Chroma Key Basics


Chroma Keying, which is commonly called Green Screen is a bit of “movie magic”, which has grown in popularity and use since movie making and video work has entered the digital age. In a nutshell, Chroma keying is a technique used for combining two frames or images by replacing a color or a color range in one frame with that from the another frame. It is often used to replace a scene’s background by using a blue or green screen as the initial background and placing the actor in the foreground.

For Optimal results, chroma keying needs to encompass three important elements – (1) shooting in a format which is chroma key friendly, (2) proper lighting techniques and (3) using an adequate editing workstation and having advanced editing skills. This is not to say that green screen magic can’t happen on the cheap, but attempting to do so may not give you the finished cinematic look which is commonplace nowadays.

1. Format – Subsampling Schemes

This is perhaps the most technical aspect. However, instead of boring you with all the technical gobbledygook regarding chroma subsampling, let’s go straight to the bottom line. Shooting in 4:2:2 is an ideal rate for chroma key projects. Anything more could be considered overkill and anything less may be considered inferior. Many DSLR and consumer video cameras unfortunately fall into the inferior category as the better ones have a best rate of 4:2:0. That’s not to say shooting at 4:2:0 or less will prevent you from chroma keying, but there may well be noticeable issues with your final product.

Please keep in mind, even the leanest version of Apple’s ProRes 422 Mov files are very large (45-50 MB per second). This may be an issue for your computer during editing.

2. Lighting

Using a basic 3-point lighting technique after first achieving flat and even lighting of your green screen with 2 lights will in most cases produce the results you desire. So in essence, you have a 5-light setup. Unlike traditional photography and videography, the hair light or back light has an increased importance as it will greatly help separate the foreground from the background.

It’s also important to provide some physical distance between your talent and the actual green screen to minimize shadows and spill. Five feet is a good rule of thumb to use as a minimum distance. Anything less will most likely come with issues. 8-10 feet would be better.

As a final note on lighting, the above basically pertains to a single talent on camera. If your scene has 2 or more people, the complexities involved in getting it right may seem to be overwhelming. The key is to have an abundance of both patience and lights.

3. Editing

Your editing workstation consists of a non-linear editing (NLE) software product or products installed on a robust computer.

Software. Currently there are many powerful NLE software choices. Most modern NLE software products have chroma key capabilities built in. It is recommended that you familiarize yourself with this part of your specific product. There are tons of tutorials online to help you out. Just search your product name followed by “chroma key”. For example if you use Adobe’s After Effects, your search string will be “after effects chroma key”.

If your clip has any camera movement whatsoever, in addition to keying your clips, you will also have to become adept at 3-D motion tracking. This is to insure your new background moves in perfect symmetry with the camera’s movements. Similarly, a certain amount of masking is often required in chroma key editing. Both of these features should also be a built in feature of your NLE software.

In some instances, you may find using more than one software editor is required to get your desired end results.

Hardware. The advanced editing techniques mentioned above combined with the large file sizes in 4:2:2 video will bog down your average desktop or laptop computer. It is therefore recommended you have access to a true workstation with one (or more) multi core processors such as those from the Intel Xenon family, and 16 or more GB of RAM. A high powered dedicated graphics card and multiple monitors will also make your life easier. If you’re thinking of buying a new system, your budget should start at $3,000 (US) for an entry level HD editing workstation. Workstations used in big movie studios and post houses are very powerful and typically cost 4 times as much.

A couple final thoughts on (chroma key) editing.

One is to keep your video files in their largest uncompressed form throughout the editing process, even though your final product may be destined to the internet and rendered at 1280×720 using a H.264 codec (Best for YouTube).

Another is to have at least 2 hard drives in your system to work with. When rendering your work, being able to read from one while writing to the other is definitely preferable when working with large HD video files.