Green Screen

We have a green screen, primarily for the use of chroma keying.

Setup

Lighting is extremely important for green screen use. The aim is to light the screen as evenly as possible with minimal hot spots (use the zebra function on cameras or the video assist to check for hot spots). The subject should be set with a standoff from the screen to separate them from the green better, and there should be a difference in colour temperature between the subject and the screen (again for better separation). Both of these will make the keying easier.

The camera setup is also important. The shutter angle should be lower than the usual 180 degrees to introduce less motion blur to the subject which makes the keying a lot harder. The recommended angle is about 60 degrees, which is a decent balance between adding too much stuttering to the subject and reducing the inherent motion blur. This will naturally also reduce the brightness of the image by limiting the incoming light. Also, stopping the iris to as low as possible (depending on the scene the green screen subject is being composited into) and thus blurring screen will also help the keying as it will crush down the detail on the surface of the screen.

Post and keying

Some practice is recommended for post green screen keying, but here are the basics:

• Use After Effects
• Use keylight within After Effects
• Use the duplicated keying layer trick
• Ask people who do a lot of either keying or After Effects post work.

Issues

• Aliasing effects cause some software to incorrectly key out parts of the subject. The solution to this might be to use an anti-aliasing filter on the camera.