoot your subject from the waist up rather than in full view.