Excellent Guide to Chroma Key and Green Screen Footage

Green screen is an essential technique for most productions. It can be used in everything from independent YouTube videos, to the latest Marvel film.

Premiumbeat.com recently released a great guide to green screen and chroma key implementation. Let's take a look at some of their major points.

What is Green Screen and Chroma Keying?

To understand what green screen is, there are certain terms we have to be familiar with.

  1. Green Screen- This term refers to a single colored background, which will be made transparent in post production. Blue screens and even red screens are also frequently used.
  2. Chroma Key- This is the actual technique editors use to remove the green color from recorded footage. It involves layering and compositing two images based on one colors chroma range.
  3. Keying- The process of removing the green from the background once the chroma keying process has begun. Once a color has been keyed it will be transparent, and can be replaced with an image.
  4. Spill- This term refers to the colored light reflecting back on the subject. Certain steps must be taken to prevent spill.

Rules of Green Screen in Modern Production

1. Pick the correct color.

When implementing green screen in a production, the actual green screen material must be non reflective. Colors such as "chroma key green" and "digi green" are good identifiers in picking your green screen.

While green is the most common color for the chroma key technique, some productions also use blue screen. This is usually based on what colors are present in the subject of the shot, or when the scene takes place. Many times night scenes will use a blue screen.

2. Separate Your Subject

Your subject should be at least six feet away from the green screen. This will mean less spill, as well as fewer shadows on the green background. This is important, because a flat single background color is essential for this process.

3. RAW Footage is the Best Footage

Recording with the highest bit-rate with the least compressed codec possible to enhance picture quality, and therefore make the chroma key process that much easier.

4. Light/Expose the Background Properly

Both your foreground and background must be properly lit. Proper lighting exposure means less spill.

5. Eliminate Motion Blur

When creating green screen footage, filming with the fastest shutter speed available will reduce motion blur. This will make the footage cleaner, helping to create a better key.

6. Leave the Work to the Software

Ultimately, the chroma key quality will come down to the software you chose. Once you've found the NLE of best fit for you, you can get to work on creating your green screen shot.

Image Via Warner Brothers

Shooting on Green Screen

Plan your shot

Planning ahead will entirely determine the quality of your green screen production. Concept art, storyboards and visualizing your story will help in your process. The first question to ask is: "what needs to be real, and what doesnt?"

What will actually be present in your scene, and what will be computer generated? If you're working on a complex scene, you'll want to create a guide for you and your crew, covering every element.

It is also important to determine your scale. You must shoot your subject on the same scale as you are applying for your effects. If you are shooting a wide shot, shoot with a wide angle lens, etc.

While shooting, you will also want to prep your actors and objects. Avoid green on your actors and subjects, don't include shiny or reflective objects, because this could lead to manually masking the footage.

Lighting a Green Screen

The essential rules to lighitng your green screen are as follows:

  1. Lighting MUST be even
  2. Lighting must be soft and diffuse
  3. Light the green screen separately from the subject.

Remember that the most important part of shooting live action green screen footage, is even lighting. Without it, your footage will look sloppy, unfinished and unprofessional- as the chroma key effect will not work to it's best potential.

Getting a Clean Chroma Key

Feeding from this idea, chroma key relies on a clean picture. Color hues, lighting and distance will create a clean green screen effect. Try to remember these points.

  1. Prevent spill- spill will mean you will have to manually fix elements of your shots, meaning you will increase the time consumption of your post production process.
  2. Avoid motion blur when possible- the green screen technique relies on sharp edges.
  3. Film with a Low ISO- When filming on a green screen, it is important to film with a low ISO setting. In fact, it is recommended to use the lowest ISO available on your camera.

Keying a Green Screen Scene

Once you've shot your footage. If you have followed all of the steps above, the keying process will be significantly easier. Perhaps the most important step in this process, is picking the correct NLE.

Here are some great options

  1. Adobe Premiere Pro- This is the industry standard for a non linear editor. It also is built with top quality keying effects.
  2. Final Cut Pro- While still a powerful tool, this NLE has a more casual workflow- but is still incredibly effective.
  3. Avid Media Composer- Avid also features a powerful chroma key plug in.
  4. Magix Vegas Pro- An underrated, very flexible chroma key module is included in this NLE. It is worth noting, that as a film maker this is the chroma key module that I use most often.

For more information on the chroma key process, check out this article

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